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Food Reviews International
ISSN: 8755-9129 (Print) 1525-6103 (Online) Journal homepage: http://www.tandfonline.com/loi/lfri20
Raisin Processing: Physicochemical, Nutritional
and Microbiological Quality Characteristics as
Affected by Drying Process
Ramla Khiari, Hassène Zemni & Daoued Mihoubi
To cite this article: Ramla Khiari, Hassène Zemni & Daoued Mihoubi (2018): Raisin Processing:
Physicochemical, Nutritional and Microbiological Quality Characteristics as Affected by Drying
Process, Food Reviews International, DOI: 10.1080/87559129.2018.1517264
To link to this article: https://doi.org/10.1080/87559129.2018.1517264
Published online: 25 Sep 2018.
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Raisin Processing: Physicochemical, Nutritional and
Microbiological Quality Characteristics as Affected by Drying
Process
Ramla Khiari
a,b,c
, Hassène Zemni
c
, and Daoued Mihoubi
b
a
Higher School of Food Industries of Tunis (ESIAT) - 58 Avenue Alain Savary, 1003 Tunis El Khadra, University
of Carthage, Tunisia;
b
Laboratory of Wind Energy Management and Waste Energy Recovery, Research and
Technology Center of Energy (CRTEn) - B.P. N°95, Hammam-Lif, Tunisia;
c
Laboratory of Molecular Physiology
of Plants, Center of Biotechnology of Borj-Cedria (CBBC) - B.P. 901, Hammam-Lif, Tunisia
ABSTRACT
Processing and conservation of grapes by suitable techniques has
been a major challenging issue for a long time. Optimization of
drying and pretreatment operations of this fruit have been exten-
sively studied. However, in order to achieve the production of high-
quality raisins and reach consumers’acceptance, special attention for
quality attributes should be taken into account. Quality characteris-
tics of grapes such as color, texture, vitamins, phytochemicals, aroma
profile and microbial stability are of paramount importance since
they could vary throughout the dehydration procedure and would
directly determine quality perception and consumer choice. This
paper presents a comprehensive review of the physicochemical,
nutritional and microbiological characteristics of dried grapes as
affected by the drying process. In addition, it investigates the
changes of different grapes quality attributes (mainly nutritional
and aromatic proprieties) during processing, which enables profes-
sionals and scientists to better choose and optimize grape processing
to deliver the highest raisin quality to consumers.
KEYWORDS
Drying process; grapes;
quality; raisins
Introduction
Grapes are one of the main prevalent agricultural crops; they have been cultivated since
prehistoric times. The global grape production currently amounts to more than 75.8
million tons (Mt) according to Food and Agriculture Organization
[1]
and International
Organization of Vine and Wine (OIV)
[2]
data for 2016. The world’sfive largest grape
producers are: China (about 14.5 Mt), Italy (about 7.9 Mt), United States of America
(about 7.1 Mt), France (about 6.4 Mt) and Spain (about 6.0 Mt).
[1]
Around 71% of this
production is destined for wine making, while the remainder is consumed fresh as table
grapes and juice or dried as raisins.
[3]
Due to their high moisture and sugar contents, grapes are very perishable and even
stored under best refrigerated conditions, they still remaining highly susceptible to con-
tamination with spoilage and pathogenic microorganisms. That is why these fruits should
be consumed or converted to other derived products within few weeks after harvest
CONTACT Daoued Mihoubi daoued.mihoubi@crten.rnrt.tn Laboratory of Wind Energy Management and Waste
Energy Recovery, Research and Technology Center of Energy (CRTEn) - B.P. N°95, Hammam -Lif 2050, Tunisia
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lfri.
FOOD REVIEWS INTERNATIONAL
https://doi.org/10.1080/87559129.2018.1517264
© 2018 Taylor & Francis
otherwise, their marketability would be jeopardized, which could result in conspicuous
economic losses.
[4]
The improvement of grape management, processing and marketing is
required which, in turn, emphasizes the need for the adoption of more effective preserving
approaches. In this respect, drying, as the oldest food-preservation method, should be one
of these techniques that may fulfill this need by enhancing derived grapes product quality,
widening their availability and diversifying their trade.
[3]
Drying basically removes the excess of water until an appropriate moisture level is
reached that inhibits the growth of bacteria, molds, and yeasts, slows down the enzyme
degradation and inactivates the majority of the physical and biochemical reactions.
[5]
It is
believed that the first raisins were produced in the near east by simply burying the grapes
in the sand. The aim of grape drying was to extend their shelf life and, due to their high
sugar content, to provide excellent sources of energy for workers executing hard tasks.
[6]
Preserving grapes in the form of raisins has other advantages including reduction of
weight and bulk, which contribute to the lessening of packing, storage and transportation
costs, as well.
[7]
World production of dried grapes (Raisins, Sultanas and Currants) reached 1.24 million
metric tons during the season 2016–2017. Turkey was the major producer, accounting for
310,000 tons (25%), followed by the United States with 297,738 tons (24%), China with
185,000 tons (15%) and Iran with 170,000 tons (14%). The four countries together account
for 78% of the world production, according to the latest report of the United States
Department of Agriculture (USDA).
[8]
Commonly, dried grapes are used as an ingredient in baking, snacks, breakfast cereals
and confectionery industry.
[8]
In 2016, about 1.2 million tons of raisins were consumed in
the world, an increase of 17% from 2000. With more than 250,000 tons consumed, the
United States and Turkey are the leading domestic markets, accounting for one quarter of
global consumption. China, with 203,100 tons of dried grape consumed in 2016, was in
third place.
[1]
World raisin consumption is in fact steadily increasing due to their nutri-
tional quality well recognized by consumers.
[8]
Many reviews concerning grapes phytochemical characteristics and their health benefits
have been published.
[9–12]
Grape drying has been well studied and the literature is rife
with very interesting examples, but we noticed the lack of a synthesis review dealing with
grape dehydration and its effect on quality attributes. However, when compared to other
food materials, grape processing is a more complicated bioprocess because the grape
berries should undergo a pretreatment prior to dehydration. Predrying treatments help
to soften grapes’waxy skin, which prevents the moisture diffusion and restrains the
procedure of drying.
[13]
Chemical and physical pretreatments have been extensively
proven to accelerate the grape drying process and improve the quality of the final dried
samples.
[14,15]
Chemical predrying treatments consist of putting grape berries in some
chemical preparation, which enables dissolution of the wax layer and affects the drying
kinetics in addition to procuring some antimicrobial properties.
[16–18]
According to Deng
et al.
[19]
, chemical pretreatments could be categorized into two types: chemical solution
(hyperosmotic, alkali, sulfite and acid, etc.) and gas (sulfur dioxide, carbon dioxide and
ozone) treatments. While physical pretreatment entails the use of thermal or nonthermal
methods including blanching (hot water, steam, superheated steam impingement, ohmic
and microwave heating, etc.), superficial abrasion, ultrasound, pulsed electric fields and
2R. KHIARI ET AL.
high hydrostatic pressure, which often targets the waxy fruit skin forming small crevices to
speed the drying process.
[19–21]
Several drying operations have also been widely studied. Sun and solar drying are the
two methods traditionally used for drying of commercial raisins. However, these processes
are very slow and depend mainly on weather conditions, which can induce microbial and
insect contamination of the resulting dried fruits and hence, lower their quality.
[22]
More
recently, advanced drying techniques such as oven drying, microwave drying, vacuum
pulsed drying, infrared drying and many others have been employed in order to enhance
the dehydration rate and guarantee a better quality of the raisins.
[7]
Nevertheless, in order to achieve the production of high-quality raisins and reach
consumers’acceptance, special attention of all processing phases should be taken into
account. The variation of the quality characteristics during processing should be
explored.
[23]
Additionally, it is widely known that drying may affect the quality attributes
of the raw materials either positively or negatively, and at the same time, partially or
totally.
[5]
Therefore, a better understanding of grape-drying operations and their impact
on these quality properties may help to bring new insights to deliver raisins of the highest
quality to consumers and satisfy their demand for this commodity.
The aim of this review is to present an overview on the effects of drying methods on the
quality characteristics of grapes emphasizing the physicochemical, nutritional and micro-
biological aspects.
Processing of grapes into raisins
At the ripening stage, grapes are harvested and transported to consumption or processing.
Ripening is characterized by the formation of a waxy layer on berry skin, softening and
development of the specific grape variety color as well as changing of taste from sour to
sweetness.
[24]
In the case of drying, the berries are first cleaned, washed, selected and
sorted, and then subjected or not to a pretreatment that accelerates the dehydration
process. After drying, the resulting raisins are sorted once again before being packaged
and marked (Figure 1).
With a waxy cuticle, grape berries are normally protected from biotic and abiotic
stresses. Indeed this skin layer plays an important role in controlling the drying process
because the waxes laid on the skin surface are hydrophobic and serve as impermeable
barriers to moisture movement through the cuticle.
[25]
With the intention to decrease skin
resistance, facilitate water evaporation and hence ameliorate drying kinetics, the use of
pretreatments prior to grape dehydration is highly recommended.
[26]
Pretreatments of grapes
As far as is currently known, there are two types of pretreatments that have been
frequently applied in raisins processing: chemical and physical treatments.
[26]
Both of
these pretreatments have been the subject of numerous investigations and have as main
objectives the modification of the grape skin permeability, enhancement of moisture rates,
swiftness of the drying process as well as improvement of raisins’quality.
[13]
Some
examples of grape pretreatments described in the literature and their relevant findings
are summarized in Table 1.
FOOD REVIEWS INTERNATIONAL 3
Chemical pretreatments
Chemical pretreatments involve treatment of grapes with sulfur dioxide (SO
2
) or dipping
berries into certain chemical solutions. These include sodium carbonate (Na
2
CO
3
), potas-
sium carbonate (K
2
CO
3
), sodium hydroxide (NaOH) and oil emulsion (e.g. ethyl oleate
and olive oil) at different concentrations and times.
[15,42]
It is worth noting that in the grape industry, the sulfiting technique (i.e. gas SO
2
fumigation or immersion in SO
2
solutions) was by far the commonly-used method for
bleaching grapes during storage and drying since sulfur dioxide treatment was known to
ameliorate food color development and conserve their quality.
[42]
However, excessive SO
2
usage may alter the quality of the processed raisins and could cause potential environ-
mental problems such as air pollution
[42,43]
or certain health concerns about reactions
associated with some types of asthma.
[44]
In order to alleviate these incidents, many
authors have assessed other alternative chemical pretreatments.
Figure 1. Diagram of grapes drying.
4R. KHIARI ET AL.
Table 1. Effects of different pretreatments on the drying of grapes.
Pretreatment
methods
Raw material
characteristics Predrying treatment conditions Drying method characteristics Analyzed parameters Major findings Reference
Chemical Sultana grapes
Average bunch
weight: 0.185 kg
Case 1: untreated;
Case 2: pretreated with potasa
solution;
Case 3: pretreated with solution
composed of 94% water + 4%
K
2
CO
3
+ 2% mixture A
Mixture Aconsisted of 70% olive oil
+ 20% ethanol + 10% (KOH:pure
water, v:v)
Sun drying Dimensionless weight
loss, dry matter, color,
pH and total acidity
–Drying time of different
pretreatments:
Case 1 = 436 h,
Case 2 = 172 h and
Case 3 = 128 h,
–Drying rate for pretreated grapes
with Case 3 solution was 1.375
times faster than the case with
potasa solution and 3.4 times faster
than the untreated case.
[27]
Thompson
seedless grapes
Average berry
weight: 4 ± 0.25 g
The initial
moisture content:
78%–81% (w.b.)
C
1
: Dipping for 30 s at 80°C in 0.5%
sodium hydroxide (NaOH) solution;
C
2
: Dipping for 30 s at 80°C in 2.0%
ethyl oleate (EO) + 0.5% NaOH;
C
3
: Dipping for 30 s at 80°C in 3.0%
EO + 0.5% NaOH;
C
4
: Dipping for 3 min at 40°C in 3.0%
EO + 2.5% potassium carbonate
(K
2
CO
3
) solution;
Control: Untreated grapes.
Microwave drying at air
temperature of 50°C and air
velocity of 2.0 m/s;
The final moisture
content: 0.18 kg/kg (d.b.)
Drying rates, color and
appearance attributes
–Drying time of different
pretreatments:
C
1
= 11.7 h,
C
2
= 8.7 h,
C
3
= 8.3 h and
C
4
= 11.0 h;
–NaOH dipping pretreatment (C
1
)→
pale and dull colored raisins.
[28]
Sultana grapes Traditional solution:5%
K
2
CO
3
+ 15% olive oil;
Solution A:4%K
2
CO
3
+ 2% ethyl
oleate;
Solution B:5%K
2
CO
3
+ 2% ethyl
oleate;
Solution C:6%K
2
CO
3
+ 2% ethyl
oleate;
Solution D:7%K
2
CO
3
+ 2% ethyl
oleate;
SDSO2: grapes first treated with SO
2
gas then was dipped into alkaline
ethyl oleate solution B;
NATUR: Untreated grapes.
Solar drying with air
temperature in the range
17–40°C and air velocity of
3 m/s;
Sun drying on concrete
ground, on wooden racks and
on polypropylene canvas
sheets.
Drying time and color –↑of drying rates with 5%
K
2
CO
3
+ 15% olive oil pretreatment
and Solution A;
Dying rates classification:
solar dying > sun dying on concrete
ground > sun dying on wooden
racks, or on polypropylene canvas
sheets;
–Nonuniform moisture content and
color intensities of the sun-dried
grapes;
↑of the drying rates and excess
lightning of raisins made with
SDSO2 pretreatment: not
acceptable in the market.
[29]
Grapes of Italy
variety
Pretreatment: an emulsion of 0.5%
NaOH solution + 1.5% ethyloleate for
30 seconds at 50°C
(Continued)
FOOD REVIEWS INTERNATIONAL 5
Table 1. (Continued).
Pretreatment
methods
Raw material
characteristics Predrying treatment conditions Drying method characteristics Analyzed parameters Major findings Reference
Thompson
seedless
grapes
Average berry
diameter:
18 ± 1 mm
D
1
: 0.5% NaOH solution at 93°C ± 1.0°
C for 5 s;
D
2
: 2.0% commercial dipping oil
+ 2.5% K
2
CO
3
solution at ambient
temperature for 3 min;
D
3
: 2.0% ethyl oleate + 2.5% K
2
CO
3
at
ambient temperature for 3 min;
D
4
: 0.4% olive oil + 7.0% K
2
CO
3
solution at ambient temperature for
3 min and
D
5
: untreated grapes.
Experimental dryer at
laboratory scale with
temperature of 60°C and air
velocity of 0.5 m/s
Drying rate and
organoleptic quality
–Pretreatment affected significantly
the drying kinetics: hot dipping
more effective than cold dipping
and no treatment assay;
–Cold dipping ↓considerably the
drying time vs. untreated raisins
Organoleptic quality from the best
to the worst: cold dipping
pretreatment > untreated grapes
> hot dipping pretreatment;
Pretreated grapes were fitted the
most with Page’s model to predict
the drying behavior.
[30]
Sultana seedless
grapes
A1) NaOH 30 g/L solution (85°C for
2 s) + washed with water (25°C for
5 min)
A2) NaOH 20 g/L solution (85°C for
2 s) + washed with water (25°C for
5 min)
A3) NaOH 3 g/L solution (85°C for 2 s)
+ washed with water (25°C for 5 min)
A4) NaOH 1.5 g/L solution (85°C for
30 s) + washed with water (25°C for
5 min)
A5) NaOH 1.5 g/L solution (100°C for
15 s) + washed with water (25°C for
5 min)
A6) K
2
CO
3
50 g/L + 4 mL/L olive oil
emulsion (5 min at 42°C)
A7) K
2
CO
3
40 g/L solution (42°C for
1 min)
A8) K
2
CO
3
5 g/L + K
2
CO
3
45 g/
L + 4 mL/L olive oil emulsion (5 min
at 42°C)
A9) NaOH 3 g/L + K
2
CO
3
5g/
L + 4 mL/L olive oil emulsion (2 s at
85°C)
A10) No treatment
Laboratory dryers at air
temperatures of 50, 59, and
70°C under constant air
velocity (2 m/s)
Drying rate and color –Dipping grapes in an NaOH solution
↑the drying rate substantially;
–Drying time between 450–900 min
depending on pretreatment and air
temperature;
–Lighter color of the treated grapes
vs. untreated ones;
–Shorter drying time and best
quality dried product: grapes
dipped in a solution of 5% K
2
CO
3
of at 42°C.
[31]
6R. KHIARI ET AL.
Sultana seedless
grapes
The initial
moisture content:
77.3% −80.5% (w.
b.)
POTAS: Potassium carbonate solution
(0.5 kg K
2
CO
3
dissolved in 10 L water
+ 0.05 kg olive oil);
AEEO: Alkaline emulsion of ethyl
oleate (0.5 kg K
2
CO
3
dissolved in 10 L
water + 0.2 kg ethyl oleate);
Both treatments for 1 min.
Control: Untreated grapes.
Batch drying at air
temperatures of 50°C, 55°C, 60°
C and 70°C and air velocity of
1.2 m/s;
Untreated samples were dried
at 60°C and 70°C air.
Drying time and color –↓of drying times of grapes dipped
in AEEO pretreatment vs. untreated,
or pretreated with potassium
carbonate solution;
–AEEO pretreatment ↑drying rates
better than POTAS dipping;
Pretreatment with ethyl oleate and
drying at 60°C →best color results.
[17]
Black grapes (var.
Muscat)
Average berry
radius: 1.83 cm;
Average berry
length: 2.78 cm;
Average berry
weight: 5.85 g;
The initial
moisture content:
79.3 ± 0.2% (w/w)
POTAS: Potassium carbonate
solution: (5% K
2
CO
3
+ 0.5% olive oil);
EO1: 2% Ethyl oleate (EO) + 2.5%
K
2
CO
3
;
EO2: 2% EO + 2.5% KOH;
EO3: 2% EO + 2.5% Na
2
CO
3
;
All treatments for 1 min;
Control: Untreated grapes.
Drying in a cabinet dryer at air
temperature of 60°C and air
velocity of 1.1 m/s;
The final moisture content:
25% ± 0.2 (w/w)
Drying time and drying
kinetics
–↑of the drying rate and ↓of drying
times of raisins pretreated with the
EO1 solution;
Page’s model →the most
appropriate model for drying
mechanism estimation vs.
Henderson, Pabis and Lewis’
models.
[32]
Seedless grapes
Dry matter:
23.62 ± 1.38%
Total sugar:
19.97 ± 1.06%
Equivalent radius
of
berries:
0.6657 ± 0.025 cm
EO: 2% ethyl oleate + 5%
K
2
CO
3
solution at ambient
temperature for 60 s;
PA: 4% PAKSAN oil (contains free
oleic acid and chiefly ethyl esters of
fatty acids; C
14
-C
18
)+K
2
CO
3
solution
at ambient temperature for 60 s;
HW: Hot water dipping at 95°C for
15 s
Laboratory-scale tray dryer at
air temperature of 40°C, 50°C,
60°C and 70°C; air velocity of
1 m/s and humidity ranging
from 10% to 15%.
Thermal
diffusivity, moisture
diffusivity,
and heat and
mass transfer coefficients
–Dipping pretreatments strongly
affected the effective moisture
diffusivity depending on the
moisture content and the
temperature of the product;
–Thermal diffusivity of the grapes
changed with the moisture content
of the grapes;
[18]
(Continued)
FOOD REVIEWS INTERNATIONAL 7
Table 1. (Continued).
Pretreatment
methods
Raw material
characteristics Predrying treatment conditions Drying method characteristics Analyzed parameters Major findings Reference
Sultana seedless
grapes
Average berry
radius:
1.60 ± 0.02 cm;
Average berry
length:
2.24 ± 0.02 cm;
Average berry
weight:
3.55 ± 0.03 g;
The initial
moisture content:
78 ± 0.1% (w.b.)
POTAS solution: 4% potassium
carbonate + 1% olive oil;
CONTROL: Untreated grapes.
Drying in a cabinet dryer at air
temperatures of 55°C, 65°C and
75°C and airflow of 2 ± 0.1 m/
s;
The final moisture content:
20%
Drying time and color –POTAS pretreatment ↓the
resistance to the moisture
movement and ↑the drying rate;
The ↑in drying air temperature
resulted in ↓in drying time;
↑of L (lightness) values and ↓of a/b
(redness/yellowness) measures
recorded in pretreated dried grapes
at 75°C;
The Parabolic model →the best
fitting model vs. Lewis, Henderson
and Pabis models.
[33]
Thompson
seedless grapes
Average berry
diameter: 17.5–
18.5 mm
The initial
moisture content:
80.3–82.6% (wb).
Dipping in 5% (w/v) of K
2
CO
3
+2%
(v/v) ethyl oleate solutions for
different durations (1, 2, and 3 min) at
several temperatures (30, 40, 50, and
60°C)
Tray dehydrator at 60°C and air
velocity of 0.6 m/s;
The final moisture content:
0.080 (db).
Rrying rate and color
kinetics
–Dipping into the ethyl oleate
solution at 60°C for 2 and 3 min →
best drying rate;
–The Midilli equation →best
description of grapes drying curves
for all dipping pretreatments;
Varying degrees of brown
coloringof all the resulting raisins.
[34]
Thompson
seedless
grapes
The initial
moisture content:
79.94% (w.b.)
Dipping solution: 25 g potassium
carbonate + 15 mL ethyl oleate in 1 L
of distilled water
G
1
: Dipping alkaline solution at
temperatures of 20°C;
G
2
: Dipping alkaline solution at
temperatures 30°C;
G
3
: Dipping alkaline solution at
temperatures 40°C;
G
4
: untreated grapes.
Laboratory scale hot air dryer
at air temperature of 60°C and
air velocity of 0.82 m/s;
The final moisture content:
18% ± 0.2 (w.b.)
Drying rates, color and
appearance attributes
–↑of the grapes’drying rate and ↓of
the drying time according to the
temperature of dipping solution;
The drying time of different
treatments:
G
1
=28h,
G
2
=25h,
G
3
= 19 h and
G
4
=51h;
–↑of the appearance and the
softness of pretreated raisins vs.
untreated ones.
G
3
pretreatment →raisins with
lighter color (↑value of Hunter
L = 21.96 and ↓value of a/
b = 0.90);
Exponential model →best
described drying proprieties of
Thompson seedless grapes.
[35]
8R. KHIARI ET AL.
Thompson
Seedless (V
1
) and
Perlette (V
2
)
grapes
P
1
: soda dip (bunches were dipped in
0.3% sodium hydroxide solution at
100°C for 3 s and immediately rinsed
in cold water then dried);
P
2
: golden bleach (bunches were
dipped in 0.3% sodium hydroxide
solution at 100°C for 3 s and
immediately rinsed in cold water,
fumigated with sulfur with 2g/kg for
2 h then dried);
P
3
: sugar syruping method (berries were
dipped in 0.3% sodium hydroxide
solution at 100°C for 3 s and after
draining the water bunches were
fumigatedwithsulfurwith1gkg
−1
for
2 h. Thereafter sulfured grapes were
transferred and kept in a syrup
containing 0.25% citric acid for 48 h at
70°C then dried).
D
1
: electrical heated
tray dryer (inside the dryer
60–65°C temperatures was
maintained during the drying)
D
2
: natural air dryer (treated
berries were dried under the
shade in the field conditions)
D
3
: shade drying under
ambient condition and
D
4
: solar cabinet dryer
The final moisture content:
15%.
Raisins yield, titrable
acidity and organoleptic
proprieties
–Sugar syruping →the highest yield
(283.51g kg
−1
) of Perlette grapes;
–Perlette raisin treated with golden
bleach and dried in shade →higher
titrable acidity;
–Drying time of Perlette raisins in
electrical traydryer: pretreatment
with soda dip and golden bleach
(9.5 h) < sugar syrup dipping
(10.5 h);
–Superior organoleptic quality
obtained with Perlette grapes
pretreated with golden bleach
method and dried in solar cabinet
dryer.
[36]
Physical Seedless white
grapes (var.
Nevado)
–The initial
moisture
–content:
84.0 ± 1.6%
Abr: Abrasion of the grape peel in a
shaker that walls were covered by
coating with abrasive sheets Shaker
for 10 min;
–EtOl: 2% (v/v) ethyl oleate + 2.5% (v/
v) K
2
CO
3
at 40°C for 3 min;
–UT: untreated grapes.
Drying in a convective oven at
a temperature of 50° and air
speed of 0.5 m/s;
–The final moisture content:
20% (w/w).
Drying rate, color and
microstructure
–Similar drying behaviour of both
physical and chemical
pretreatments;
–The mass transport coefficient for
physical pretreated samples was ↑
about 4 times than untreated
samples;
–Physical predrying →darker raisins
vs. chemical pretreatment.
[16]
Red and White
grapes
Abrasion pretreatment: Abrasion of
the grape peel in a shaker that walls
were covered by coating with
abrasive sheets Shaker for 10 min;
–UT: untreated grapes.
Drying in a convective oven at
a temperature of 50° and air
speed of 0.5 m/s;
–The final moisture content:
20% (w/w).
Drying characteristics,
moisture diffusion, color
–and shrinkage behaviour
–Peel abrasion pretreatment: ↓of
drying times;
–Modeling of shrinkage behavior
with respect to drying time:
correlation to a nonlinear model.
[37]
(Continued)
FOOD REVIEWS INTERNATIONAL 9
Table 1. (Continued).
Pretreatment
methods
Raw material
characteristics Predrying treatment conditions Drying method characteristics Analyzed parameters Major findings Reference
Red grapes cv. Red
Globe
–Average diameter:
24.4 ± 1.95 mm
Initial moisture
–content:
6.43 ± 0.02 kg
water/kg db
TRAbr: Abrasion of the grape peel in
an amotorized rotating drum made of
plexiglass, lined inside with
sandpaper at a rotation speed of was
10 rpm for 15 min
–TREtOl: 2% (v/v) ethyl oleate + 2.5%
(v/v) Na
2
CO
3
at 40°C for 3 min;
–UTR: untreated grapes.
Drying in a convective dryer at
air temperatures of 40°C, 50°C,
60°C and 70°C and air velocity
of 2.3 m/s;
–The final moisture content:
0.30 kg water/kg db
Drying kinetics and
quality parameters
(color, shrinkage,
phenolic content and
antioxidant activity)
–Peel abrasion pretreatment
+ drying at 50°C: ↓of color
changes, ↓of shrinkage, best
rehydration, ↑phenolic content and
↑antioxidant activity;
–The logarithmic model →the best
in describing the drying kinetics of
abraded grape at all the
temperatures except at 70°C;
–The Page model →the highest
correlation factor;
–The quadratic model →acceptable
to fit data for all the samples and
temperatures examined for
shrinkage.
[15]
Seedless red grape The treated bulk samples were
ohmically heated in a solution
containing 2% citric acid to a final
medium temperature of 60°C using a
field strength of 15 V/cm. The ohmic
pretreatment was conducted at
30 Hz, 60 Hz, and 7.5 kHz.
Drying in a food dehydrator at
a temperature of 57°C.
Drying rate and
adsorption isotherms
–Ohmic pretreatment significantly
↑grape drying rate, especially at
low electrical frequencies;
–The extent of the drying rate
depend on the frequency of
alternating current, being ↑at low
frequencies (30 and 60 Hz) and ↓at
a high frequency (7.5 kHz);
–Ohmic pretreatment→a shift in the
sorption isotherm.
[38]
Sultana seedless
grape
–Length:
15–18 mm;
–Diameter:
12–14 mm;
–Average weight:
1.28 g.
Microwave pretreatment of fresh,
dipped (2.5% K
2
CO
3
+ 0.5% olive oil
for 1 min) and blanched (boiling
water for 0.5 min) for 0.5–2 min at
215 W, 325 W and 420W.
Sun drying with average
daylight temperature of 22°C;
–The final moisture content:
16% (wet basis).
Drying rates, color and
appearance
–Microwave pretreatment ↓the
moisture content by 10 to 20%;
–Microwave pretreated grapes were
dried nearly 2 times faster than
untreated ones;
–Color and appearance of the
pretreated grapes: similar to
commercial raisins.
[39]
10 R. KHIARI ET AL.
Grapes (raisins
variety)
–The initial
moisture content:
Approximately
75% (w.b.)
Pulsed electric fields (PEF)
pretreatment: grapes were placed in
the PEF chamber one after the other
and 100
–pulses of the exponential decay
waveform at approximately 1 kV/cm
electric field was applied;
–Microwave pretreatment: grapes
were treated in a microwave oven
intermittently at power densities of 2
and 5 W/g for 5 min at the drying
temperature of 65°C;
–Chemical pretreatment: solution of
0.5% sodium hydroxide + 2% ethyl
oleate heated to 80°C in a water bath
for 30 s.
Drying in a convective drier at
65°C;
–The final moisture content:
Approximately 20% (w.b.)
Drying rate, color,
–total soluble solids (TSS),
appearance and market
quality.
–Chemical treatment →the drying
rate ↑in comparison to PEF and
microwave pretreatment;
–PEF and microwave-treated
samples: ↑TSS + good appearance
and market quality.
[40]
Red Globe grapes
–Average berry
weight:
10 ± 0.46 g
–Average berry
diameter:
15 ± 0.97 mm.
–The initial
moisture content:
7.62 ± 0.08 g
water/g dry matter
(db)
Carbonic maceration (CM)
pretreatment: grape berries were
put into three maceration tanks, and
in each tank 105 g of yeast solution
was added, then CM of the grapes
was carried out under 0.3 MPa at 40°C
for 12 h;
–Ethyl oleate solution (AEEO)
pretreatment: Dipping in alkaline
emulsion of 5 mL ethyl oleate in
500 mL water and adding 15 g
potassium carbonate for 5 min at
room temperature;
–AEEO + Freezing pretreatment:
Dipping in AEEO then freezing at
−18°C for 12 h.
Infrared drying of grapes was
carried out at 70°C in an
infrared oven which was
heated by three infrared pipe
lamps with a power level of
225 W each.
Drying rate, cell
permeability, rehydration
ratio, color, total
phenolic content (TPC)
antioxidant activity
(DPPH + FRAP tests)
–CM process →a strong and
beneficial effect on drying kinetics
of red grapes and physicochemical
properties and antioxidant ability of
raisins;
–CM vs. AEEO and AEEO + Freezing
→the best pretreatment in terms
of production time, ↑TPC, best
oxidation resistance ability and best
rehydration ratio;
–CM pretreatment vs. direct infrared
drying →the drying time ↓by 31%,
the TPC of raisin ↑by 28.43%, the
DPPH radical scavenging activity ↑
by 11.75%, the Ferric reducing
antioxidant power ↑by 73.9% and
rehydration ratio ↑by 32.24%;
–CM treatment →the most desirable
color of red grape raisins;
–The drying rate ↓with the moisture
content.
[41]
(Continued)
FOOD REVIEWS INTERNATIONAL 11
Table 1. (Continued).
Pretreatment
methods
Raw material
characteristics Predrying treatment conditions Drying method characteristics Analyzed parameters Major findings Reference
Thompson
seedless grapes
–Average berry
length: 18.4 mm
–Average berry
width: 12.3 mm
–Average berry
weight: 3.34 g;
–The initial
moisture content:
3.95 kg/kg (d.b.)
High-humidity hot air impingement
blanching (HHAIB) at different
temperatures (90, 100, 110, and 120°
C) and times (30, 60, 90 and 120 s);
–Relative humidity: 40–45%
Drying in an air impingement
dryer at the temperatures of
55°C, 60°C, 65°C and 70°C.
Drying rate, polyphenol
oxidase (PPO) activity,
moisture
–diffusivity and color
–The HHAIB ↓the drying time of
Thompson seedless grapes;
–HHAIB pretreatment at 110°C for
90 s followed by air drying at 60°C
→the most favorable conditions
for drying grapes;
–The obtained raisins pretreated by
HHAIB →desirable green-yellow or
green color.
[21]
↓: decrease/low; ↑: increase/high; →: resulted in/cause.
12 R. KHIARI ET AL.
The effect of different pretreatments (C
1
: Dipping in 0.5% sodium hydroxide (NaOH)
solution; C
2
: Dipping in 2% ethyl oleate (EO) + 0.5% NaOH solution; C
3
: Dipping in 3%
EO + 0.5% NaOH solution and C
4
: Dipping in 3% EO + 2.5% potassium carbonate
(K
2
CO
3
) solution) has been studied by Tulasidas et al.
[28]
on drying rates, color and
appearance attributes of Thompson seedless grapes. Their results showed that C
2
and C
3
pretreatments exhibited shorter drying times and resulted in good quality raisins com-
pared to C
1
and C
4
predrying treatments. C
1
pretreatment was judged to generate raisins
of inferior quality in terms of color and appearance (dull and pale raisins). This could be
attributed to the specificeffects of each treatment.
[45]
Dipping in NaOH solution causes
solubilization of the waxy surface and physical damage of the skin, hence accelerating only
the first stage of drying. However, dipping in EO alkali solution results in both decreasing
the resistance of skin tissues and increasing internal diffusion, thus allowing moisture to
more readily evaporate from grapes in addition to reducing browning and other degra-
dative reactions affecting quality attributes.
A survey was conducted by Doymaz
[32]
in which the influence of various dipping
pretreatments was tested including treatment of black grapes with EO or olive oil plus
potassium carbonate (K
2
CO
3
), potassium hydroxide
[46]
and sodium carbonate (Na
2
CO
3
)
solutions on drying time and drying kinetics of raisins. EO plus K
2
CO
3
pretreatment
enhanced drying rate to a greater extent and displayed shorter drying times than the other
pretreatments (K
2
CO
3
plus olive oil, ethyl oleate plus KOH, ethyl oleate plus Na
2
CO
3
and
untreated samples).
The use of alkaline emulsion of EO, commercial emulsion (alkaline emulsion of
‘‘PAKSAN’’ oil) and hot water (HW) has been examined by Esmaiili et al.
[18]
as pretreat-
ments before drying seedless grapes. They found that the three treatments strongly
affected raisin moisture diffusivities (average effective moisture diffusivities ranged from
3.34 to 8.46 × 10
−10
m
2
s
−1
at 50°C). On the other hand, mass transfer coefficients at a
given moisture content and different temperatures for the EO pretreated raisins were
revealed to be two times greater than HW-pretreated ones during drying.
With reference to the study of Doymaz and Altıner
[33]
, the effect of POTAS solution
(composed of 4% potassium carbonate + 1% olive oil) pretreatment on drying and color
characteristics of Sultana seedless grapes was investigated. The authors indicated that
when applying POTAS pretreatment, a reduction of the resistance to the moisture move-
ment associated with an increase of the drying rate were noted. Regarding color attributes,
POTAS solution was found to influence significantly the color characteristics of seedless
grapes by producing lighter raisins in comparison with untreated samples. This might be
ascribed to the action of potassium carbonate by saponification of fatty acids such as oleic,
stearic and oleanolic acids, that are known as constituents of grape wax.
[47]
These fatty
acids and their esters, characterized by the presence of lipophilic and hydrophilic groups
on the same molecule, were suggested to yield light-colored raisins.
[48]
The effect of dipping Thompson seedless grapes in potassium carbonate and EO
solutions for different durations (1, 2 and 3 min) at several temperatures (30, 40, 50 and
60°C) on drying rate and color kinetics was studied by Bingol et al.
[34]
Grapes dipped into
the solution at 60°C for 2 and 3 min had the fastest drying rate. Regardless of the dipping
time and temperature applied, all raisins had varying degrees of brown coloring. As
Grncarevic and Hawker
[49]
suggested, this could be explained by the loss of the grape
cell integrity, which was dependent on pretreatment conditions. Cell integrity was found
FOOD REVIEWS INTERNATIONAL 13
to be maintained for about 50% of weight loss and up to this time browning could be
initiated.
Regarding the work of Mandal and Thakur
[36]
, various pretreatments (P
1
: soda dip, P
2
:
golden bleach and P
3
: sugar syruping method) for the processing of raisins from two grape
varieties (Thompson Seedless and Perlette) have been tested. In general, the effective
pretreatment was P
3
, which resulted in raisins, obtained from the Perlette variety, with
higher yield (283.51 g. kg
−1
), titrable acidity (0.60%) and organoleptic proprieties (attrac-
tive and golden-yellowish raisins according to sensory analysis). However, it required
more time than the two other predrying treatments, which could explain the increase in
the acidity levels. On the other hand, rising sugar concentrations due to sugar syruping
pretreatment could result in the inhibition of polyphenol oxidase and thereby reduction in
browning as reported by Grncarevic and Hawker
[49]
and Radler.
[48]
Physical pretreatments
Physical pretreatment is the second predrying method that has been successfully used to
ameliorate grape drying. It encompasses the employment of thermal or nonthermal
techniques such as blanching, superficial abrasion, microwave or ohmic heating, pulsed
electric fields, and carbonic maceration.
[7,19,50]
The use of a physical pretreatment (superficial abrasion of the grape peel) has been
proposed by Di Matteo et al.
[16]
instead of a chemical one (traditional EO dipping) for
improving the drying rate of seedless grapes. The results of their study showed that both
pretreating methods manifested a quite similar drying behavior. The mass transport coeffi-
cient for physically pretreated grapes was 4 times superior than that estimated for untreated
samples (drying time about 35 h). However, physical predrying method gave darker raisins
than chemical processed ones. In fact, while chemical pretreatment inhibited the activity of the
polyphenol oxidase (PPO), the physical pretreatment allowed its activation because it is
mainly located in the peel. In the presence of oxygen, PPO catalysis leads to the formation
of brown pigments and thus, enhances the degree of grape browning.
Similarly, Senadeera et al.
[37]
and Adileltta et al.
[15]
conducted experiments on the effect
of abrasive pretreatment on the drying rate and the quality of dried grapes. Better drying
characteristics and quality were recorded in raisins pretreated by peel abrasion in com-
parison to those pretreated chemically (dipping in EO solution) or untreated ones.
The use of ohmic pretreatment before drying seedless red grapes has been assessed in the
study of Salengke and Sastry.
[38]
Ohmic heating affected the drying process by increasing the
dehydration rate, which was largely attributed to breaking action on the skin of the treated
samples. In turn, this affected their permeability and increased the moisture diffusion rate. In
addition, a shift in the sorption isotherm of the raisins produced was observed, which was
possibly attributed to a leaching of a small amount of solute sugar from the grape berries
during ohmic pretreatment, thus reducingthe amount of water required for sugar dissolution
during adsorption at high water activities. Since the shift was to the right, this indicated that
the equilibrium moisture contents of the treated samples were less than that of the untreated
samples at the same water activity levels.
With reference to the work of Kostaropoulos and Saravacos,
[39]
the feasibility of
microwave heating as a predrying technique of Sultana seedless grapes has been evaluated.
Their results revealed that microwave pretreated grapes dried nearly two times faster than
untreated ones. This might be assigned to the absorption of microwave energy by water
14 R. KHIARI ET AL.
molecules in the interior of the berries, which resulted in rapid evaporation, causing
partial puffing. Thus, the moisture diffusivity during drying may increase considerably,
due to increased porosity of the treated grapes. The color and appearance of the pretreated
grapes were similar to commercial raisins presumably due to a partial inactivation of
browning enzymes by the absorption of microwave energy.
The effects of microwave, pulsed electric fields (PEF) and conventional chemical
pretreatments has been investigated by Dev et al.
[40]
on grape drying rate and quality.
Drying rate increased significantly due to PEF and microwave pretreatments to the same
extent, but the highest dehydration rate was registered in chemically pretreated grapes.
This could be ascribed to the different effects of each pretreatment: while chemical
treatments enable the breakdown of the waxy skin, physical pretreatments act by forming
small cracks in the grape peel so that the drying rate will be accelerated. On the other
hand, the appearance and market quality of PEF and microwave pretreated samples were
better than the chemical treated and the untreated samples because of their soft texture
and shiny eye appeal, which may be due to increased porosity.
A new predrying method, carbonic maceration (CM), has been recently investigated for
raisin making.
[41]
CM involves grapes setting in a closed tank with a carbon dioxide-rich
atmosphere, which allows an intracellular fermentation and a CO
2
impregnation in the fruits
and vegetables under the rich CO
2
anaerobic conditions. In this manner, the plant tissues loosen
while keeping the fruit intact, thus enhancing the drying rate.
[50]
The effects of CM, dipping in
alkaline emulsion of EO solution (AEEO) and dipping in AEEO then freezing at −18°C for 12 h
(AEEO + Freezing) on infrared drying kinetics of red grapes were studied.
[41]
CM-treated raisins
had the shortest production time, the highest total phenol content, the best oxidation resistance
ability and the best rehydration ratio. In fact, CM could decrease the pH of the cytoplasm,
decompose the cell structure, and increase the cell wall and membrane permeability so that high
polymers are broken down into smaller ones releasing water, and thus, improving the drying
kinetics as well as the quality of the dried grapes.
[50]
Water blanching has been also used to soften the grape waxy skin and to increase mass
transfer during dehydration. This pretreating method is traditionally the most adopted
technique in the food industry because it is a simple, easy and cost-effective technology.
[51]
The effects of high-humidity hot air impingement blanching (HHAIB) on drying kinetics
and color of seedless grapes at different temperatures (90, 100, 110 and 120°C) and times
(30, 60, 90 and 120 s) have been assessed.
[21]
HHAIB not only reduced the drying time but
also effectively inhibited enzymatic browning as the obtained raisins had desirable green-
yellow or green color. During blanching, the grape skin will generate microcrevices when
the pressure inside the grapes is greater than the ambient pressure. These crevices could
greatly reduce resistance to water diffusion from the peel to the drying air. In addition,
blanching could expel the intercellular air entrapped inside the sample tissues and reduce
the resistance of cell membranes and cell walls to water diffusion by structure softening.
As regards browning prevention, HHAIB acts by decreasing the PPO residual activity.
Drying methods
Numerous drying methods have been employed for raisin making with the main
objectives to prolong their shelf life, to produce high-quality dried grapes as well as
to diminish postharvest losses. Currently, the most frequently used grape drying
FOOD REVIEWS INTERNATIONAL 15
techniques include sun and solar dehydration, oven and microwave drying, vacuum
drying and infrared drying. Some examples of the most assayed techniques for grape
drying are outlined in Table 2.
Drying kinetics
The best way to characterize the behavior of a food product during drying is to evaluate
experimentally its drying kinetics. The drying kinetics are also used as a tool to select for
the suitable drying techniques and for controlling, optimizing and engineering the food
dehydration process.
[63]
The drying characteristic expresses generally the moisture ratio as a function of time,
which can be illustrated by either moisture content of grapes ‘M’versus time ‘t’, drying
rate ‘dM/dt’versus time ‘t’or drying rate ‘dM/dt’versus moisture content ‘M’.
[64]
While
the drying curve (plot of ‘M’versus time ‘t’) outlines the dehydration kinetics as a function
of time, the drying rate curve (plot of ‘dM/dt’versus time ‘t’) describes the rate of moisture
content change over the time. Both of these representations are used to get an overview of
moisture content changes during drying. On the other hand, when plotting drying rate
‘dM/dt’versus moisture content ‘M’at different times, a curve illustrating the drying cycle
is obtained. The typical drying cycle consists of three stages: heating the food to drying
temperature, evaporation of moisture from product surface (constant rate period) and
falling of drying rate (falling rate period).
[65]
As regards grape drying, the constant-rate
drying period was generally absent in all drying methods. The dehydration took place
principally during the falling drying rate period where internal diffusion mechanisms
dominate.
[15,37,52]
In order to describe the kinetics of the drying process, many thin-layer drying models
have been developed. These models can be classified into three categories: theoretical,
semitheoretical and empirical.
[54]
Mathematical modeling of drying agricultural products
is important in understanding the heat and mass transfer phenomena involved in the
production and processing of dried foods. Mathematical modeling and numerical simula-
tion not only reduce the need for expensive and repetitive experimentation, but also help
to design new and improve existing commercial drying processes. They are used to
estimate drying time of several products and also to generalize drying curves.
[66]
As
pertains grape drying, many authors have developed mathematical models based on the
diffusion theory to predict the drying kinetics of moist substances in thin layers, as shown
in Table 3. The constants in the fitted models have been found to be functions of air
temperature, air velocity, heat of sorption and so forth. Model fit is generally evaluated
based on statistical parameters including the correlation coefficient (R
2
), the sum squared
errors (SSE) and the root mean square error (RMSE). The higher R
2
and the lower SSE
and RMSE values represent the goodness of model fitting and provide a better prediction
of the drying characteristics of grapes.
[64]
Traditional drying methods
Historically, sun drying grapes into raisins dated back to 1490 B.C.
[78]
To date, such
process is by far the most traditionally used method for preserving food and agricultural
crops, especially in the developing countries owing to its low cost and ease of handling.
[60]
Natural open sun drying consists of exposing grape bunches directly to sunlight (with or
without cover) until thorough dehydration.
[79]
It is an easy-to-use and practical drying
16 R. KHIARI ET AL.
Table 2. Comparison of different drying methods on raisins processing proprieties.
Type of
drying
technique Drying technique characteristics Pretreatment Duration of drying Major findings Reference
Sun drying Sultana grapes dried on:
- concrete ground
- polypropylene canvas sheets
- wooden racks
- Traditional solution: 5%
K
2
CO
3
+ 15% olive oil;
- Solution A: 4% K
2
CO
3
+ 2% ethyl
oleate;
- Solution B: 5% K
2
CO
3
+ 2% ethyl
oleate;
- Solution A;
- Solution B;
- NATUR: Untreated grapes.
9 days (220 h)
8 days (200 h)
8.5 days (210 h)
10 days (240 h)
10 days (240 h)
18 days (440 h)
–Lighter color of the treated grapes vs.
untreated ones;
–Drying on polypropylene canvas sheets or on
wooden racks →lighter product compared
to drying on concrete ground.
[29]
Sultana seedless grapes dried on plastic sheets
spread on the ground
Initial moisture content: 78% w.b.
Black seedless grapes (currents)
Initial moisture content: 70% w.b.
Temperature range: 23–35°C
Air humidity: 72%
- No pretreatment
- Treated with an emulsion of 2%
KHCO
3
and 0.2% olive oil for 2 min
- No pretreatment
31 days (740 h)
7.5 days (179 h)
10 days (240 h)
–The drying rate ↓in natural grapes than in
grapes pretreated with the emulsifying
solution;
–Final moisture content: 15% w.b.
[52]
Sultana grapes
- spread over a grid support
Initial moisture: 5–6.2 w.b.
Temperature range: 20–45°C
- Immersion 2–3 times
for 2–3 s in 1% NaOH solution at 90°C
10.5 days (250 h) –Final moisture content: 16% (d.b.)
[53]
Open sun drying of seedless and seeded
grapes
Average berry diameter of of seedless grapes:
1.72 ± 0.1 cm;
Initial moisture content: 78.2 ± 0.2%;
Average berry diameter of of seeded grapes:
2.20 ± 0.1 cm
Initial moisture content: 79.5 ± 0.2% (w.b.)
- Dipping into the solution of
potassium carbonate and olive oil
(2.5% K
2
CO
3
+ 0.5% olive oil) for
1 min.
- Seedless grapes
- Seeded grapes
7.5 days (176 h)
9.5 days (228 h)
–The drying time ↑with increased berry size.
–Fick’sdiffusion model used to estimate the
effective moisture diffusivity values;
–Final moisture content: 22% (w.b.)
[54]
Solar
drying
Solar drying with air temperature in the range
17–40°C and air velocity of 3 m/s;
- Traditional solution: 5%
K
2
CO
3
+ 15% olive oil;
- Solution A: 4% K
2
CO
3
+ 2% ethyl
oleate;
- Solution B: 5% K
2
CO
3
+ 2% ethyl
oleate;
- SDSO2: grapes first treated with SO
2
gas then was dipped into alkaline
ethyl oleate solution B;
-NATUR: Untreated grapes.
5 days (120 h)
5 days (120 h)
5 days (120 h)
Around 6 days (140 h)
18 days (440 h)
–Lighter color of the treated grapes vs.
untreated ones;
–SDSO2 pretreatment ↑the drying rates, but
the color was too light: not acceptable in the
market.
[29]
(Continued)
FOOD REVIEWS INTERNATIONAL 17
Table 2. (Continued).
Type of
drying
technique Drying technique characteristics Pretreatment Duration of drying Major findings Reference
Forced air drying of Sultana seedless grapes at
60°C
Initial moisture content: 78% w.b.
Forced air drying of black seedless grapes
(currents) at 50°C/6 h, 60°C thereafter
Initial moisture content: 70% w.b.
Air velocities: 0.5–1.5 m/s
- No pretreatment
- No pretreatment
Around 2 days (56 h)
1–2 days (45h)
–Acceptable raisins
–Final moisture content:16% w.b.
[52]
Thompson seedless spread on plastic net
Initial moisture: 349.59% d.b.
Temperature range: 25.9–40°C
Solar radiation: 605–673 W/m
2
- Dipped for 3 min into
a solution of 2.5%
K
2
CO
3
and 2% dipping oil
4 days –The drying time of the grapes ↓by 43%
compared to the open sun drying;
–Solar drying produced better-quality raisins;
–Final moisture content: 17% d.b.
[22]
Seedless grapes dried in:
- Indirect natural convection solar dryer;
Ambient temperature: 27–31°C; Inlet drying air
temperature 45.5–55.5°C
Max. solar radiation: 988 W/m
2
- Indirect natural convection solar dryer + sand
as storage material
- Indirect natural convection solar dryer + sand
as storage material
Initial moisture: 80%
- No pretreatment
- No pretreatment
- Dipping grapes into boiling water
containing 0.4% olive oil and 0.3%
NaOH for 60 s
3 days (72 h)
2.5 days (60 h)
0.5 days (8 h)
–The storage material (sand) →accelerated
the drying process by 12 h;
–The storage + chemical pretreatment →
significant ↓of the drying time;
–The designed system →capable of drying
10 kg of chemically treated grapes during
20 h of sunshine;
–Final moisture content: 18%
[55]
Sultana grapes dried in:
- Indirect natural convection solar dryer;
Temperature range: 20–45°C
- Solar tunnel greenhouse
Maximum temperature: 60°C
Initial moisture: 5–6.2 w.b.
- Immersion 2–3 times for 2–3 s in 1%
NaOH solution at 90°C
3.5 days (77 h)
5 days (119 h)
–Solar tunnel greenhouse drying →
satisfactory and competitive to the natural
convection solar drying process
+ production of best quality raisins with the;
–Final moisture content: 16% (d.b.)
[53]
- Natural convection walk-in type solar tunnel
dryer for Thompson seedless grapes drying
Temperature range: 55–70°C
- Maximum allowable temperature: 65°C;
- Incident solar radiation: 2.3 MJ/h/m
2
Initial moisture: 85% (w.b.)
- No pretreatment 7 days –Solar tunnel at 65°C temperature →the best
moisture removal rate;
–Final moisture content: 16% (w.b.)
[56]
18 R. KHIARI ET AL.
V. vinifera cv. “Sultanina”dried in:
- Direct sunlight drying
- Polythene tunnel type rack systems solar
dryer
- Polythene tunnel type rack systems solar
dryer
- No pretreatment
- 1%, 2%, 3% and 5% of sodium
hydroxide + 1% olive oil
14 days
13 days
7 days
–Tunnel drying →satisfactory and
competitive to traditional drying method
(direct sun drying)
–Grapes in the solar drying tunnel →dried
faster + better color quality than samples
dehydrated with direct sunlight.
[57]
Oven
drying
Italia Muscat grapes dried in:
- Oven drying (O) at temperature of 50°C and
air velocity of 0.5 m/s;
- Greenhouse drying (G) under a temperature
range of 20–40°C and an air velocity of 1 m/s;
- Pretreatment I: berries dipped in
0.5% olive oil + 6% K
2
CO
3
solution at
50°C for 2 min;
- Pretreatment II: berries dipped into
an alkaline solution of NaOH (1%) at
90°C for 3 s
- Pretreatment I
- Pretreatment II
5 days
6 days
13 days
13 days
–↓of drying time with oven drying at 50°C vs.
greenhouse drying →raisins with better
quality characteristics
–Significant differences in the pH value (3.85–
4), acidity (1172.5–2730 mg TA/100 g DW)
and total sugars (31.5–49.7%) between raisin
samples subjected to both treatments;
–Mycological analysis →a noteworthy fungal
flora distribution among raisin samples,
+ abundance of ochratoxinogenic species
such as Aspergillus ochraceus (15.56%) and
Aspergillus carbonarius (10.41%).
[3]
Microwave
drying
Spherical, homogeneous and isotropicrapes
grapes dried were exposed to microwave (MW)
radiation at 2450 MHz, power density of 0.5 W/
g dry mass basis and air velocity 2.0 m/s at
temperatures of:
* 30°C
* 40°C
* 50°C
* 60°C
- Not mentioned 11.5 h
8.5 h
5.5 h
4h
–MW drying →energy efficient with a low
specific energy requirement, ↓of drying time
+ adequate quality raisins production.
[58]
Pulsed
Vacuum
drying
(PVD)
Thompson seedless grapes dried in a pulsed
vacuum with optimum drying conditions:
temperature of 65°C, vacuum duration
time = 15 min and normal pressure
duration = 4 min.
Solutions of NaCl●I: 120 g/L,
–II: 130 g/L,
–III: 140 g/L,
–IV: 150 g/L,
–V: 160 g/L,
–VI: 170 g/L.
58 h
44.3 h
32.3 h
18.8 h
14.7 h
12.1 h
–With ↑of ripness, ↑of total phenol content,
soluble solid content and pH + ↓of the
titratable acidity content;
–↑of drying rate + moisture effective
diffusivity;
–PVD drying →better color and texture quality
of raisins.
[59]
(Continued)
FOOD REVIEWS INTERNATIONAL 19
Table 2. (Continued).
Type of
drying
technique Drying technique characteristics Pretreatment Duration of drying Major findings Reference
Microwave
&
Vaccum
drying
(MWVD)
Thompson seedless grapes dried in a
MW2450 MHz, power density of 3 kW at
temperatures of:
* 54°C
* 60°C
* 66°C
* 71°C
* 77°C
The vacuum vessel, subjected to a negative
pressure of 2.7 kPa, and exposed to 3 kW of
MW power.
- No pretreatment 1.53 h (92 min)
1.36 h (82 min)
1.56 h (94 min)
1.2 h (72 min)
1.3 h (78 min)
–MWVD process →a discrete, real-time
control of the process to produce dried
grapes with better retention of fresh
character (nutritional composition) vs. sun-
dried raisins;
–Vitamin A was only found in the MWVD
raisins vs. sun-dried ones;
–↑levels of Vitamin C, thiamine and riboflavin
in the MWVD grapes vs. the sun-dried raisins.
[60]
Microwave
oven &
hot air
cabinet
drying
Thompson seedless grapes dried in:
- Microwave oven followed by hot air cabinet
dryer
- Hot air cabinet dryer followed by microwave
oven
- Hot air cabinet dryer alone
- Immersion of grapes in boiling
solution (about 80°C) of 0.2% sodium
hydroxide for 30 s, and immediately
washed by immersing in cold water,
then in 0.2% solution of citric acid.
- Grapes sulfuring was done by
immersing the samples in 1000 ppm
solution of potassium metabisulfite
(K
3
S
2
O
5
) for 4 h.
6.8 h (413 min)
7.9 h (474 min)
12 h (720 min)
–The drying time in the hot air
cabinet alone< hot air cabinet+ microwave
power range (75–900 W);
–The ↑in microwave power →speeding up of
the drying process, thus ↓of the drying time.
–Microwave oven + hot air cabinet drying →
the optimum selection percentage (78%):
considered as reasonable.
[61]
Infrared
drying
Seedless grapes dried at four different
temperatures between 50 and 80°C
The infrared drying was carried with an
electromagnetic radiation in the wavelengths
range of 2 and 3.5 µm and at the frequency
range of 50–60 Hz
- No pretreatment
- Dipping into the solution of
potassium carbonate and olive oil (5%
K
2
CO
3
and 0.5% olive oil in 1 L water)
for 5 min.
Not mentioned –↑of moisture diffusivity and thermal
diffusivity with the increase in temperature.
–The studied model equations →significant
for use in calculation and design of other
heating processes (thermal processing and
cooling/freezing) related to seedless grape.
[62]
↓: decrease/low; ↑: increase/high; →: resulted in/cause.
20 R. KHIARI ET AL.
method that does not require high capital cost. Nevertheless, despite its simplicity, feasibility
and cost effectiveness, sun drying has certain drawbacks including long duration of biopro-
cessing, reliance on weather conditions, and risk of mold and insect contamination.
[22]
Therefore, solar dehydration, which involves the harnessing of solar radiation as source of
energy, has been developed as an alternative technique to sun drying. Basically, there are
three types of solar dryers: direct type, indirect type and mixed type.
[78]
In the direct type of
solar dryer, grape berries are subjected to solar radiation that would be converted through a
transparent material into heat and then directly absorbed by the fruits. In the indirect mode
of solar dryer, the sun’s radiation is collected in a solar collector where the air is heated then
passed into the drying cabinet to dehydrate the grapes.
[79]
The mixed mode of solar dryer
combines the use of both the direct and the indirect types. It implies the fruit materials
dehydration by two ways: through the direct absorption of incident solar rays by the grapes
in addition to the preheated air flow emanating from the solar air collector.
[80]
Each of the aforementioned procedures has been extensively investigated in many
studies for grape drying. The study of Mahmutoğlu et al.
[29]
was conducted to evaluate
the effect of two drying methods: sun (on concrete ground, on polypropylene canvas
sheets and on wooden racks) versus solar drying and different pretreatments on the quality
of Sultana grapes (Table 2). Drying rates were classified for the tested dehydration
techniques: solar dying > sun dying on concrete ground > sun dying on wooden racks,
or on polypropylene canvas sheets. The color of the pretreated dried grapes was lighter
than that of the untreated ones. Drying on polypropylene canvas sheets or on wooden
racks or by solar drying gave a relatively lighter product compared to drying on concrete
ground due to reduced drying times. In addition, the overheating of the concrete results in
the darkening of the bottom layers of the grapes.
The use of sun and solar drying methods to dehydrate Sultana seedless and black
seedless grapes (currents) was also examined by Karathanos.
[52]
This study showed that
solar drying obviously improved the drying rates of both varieties and exhibited shorter
drying times than sun dehydration. Sun drying of Sultana raisins took 31 days (without
pretreatment) and 7.5 days (when they were pretreated with an emulsion of salt and olive
Table 3. Mathematical thin-layer models used in fitting grapes drying kinetics.
Model name Model equation Reference
Lewis MR = e
(-k.t) [67]
Handerson and Pabis MR = a e
(-k.t) [68]
Modified Handerson and Pabis MR = a e
(-k.t)
+be
(-g.t)
+ce
(-h.t) [69,70]
Logaritmic MR = a e
(-k.t)
+ce
(-k.t) [15,71,72]
Two term MR = a e
(-k0.t)
+be
(-k1.t) [15,73]
Two term exponential MR = a e
(-k.t)
+(1-a) e
(-k.a.t) [15,74]
Verma et al. MR = a e
(-k.t)
+(1-a) e
(-g.t) [75]
Approximation of diffusion MR = a e
(-k.t)
+(1-a) e
(-k.b.t) [73]
Page MR ¼eðk:tnÞ
[15,30,32]
Modified Page MR ¼eðk:tÞn
[54]
Midilli et al. MR ¼eðk:tnÞþb:t
[34,54]
Parabolic MR = a + b.t + c.t
2[33]
Wang and Singh MR = 1 + a.t + b.t
2[76]
Guggenhein, Anderson and De Boer (GAB) MR ¼MeqCkaw
ð½ð1kawÞð1kawþCkawÞÞ
[77]
Brunauer, Emmett and Teller (BET) MR ¼ABaw
ð½ð1awÞð1þðA1ÞawÞÞ
[77]
t: Drying time (s); MR: Moisture Ratio; a, b, c, g, h; n: dimensionless constant for drying; k, k
1
,k
0
: Drying velocity constant;
a
w
: water activity; A, B, C: Estimated parameters of the models applied to experimental sorption data.
FOOD REVIEWS INTERNATIONAL 21
oil), whereas solar drying lasted only 2 days. The drying of currants (characterized as
natural currants with no addition of any emulsifier), on the other hand, required much
shorter times (10 days and 1 to 2 days for sun and solar drying, respectively). The
discrepancy in the drying behavior between the two grape varieties may be due to the
thin skin of currants compared to the thicker one of Sultanas as well as the application of
the pretreatment, which would speed the process of drying.
With regard to the study of El-Sebaii et al.,
[55]
the usage of an indirect type natural
convection solar dryer for drying some vegetables and fruits (including seedless grapes)
has been investigated. The required time to achieve the equilibrium moisture content for
seedless grapes was 60 h and 72 h when the system was used with and without a storage
material (sand), respectively. The combination of a chemical pretreatment (dipping
samples into boiling water containing 0.4% olive oil and 0.3% NaOH for 60 s) drastically
reduced the drying time to 8 h for the system with the storage material. The authors
concluded also that the designed dryer was capable of dehydrating 10 kg of chemically
pretreated grapes during 20 h of sunshine.
Fadhel et al.
[53]
analyzed the drying of the Sultanine grape variety by three different
solar processes: natural convection solar dryer, solar tunnel greenhouse drying and open
sun drying. Grapes in the solar dryer were dried within about 4 days, those dehydrated in
the greenhouse required around 5 days, while with open sun drying the fruits took more
than 10 days to dry. The solar tunnel greenhouse drying was satisfactory and competitive
to the natural convection solar drying process (in terms of energy efficiency and opera-
tional cost); besides, it produced raisins with the best quality due to the increase of their
drying rate as well as their protection from pests, rain and dust.
The work of Rathore and Panwar
[56]
suggested the development of a natural convection
walk-in type solar tunnel dryer for the dehydration of Thompson seedless grapes. They
proposed the use of the designed drying system not only for grape but also for other
agricultural and horticulture product drying. The solar tunnel was adequate to transform
the Thompson grapes into raisins within almost 7 days and the best moisture removal rate
was recorded at 65°C temperature.
The effect of two drying methods (polythene tunnel type rack systems solar dryer and
in direct sunlight drying) as well as four different dipping alkali solution (1%, 2%, 3% and
5% of sodium hydroxide + 1% olive oil) on V. vinifera cv. “Sultanina”processing have
been studied.
[57]
Tunnel drying was found to be satisfactory and competitive to traditional
drying method (direct sun drying) since grapes in the solar drying tunnel dried faster and
had better color quality than samples dehydrated with direct sunlight.
Advanced and combined drying methods
With the progress of science and technology, novel drying techniques have been used for
raisins production and are considered as key ways in the concept of sustainable engineer-
ing processes. The improvement of these methods contributes to lift some of the conven-
tional drying technologies limitations such as reducing drying time and energy
consumption as well as improving final dried product quality and safety.
[81]
Over the years, oven and microwave drying have received considerable attention due to
their rapid processing rates, short drying times, instantaneous and precise electronic
control in addition to being as clean heating processes.
[82]
While oven drying (OD) uses
thermal energy to directly dehydrate the food product, microwave drying (MWD)
22 R. KHIARI ET AL.
involves the use of an electromagnetic radiation that would be converted into thermal
energy for evaporating the moisture from the grapes.
[81]
Another drying method, vacuum
drying (VD), has been successfully used in raisin processing, as well. VD is a process in
which moist materials are dried under subatmospheric pressure. The reduced pressure by
vacuum increases the mass transfer of water between the food and its surroundings, which
lowers the heat needed for rapid drying and procures a high quality product.
[83]
Recently,
infrared drying (ID) has become among the popular drying techniques in the food
industry owing to its diminished drying time, the acceptable quality of the final dried
product, and its greater energy savings ability, in addition to its cost-effectiveness com-
pared to MWD and VD methods.
[84]
ID implies the use of infrared radiation, which is an
electromagnetic wave that can be classified into three regions based on its wavelength: the
near-infrared (NIR; 0.78–1.4 mm), the middle-infrared (MIR; 1.4–3 mm) and the far-
infrared (FIR; 3–1000 mm).
[85]
At the NIR wavelength, the infrared radiation is trans-
ported through water, whereas the FIR wavelength assures its absorption at the surface. In
general, NIR radiation is advantageous for processing thick products, while FIR radiation
is favorably absorbed by food with thin layers.
[84]
Advanced processes. Several papers have reported the feasibility of the above cited
methods (OD, MWD, VD and ID) either alone or assisted to other techniques for raisin
drying. A mathematical model was developed by Tulasidas et al.
[58]
to describe MWD of
grapes and suggested its viability at commercial scale. The simulation was found to
describe the diffusion equation quite well as demonstrated by a good fitting with the
experimental data since it took into account shrinkage, which occurs drastically in grapes,
as well as the changes in physical and dielectric properties that occur throughout the
process due to changes in moisture content and temperature. The proposed numerical
model was based on a semitheoretical approach and therefore could be adapted to scale-
up purposes. Their findings affirmed also that MWD was energy efficient since it required
low specific energy, decreased the drying time in addition to producing raisins with
acceptable quality.
The work of Çağlar et al.
[62]
was carried out to determine the moisture diffusion
coefficient of seedless grapes under an infrared heating system. They investigated the
effects of grape drying and temperature on the moisture diffusion coefficient and the
thermal diffusivity. Nine different derived models including the effect of moisture content
and drying temperature were evaluated in order to predict moisture diffusivity, thermal
diffusivity and drying rate. These models differed from each other based on the form of
the empirical equation without physical foundation. The choice of the best model was
based on statistical parameters. The model with the highest correlation coefficient (R
2
) and
the lowest chi-square (χ
2
) was considered as the best model to choose. The results
indicated that both moisture diffusivity and thermal diffusivity increased exponentially
with increase in temperature. The studied model equations were significant for use not
only in calculation and design related to drying but also in calculation of required
parameters in design of other heating processes such as thermal processing, and cool-
ing/freezing related to seedless grape.
The effects of two drying methods (greenhouse and oven drying) as well as different
chemical pretreatments (pretreatment I: dipping in olive oil and K
2
CO
3
solution and
pretreatment II: dipping in NaOH solution) on the physicochemical, phytochemical and
FOOD REVIEWS INTERNATIONAL 23
microbiological quality parameters of Muscat Italia grapes were studied.
[3]
Oven drying at
50°C was found to have shorter drying time and resulted in raisins with better quality
characteristics (in terms of appearance and phytochemical attributes) than greenhouse
drying. This might be attributed to the shortened drying time, in the case of oven drying,
which allowed for a reduction in raisin color degradation as well as an accumulation of
phenolic compounds.
Wang et al.
[59]
analyzed the effects of ripeness on the physicochemical properties and
pulsed vacuum drying (PVD) kinetics of Thompson seedless grapes, and found that the
total phenol concentration, soluble solid content and pH increased considerably while the
titratable acidity content decreased, which could be related to the concentration of sugars
and the degradation of organic acids (mainly malic acid) during maturation of grape
berries and upon exposure to high drying temperature. The drying rate and effective
moisture diffusivity also dramatically rose with increasing ripeness due to the softening of
the fruit cell wall and the increase of its pericarp permeability, which facilitates moisture
transport according to magnetic resonance imaging analysis. During PVD processing,
most of the drying time occurs under vacuum, which partly hinders the browning reaction
and results in more desirable raisins according to color and texture quality attributes.
Indeed, during PVD, the pulsed vacuum atmosphere engenders an oxygen deficient
environment, which can reduce some biochemical reactions, such as oxidation deteriora-
tion, browning reactions and hence, improve the quality characteristics of dried grapes.
Combined processes. In recent studies, there has been substantial progress in using
combined drying methods since the combination of such techniques have shown an
increase in the dehydration process efficiency as well as an improvement in the quality
of the final dried products. Among the many possible combinations, only two combined
processes have been assayed for grape drying, combining convection with microwaves and
vacuum with microwaves.
Combined hot air forced convection and microwave drying (MWD) was studied by
Tulasidas
[86]
to assess the best operating conditions that might enhance the color quality
of Thompson seedless raisins. MWD (using a modified microwave oven apparatus at
2450 mHz) was found to be energy efficient with a low specific energy requirement
because it presented shorter drying times when compared to hot air convective drying.
However, with reference to their previous citations
[87]
and
[88]
, a selection of suitable levels
of temperature and power (by means of response surface models) were needed to achieve
a desired level of quality in the finished product.
Clary et al.
[60]
investigated the drying of grapes using a microwave vacuum dehydration
(MWVD) technique versus sun-dried grapes. The MWVD process provided a discrete,
real-time control of the process to produce dried grapes with better retention of fresh
character, including nutritional composition compared to sun-dried raisins. With refer-
ence to a temperature control system, the MW power was controlled to ensure the grapes
did not exceed the set treatment temperature. When the surface temperature of the grapes
approached the set point, the MW power was automatically reduced. The results showed
that MWVD of grapes using temperature monitoring controlled MW power improved
product quality compared to using fixed levels and incrementally staged MW power
applications. Vitamin A was found in the MWVD raisins but not detected in the sun-
24 R. KHIARI ET AL.
dried ones, and vitamin C, thiamine and riboflavin were also higher in the MWVD grapes
than in the sun-dried raisins.
Kassem et al.
[61]
evaluated the drying characteristics of Thompson seedless grapes using
combined microwave oven and hot air cabinet dryer. The hot air cabinet alone as a drying
method required more time to dry grapes while within a certain microwave power range
(75–900 W), the increase in microwave power resulted in speeding up of the drying
process, thus shortening the drying time. To select the best drying method, which has the
highest selection percentage, a number of criteria have been chosen including rehydration
ratio, total soluble solids, drying ratio, final drying time, final moisture content and
specific energy consumption. When grapes were dried in the microwave oven followed
by hot air cabinet drying method, 78% of the optimum selection percentage (100%) was
achieved, which was considered as reasonable (in comparison with hot air cabinet
followed by microwave oven drying or hot air cabinet dryer alone methods with respective
percentages of 67% and 56%).
It is noteworthy that there are also many other advanced drying techniques (e.g., freeze
drying, swell drying, ultrasonic drying, etc.) that have been effectively employed for
dehydrating some fruits and vegetables. However, to the best of our knowledge and
with reference to the available data, these methods have only been studied for drying
grape by-products from wine and juice production (i.e., grape seeds, skins, pomace, stems
and stalks) and not for raisin processing.
Effect of drying on raisin quality characteristics
Appearance, texture and flavor (taste and aroma) along with nutritional value are parti-
cularly the most intriguing quality attributes that influence consumers’acceptance of agri-
food products.
[89]
The quality parameters of raisins, as for other dried fruits and vege-
tables, can be substantially affected during the dehydration process, depending on the
operating and pretreatments conditions as well as the applied drying technology.
[90]
Effect of drying on the physicochemical characteristics
Visual appearance parameters (i.e., color, size and shape) are intrinsic quality attributes
highly associated with consumers’quality expectations and their choice of purchasing a
product, since these attributes are the first characteristics used to assess food quality, to
some extent.
[91]
Color
Raisin color seems to be one of the most important quality attributes relevant to market
acceptance. Depending on the grape cultivar, pretreatment conditions as well as drying
method used, raisins may be found in a variety of sizes and colors including green, yellow,
brown and black.
[92]
The brown color of dried grapes is formed consequently due to the
effects of enzymatic reaction (action of the enzyme polyphenol oxidase (PPO)) and
nonenzymatic browning (Maillard reaction) taking place during the dehydration
process.
[93]
The appraisal of color can be achieved via destructive or nondestructive methods.
Destructive method consists of quantifying the color pigments present in dried foods
FOOD REVIEWS INTERNATIONAL 25
spectrophotometrically or using high performance liquid chromatography.
[90]
On the
other hand, the nondestructive method describes the food color in terms of CIELAB
parameters (L*a*b*).
[94]
CIELAB color notation locates a color in a three-dimensional
space defined by lightness (L*) and the chromaticity coordinates a* (redness/greenness)
and b* (yellowness/blueness). L* represents light-dark spectrum with a range from 0
(black) to 100 (white). a* is the red-green spectrum with a range from −60 to + 60, and
b* indicates yellow-blue spectrum with a range from −60 (blue) to + 60 (yellow).
[59]
Color properties of Sultana seedless grapes as affected by two pretreatments, POTAS
(traditional potassium carbonate solution) and AEEO (Alkaline emulsion of EO) solutions
as well as different drying temperatures, were studied by Doymaz and Pala.
[17]
The use of
AEEO solution led to better color (lighter and brighter raisins as indicated by the higher L
and lower a/b values, respectively) in comparison with POTAS or untreated samples.
When grapes were pretreated with AEEO and dried at 60°C, lightest raisins’hue was
observed. However, further increase in drying temperature resulted in undesirable
darkening.
The appearance and market quality of dried grapes were evaluated by Dev et al.
[40]
in
response to MW, PEF and conventional chemical pretreatments. The appearance and
market quality of PEF and MW-treated samples were superior (higher aesthetic appeal
according to their shiny appearance) to the chemical treated and the untreated samples.
Statistical analysis revealed that MW-treated raisins had a significant difference in light-
ness in comparison with the other applied treatments. PEF-treated raisins were signifi-
cantly darker than MW but of close lightness value with untreated and chemical treated
ones. This could be attributed to the effect of MW pretreatment, which has been reported
to prevent enzymatic browning in some fruits and vegetables and thus, enhance the color
of the dried products.
[51]
Bingol et al.
[34]
investigated the effect of chemical pretreatment (dipping into 2% ethyl
oleate and 5% K
2
CO
3
solution) at different temperatures and times of dipping of
Thompson seedless grapes on color kinetics. The drying was carried out in a tray
dehydrator at 60°C. Slight color changes occurred immediately after dipping; for example,
the grapes’green color changed to greenish/yellowish at 40°C/3 min, 50°C/2 min, and 50°
C/3 min. However, at 60°C dipping temperature and 2- and 3-min dipping times, the
color of the grapes was visibly yellowish following the dipping pretreatment and the final
color of the raisins was lighter brown. This could be correlated with the inactivation of the
PPO enzyme at higher temperatures, which led to the minimizing of browning.
[48]
The effect of high-humidity hot air impingement blanching (HHAIB) at different
temperatures and times was studied on the color changes of seedless grapes.
[21]
Increasing the blanching time and temperature and decreasing the drying temperature
effectively inhibited enzymatic browning and resulted in raisins with brighter color
(desirable green-yellow or green color). It was concluded also that the color changes
caused by enzymatic browning were more severe than nonenzymatic browning.
Wang et al.
[41]
analyzed the color characteristics of infrared dried red grapes upon
dipping in alkaline emulsion of ethyl-oleate (EO) solution (AEEO), AEEO dipping
+ freezing and carbonic maceration (CM) treatments. CM pretreatment resulted in the
most desirable color of raisins (characterized with the lowest color difference (ΔE))
followed by AEEO dipping + freezing treatments then AEEO dipping solution. These
26 R. KHIARI ET AL.
two latter pretreatments were found to have prolonged drying time, which induces the
darkening of the final dried fruits.
According to Guiné et al.,
[95]
the color of raisins from Crimson seedless cultivar was
affected by the drying method assessed (in solar greenhouse, in convective chamber at 50°
C and at 60°C). Convective-dried grapes were lighter than those dried in the solar
greenhouse. This could be attributed to the longer drying time, which could result in
severe browning (drying time = 47 h, 101 h and 721 h for convective at 60°C, at 50°C and
greenhouse, respectively).
As Zemni et al.
[3]
reported, the color of raisins from Muscat of Italy variety was highly
influenced by the predrying treatment used. Oven drying of grapes that were dipped in
NaOH solution produced raisins with light brown shade in comparison with greenhouse-
dried grapes (dark brown colored). Alternatively, olive oil and K
2
CO
3
emulsion dipping
resulted in reddish-brown raisins. This could be attributed to the sensitivity of the PPO to
the higher temperature of dipping (90°C) in the case of caustic soda (NaOH) pretreatment
in comparison to the potash (K
2
CO
3
) pretreatment (50°C). Indeed, the exposure of PPO
to temperatures of 70–90°C at certain times is known to destroy their catalytic activity and
prevent browning of processed products.
[96]
Shrinkage
One of the most important physical changes that biological materials also endure during
dehydration is the reduction of their volume or size, often called shrinkage. Shrinkage is
caused by structural collapse, which is generally associated with water removal.
[90]
Shrinkage assessment can be carried out using common methodologies including direct
measurement of product dimensions or volume displacement technique.
[97]
Several mod-
els and equations predicting volume changes during the procedure of drying have been
widely tested for agri-food materials, as well.
[15,37,98–102]
Novel methodologies based on
image analysis and computer monitoring have been proposed to estimate the size reduc-
tion (shrinkage) and shape change (deformation) during food dehydration. Shrinkage
deformation characteristics determined by these methods were successfully applied in the
assessment of water diffusivities and drying simulation of food products and results were
comparable with other studies where different methodologies have been used.
[97]
These
innovative technologies create the opportunity for multidimensional quality analysis; in
other words, they allow the simultaneous measuring of different quality parameters
including shrinkage, deformation, color and texture.
[103]
In the case of raisins, both of
these approaches (modeling and new technologies) have been studied for investigating
shrinkage throughout the process of drying.
Gabas et al.
[104]
evaluated the effect of a chemical pretreatment (a solution of 2%
CaCO
3
with 0–3% EO) on the physical properties of the grapes of Italy variety. They
found that the shrinkage increased with drying temperature between 40 and 80°C and
decreased with increasing concentration of EO pretreatment. The temperature effect on
shrinkage can be attributed to the temperature dependence of elastic and mechanical
properties of cell wall structures. On the other hand, EO acts on the grape skin by
dissolving the waxes and resulted in less collapsed structure, which, in turn, led to
better-dried grapes.
Azzouz et al.
[105]
compared two models of diffusion developed to evaluate the
effective diffusivity: a simplified one based on Fick’s law and a second one that
FOOD REVIEWS INTERNATIONAL 27
accounted for the shrinkage as a fundamental parameter by modeling the movement of
the solid skeleton. Shrinking was ideal since the eliminated water was replaced by the
solid, there was no appearance of pores (ε= 0) and a linear correlation between
volume changes and water content was discerned for both varieties of grapes. The
same conclusion was also ascertained by Bennamoun and Belhamri
[106]
,whofound
that the necessity of introducing shrinkage was clear and neglecting it brought about
erroneous results (in the evolution of moisture content, which was under estimated
without considering shrinkage). According to many researchers, a linear relationship
between shrinkage and moisture content and temperature is generally observed during
drying operations.
[15,107–110]
Indeed, the drying process leads to changes of foods at
microstructural level and consequently, it affects their macroscopic characteristics. Loss
of water and the segregation of components occurring during drying, result in damage
and disruption of the cellular walls, and even collapse of the cellular tissue. These
changes are generally associated with volume reduction of the product.
The work of Bingol et al.
[34]
investigated the quality parameters of Thompson seedless
grapes as affected by drying conditions (dipping temperature and time in a tray dehy-
drator). Regarding shrinkage, their findings disclosed that as drying proceeds, grapes
shrink due to the loss of moisture content and the decrease in porosity.
Senadeera et al.
[37]
expressed the shrinkage behavior as average diameter variation during
drying in a convective oven at 50°C, and found that at theend of drying,treated grapes by peel
abrasion showed lower shrinkage changes than untreated ones. The diameter change of
untreated white grapes was about 35% and only 19% for treated ones. For red grapes, the
diameter changed from about 40% for untreated samples to 30% after abrasive pretreatment.
It seems that physical pretreatment of grapes causes a loss in their initial moisture content,
which reduces the drying time and thereby, decreases shrinkage.
The influence of physical (peel abrasion) and chemical (EO dipping) pretreatments on
drying characteristics and quality of Red Globe grapes has been studied by Adiletta et al.
[15]
Peel abrasion combined with optimal drying temperature (50°C) was not only effective in
shortening the drying time, but also reducing the shrinkage of grapes. In fact, the decrease of
grape volume has been observed to be proportional to the water content decrease during
drying. Among the common used empirical models that correlate shrinkage with moisture
content, the quadratic model showed an acceptable fit to experimental data (according to the
values of statistical parameters mainly R
2
) for all the samples and temperatures studied.
Machine vision (MV) was used by Behroozi Khazaei
[103]
to measure grape shrinkage
through the drying process. An artificial neural network (ANN) was developed for
dynamic modeling of Sultana grape drying in a hot air dryer. The ANN was qualified as
multilayer feed forward neural network (MFNN) and was found to have better perfor-
mance than multiple linear regression (MLR) model (value of R
2
= 0.99952 and 0.99830,
respectively) for predicting the moisture content and shrinkage of grapes during the
process of dehydration. The MFNN model proposed by the MV methodology might be
useful for developing an automatic control system for hot air dryers.
Texture
Raisins textural parameters of importance include hardness, stickiness, moistness, springi-
ness (elasticity), cohesiveness and chewiness. These often can be measured by texture
profile analyses (TPA).
[110]
28 R. KHIARI ET AL.
Grapevine cultivars, Fiesta and Selma Pete, dried by two drying methods, dry-on-vine
(DOV) and tray-dry (TD) were analyzed for texture.
[92]
Moistness, stickiness and chewi-
ness of DOV samples were higher than TD samples; however, the grittiness of DOV
raisins was lower than TD ones. The grittiness is generally due to the formation of sugar
crystals, a phenomenon known as “sugaring.”Sugaring occurs often with stored raisins
that are overly moist or that are mechanically damaged. Since the TD raisins were less
moist than the DOV raisins, so presumably the higher grittiness noted with these raisins
was owing to mechanical damage during the tray-drying process. They concluded also that
raisins being the most sticky or chewy were generally the least moist.
The effect of air impingement drying at different temperatures (50, 55, 60 and 65°C) on
texture (expressed in terms of hardness) of Monukka seedless grapes were evaluated by
Xiao et al.
[111]
Drying temperatures (from 55 to 65°C) significantly increased the hardness
during the drying process. This observation was explained by the fact that high tempera-
ture allowed the water removal from the surface better than the migration of water from
the interior, which led to the formation of a hard layer on the surface.
According to Adiletta et al.,
[110]
the hardness and chewiness of dried white grapes
(Regina) were higher than those of red raisins (Red Globe). This may be explained by the
difference in the size and thickness of the berries, which could be determining factors,
since both varieties were compared at the same final moisture content. However, for each
cultivar, untreated and treated dried samples were similar in terms of hardness and
chewiness. With respect to springiness, no difference was detected between both cultivars.
The texture of Italia grapes dried under different temperature and time combinations
was examined in the work of Rybka et al.
[112]
Firmness was negatively correlated to
consumer’s overall acceptance. In fact, the longer drying time or higher temperature
resulted usually in harder texture, which, in turn, influences the consumer’s acceptability
and alters the mouthfeel for the worse.
The texture attributes of Crimson seedless cultivar under different drying methods was
studied by Guiné et al.
[95]
They concluded that grapes dried in a convective chamber at
60°C had higher hardness and springiness but less chewiness than raisins obtained by
solar greenhouse dehydration and convective drying at 50°C. This might be assigned to
the difference in their final moisture content (at the end of drying), which was lower in the
case of convective drying compared to solar greenhouse drying. In addition, the drying
temperature could also influence these texture proprieties. The temperature in the con-
vective drying chamber was constant and higher; however, it was variable and lower in the
greenhouse solar drying.
Aroma
Flavor is also deemed an important quality feature that influences the marketability of a
product in addition to its appearance and shape. Flavor is generally determined by taste and
aroma.
[94]
It can be also defined as a blend of several volatile and nonvolatile compounds with
a wide range of physicochemical characteristics.
[113]
While the volatile components impact
both taste and aroma, the nonvolatile ones influence only the taste.
[114]
Flavor is probably the
most elusive and subjective quality parameter because it is difficult to measure and relies
typically on consumer preference.
[113]
In the present review, we are not going to emphasis the taste determination but rather
aroma identification in dried grapes since taste perception is often accomplished by
FOOD REVIEWS INTERNATIONAL 29
sensory analysis. Indeed, sensory evaluation encompasses taste assessment along with
other organoleptic attributes such as color, texture and aroma, so that taste is usually
judged as a part of raisins’overall appreciation by panelists as reported in.
[30,95,112]
Only a few papers have focused on the characterization of raisins’aroma profile
[115–119]
although grape volatile compounds have been extensively studied.
[113,120–123]
It is worth
mentioning that almost all the other earlier researches on raisins’aroma have been
interested in the identification of the volatile components in sun or air-dried grapes and
no information is available on the aroma profiles of their corresponding fresh counter-
parts, except the work of.
[115]
Volatile compounds are low molecular weight substances (usually < 300 Da) that volatilize
rapidly with the contact of air. These volatile molecules are present in food in low concentra-
tions ranging from milligram per kilogram to nanogram per kilogram.
[124]
Raisins aroma
constituents exist as both free and glycosidically bound-form volatile compounds.
[118]
These
odoriferous components come generally from three origins: the majority of them derived
from the fresh grapes, other metabolites including furans and pyrazines are produced during
the processof drying as a result of the Maillard reaction
[117,118,125]
, while aliphatic compounds
such as acids and aldehydes are generated from the oxidative degradation of unsaturated fatty
acids.
[117,118]
The evaluation of aromatic constituents includes usually four steps: extraction, con-
centration, chromatographic separation and detection.
[126]
In the former studies of raisins’
aroma
[115,116]
, steam distillation and simultaneous distillation extraction were used as
isolation methods. The solid phase microextraction (SPME) is another technique recently
applied
[117–119]
for the extraction of raisin volatiles since it is considered as a simple,
inexpensive, solvent-free and sensitive method. For study and characterization of volatiles
in raisins, instrumental analysis (Gas Chromatography-Mass Spectrometry, GC-MS) as
well as sensory evaluation (olfactometry) have been frequently used. The combination of
these two methods is also called Gas Chromatography-Olfactometry (GC-O). GC-O
allows the detection and the estimation of each aroma-active compound based on its
odor activity value (OAV); hence, a volatile that has an OAV above 1 is considered as a
flavor contributor.
[127]
Currently, a total of 91 odorants have been distinguished in raisins
belonging typically to six categories: floral, fruity, green, roasted, fatty and chemical
aromas.
[117,118]
The common volatile compounds found in the aroma profiles of raisins
include some aliphatic acids, aldehydes, terpenoids, furans and pyrazines. The aliphatic
acids such as hexanoic, heptanoic, octanoic, nonanoic and decanoic acids along with the
aliphatic aldehydes pentanal, heptanal, hexanal, (E)-2-heptenal, (E)-2-octenal, (E)-2-non-
anal and (E,E)-2,4-nonadienal were reported to be present at high concentration in
raisins.
[117,118]
These aliphatic compounds are all well known to be derived from unsatu-
rated fatty acid oxidative degradation and to generate especially the dried grapes’green
aroma. Terpenoids (geraniol, linalool and β-damascenone) were also recorded recently as
potent volatile compounds since they strongly contribute to raisins characteristic floral,
fruity and fatty aromas. Furans (2-pentylfuran and furfural), pyrazines (3-Ethyl-2,5-
dimethyl pyrazine and 2,6-Diethyl pyrazine) as well as benzene-acetylaldeyde were also
identified in high levels. These compounds have been pointed out to come from the
Maillard reaction and to contribute to the marked fruity, roasted and floral aromas in
raisins, respectively.
[117,118]
30 R. KHIARI ET AL.
As mentioned above, most of the raisins volatiles have been reported to originate from the
fresh grapes. The process of drying contributes additionally to the generation of other
compounds through the unsaturated fatty acid auto-oxidation (UFAO) and Maillard reac-
tions. Based on these data, we propose in Figure 2 a schematic representation of the probable
pathways leading to the formation of raisins volatile compounds adapted from previous
references investigating the metabolic aroma biosynthesis in grapes and wine.
[121,123,130–132]
Many factors may affect raisins’volatile composition including the grape variety genotype, the
climatic, edaphic and agronomic conditions during production, the harvesting date, the
postharvest handling, the pretreatment conditions, the drying process and storage
conditions.
[130]
To date, we have a limited understanding of the involvement of these factors
in the volatile composition and the resulting flavor of raisins; hence, future challenges need to
focus more on this subject.
[128]
The volatile compounds ascertained in the above cited
investigations dealing with dried grapes’aroma profile
[115–119]
are delineated in Table 4.
These studies are discussed below.
The study of Ramshaw and Hardy
[115]
evaluated the aroma-active components present in
fresh and dried sultana grapes previously treated with a commercial solution. Their results
showed that the application of a pretreatment prior to drying highly influenced raisins’aroma
Figure 2. Probable pathways for the metabolism of raisins aroma (adapted from Wen et al.
[128]
; van
Boekel
[129]
; Dunlevy et al.
[
122
]
, with modifications).
AACT: acetoacetyl-CoA thiolase; HMGS: 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase; HMGR:
3-hydroxy-3-methylglutaryl-CoA reductase; MVA: mevalonate; MK: mevalonate kinase; MVP:
Mevalonate-5P; PMK: phosphomevalonate kinase; MVPP: mevalonate-5PP; MPDC: diphosphomeva-
lonate decarboxylase; 3GP: D-glyceraldehyde 3-phosphate; DXS: 1-deoxy-D-xylulose-5-phosphate
(DXP) synthase; DXR: DXP reductoisomerase; MEP: 2-C-methyl-D-erythritol 4-phosphate; MCT: MEP
cytidyltransferase; CDP-ME: 4-(cytidine 5ʹ-diphospho)-2-C-methyl-D-erythritol; CMK: CDP-ME kinase;
CDP-MEP: 2-Phospho-4-(cytidine 5ʹ-diphospho)-2-C-methyl-D-erythritol; MCS: 2-C-methyl-D-erythritol
2,4-cyclodiphosphate (ME-2,4cPP) synthase; HDS: 1-hydroxy-2-methyl-2-butenyl 4-diphosphate
(HMBPP) synthase; HDR: HMBPP reductase; IPPI: isopentenyl diphosphate (IPP,C5) Delta-isomerase;
DMAPP (C5): dimethylallyl diphosphate; GPS: geranyl diphosphate (GPP,C10) synthase; FPS: farnesyl
diphosphate (FPP,C15) synthase; GGPS: geranylgeranyl diphosphate (GGPP,C20) synthase; TPS:
Terpenoid synthase; ABA: abscisic acid; UGT: Uridine diphosphate (UDP)-dependent glycosyl
transferases.
FOOD REVIEWS INTERNATIONAL 31
Table 4. Volatile compounds reported in raisins.
Compounds Reported by
Concentration of free-form
(µg/L)
1
Concentration of bound-form
(µg/L)
1
Acids
2-Methyl-propanoic acid W1, W2 2.70–281 nd
Pentanoic acid W1, W2 43.3–171 70.8–76.8
Hexanoic acid B, W1, Bh, W2 271–675 tr
2-Ethyl-hexanoic acid W1 122 71.2–75.1
Heptanoic acid B, W1, W2 9.77–198 41.6–59.1
Octanoic Acid B, W1, W2 11.8–182 59.2–59.7
Nonanoic acid B, W1, Bh, W2 36.0–150 80.0 −109
Decanoic acid B, W1, W2 28.5–133 265–275
Dodecanoic acid W1, W2 25.4–137 3.97–5.56
Tetradecanoic acid W1, W2 122–129 4.64–7.51
n-Hexadecanoic acid W1, W2 25.5–124 tr
Aldehydes
Pentanal R, W1, W2 6.7–102 nd
Hexanal R, B, W1, W2 8.79–344 nd
Heptanal B, W1, W2 5.98–87.1 nd
(E)-2-Hexenal B, W1, W2 16.6–30.5 nd
Octanal B, W1, Bh, W2 2.56–54.5 nd
(E)-2-Heptenal R, B, W1, W2 21.8–86.1 nd
Nonanal B, W1, W2 5.35–29.5 nd
(E)-2-Octenal B, W1, W2 16.7–23.1 nd
(E,E)-2,4-Heptadienal B, W1, Bh, W2 62.1–257 nd
Decanal B, Bh, W2 0.10–86.5 nd
Benzaldehyde R, B, W1, Bh, W2 6.60–18.5 nd
(E)-2-Nonenal B, W1, W2 9.3–12.7 nd
Benzeneacetaldehyde R, B, W1, W2 10.9–398 nd
(E,E)-2,4-Nonadienal B, W1, W2 54.3–60.2 nd
Esters
Ethyl acetate W1, Bh, W2 97–395 nd
Ethyl hexanoate W1, W2 3.94–28.5 nd
Ethyl octanoate R, W1, Bh, W2 1.13–2.77 nd
Butyrolactone R, W1, W2 3.00–6.24 nd
Ethyl decanoate Bh, W2 nd - 0.11 nd
Diethyl succinate W1, W2 0.60–2.03 nd
Benzyl acetate W1, W2 1.01–1.16 nd
Methyl salicylate W1, W2 0.80–794 0.77–12.5
Phenethyl acetate W1, W2 1.04–4.11 nd
γ-Nonalactone W1, W2 2.30–48.1 nd
Methyl hexadecanoate W2 nd - 1.27 nd
Ethyl hexadecanoate W1, W2 0.15–3.99 nd
Alcohols
3-Methyl-1-butanol W1, W2 0.52–463 302–969
1-Pentanol R, W1, W2 0.60–5.40 5.21–20.7
3-Methyl-2-buten-1-ol W1, W2 0.13–0.28 0.44 −157
1-Hexanol W1, W2 12.7–141 38.3–215
1-Octen-3-ol B, W1, Bh, W2 2.47–28.2 1.16–10.4
Heptanol W1, W2 1.50–26.8 2.20–6.72
(Continued)
32 R. KHIARI ET AL.
Table 4. (Continued).
Compounds Reported by
Concentration of free-form
(µg/L)
1
Concentration of bound-form
(µg/L)
1
6-Methyl-5-hepten-2-ol W1, W2 0.19–0.36 0.17–2.41
2-Ethyl-1-hexanol W1, W2 0.37–1.90 0.96–10.4
1-Octanol B, W1, W2 0.30–7.47 0.73–7.06
(E)-2-Octen-1-ol W1, W2 1.17–26.2 0.35–1.33
1-Nonanol B, W1, W2 0.08–1.27 0.10–9.50
Benzyl alcohol R, W1, W2 4.33–77.0 126–2931
Phenylethyl alcohol R, W1, Bh, W2 10.1–397 41.3–159
Ketones
2,3-Butanedione R, W2 52.9–88.3 nd
2,6-Dimethyl-4-heptanone W1, W2 1.65–16.9 nd
Acetoin R, W1, W2 15.8–2684 18.8–1922
6-Methyl-5-hepten-2-one W1, Bh, W2 1.73–34.1 nd
6-Methyl-3,5-heptadiene-2-one B, W1, W2 1.59–66.6 nd
Acetophenone R, W2 4.13–10.1 nd
Terpenoids
Limonene W2 6.33–19.7 7.43–8.19
γ-Terpinene W2 nd - 17.0 7.40–8.37
(E)-β-Ocimene W1, W2 10.0–21.8 2.87–3.46
p-Cymene W1, W2 1.07–27. 2 nd
Terpinolene W2 7.50–19.4 7.72–7.90
Rose oxide W2 0.30–54.3 5.05–5.72
Alloocimene W2 7.0–17.3 nd
3-Ethyl-2-methyl-1,3-hexadiene W1, W2 16.4–22.1 nd
Nerol oxide Bh, W2 0.43–156 5.35–29.2
Linalool W1, Bh, W2 0.83–184 1.78–10.2
α-Terpinenol W2 nd - 0.60 nd
Hotrienol W1, W2 0.87–1071 0.56–0.79
p-Menth-1-en-9-al W1, W2 0.20–99.9 5.06–5.59
α-Terpineol R, B, W1, W2 0.30–23.6 0.24–8.59
γ-Terpineol W2 nd - 3.33 nd
Lilac alcohol W1, W2 0.14–0.43 nd
Neral Bh, W2 1.70–31.2 46.1–51.8
cis-Pyran linalool oxide W1, W2 3.30 −12.4 0.45–3.19
β-Citronellol W1, W2 0.27–3.22 0.91–2.25
Nerol W1, W2 2.30–206 2.36–170
β-Damascenone W1, W2 1.30–13.4 1.20–127
Geraniol W1, W2 0.10–558 0.16–60.5
Geranylacetone B, W1, W2 1.42–1.85 3.25–16.3
Geranic acid W1, W2 45.3–1091 180–250
Furans
2-Pentyl furan B, W1, Bh, W2 9.99–256 nd
Furfural R, B, W1, W2 82.9–3209 nd
2-Acetylfuran R, B, W1, W2 9.75–194 nd
5-Methyl-2-furfural R, B, W1, W2 4.38–57.8 nd
Pyrazines
2-Ethyl-6-methyl pyrazine W1, W2 1.97–86.7 nd
2,6-Diethyl pyrazine W1, W2 19.2–157 nd
3-Ethyl-2,5-dimethyl pyrazine W1, W2 22.0–106 nd
(Continued)
FOOD REVIEWS INTERNATIONAL 33
profile. In whole, 34 compounds were identified, among them the fresh grape volatiles mainly
furfural, methyl furfural, 2-hydroxybutan-3-one and diacetyl were found in considerable
amounts in the dipped sultanas but at far less rates in the undipped raisins. These compounds,
commonly ascribed to nonenzymatic browning, seemed to be responsible for the malty,
caramel like aroma of the undipped sultanas and their production was presumably inhibited
by the predrying treatment. On the other hand,the dipped sultanas contained several carbonyl
compounds that were not observed in the undipped ones.
Buttery et al.
[116]
recorded a total of 38 components in the volatile oil of sun-dried
grapes obtained from the Thompson seedless variety. They concluded that the process of
drying substantially affected the raisins’aromatic composition. The major components
identified included the aliphatic acids octanoic, nonanoic, hexanoic, heptanoic and
decanoic acids as well as 2-hexyl-3-methylmaleic anhydride, nonanal, phenylacetaldehyde
and 2-pentylfuran. In this study, two unusual metabolites (2-hexyl-3-methylmaleic anhy-
dride and 1-octen-3-one) were detected for the first time in raisins. The former belongs to
anhydrides, which are usually hydrolyzed by water so that the loss of moisture owing to
drying favored its generation. Alternatively, the latter odorant had been reported to be the
main characteristic aroma of mushroom and was found in cooked mushroom and
artichoke.
Wang et al.
[117]
analyzed the volatile compounds in air-dried raisins from Flame
Seedless, Thompson Seedless and Crimson Seedless varieties. In total, 77 volatiles were
identified of which 37 had never been reported as raisin volatiles before. The aroma
characters of the three varieties were quite similar except for some discrepancies in the
concentration of each aroma character. The major free-form volatiles were ethyl acetate,
hexanoic acid, (E,E)-2,4-heptadienal and geraniol, with β-damascenone, 3-ethyl-2,5-
dimethylpyrazine, 1-octen-3-ol and hexanal making up the highest contribution to the
aroma. Fruity and floral were the main characteristics of the free-form fragrances in
raisins. The major bound-form (glycosidically bound) metabolites were benzyl alcohol
and acetoin, with β-damascenone being responsible most for the bound-form aromas,
Table 4. (Continued).
Compounds Reported by
Concentration of free-form
(µg/L)
1
Concentration of bound-form
(µg/L)
1
Phenols
Phenol W1, W2 0.44–14.1 0.40–69.5
4-Vinylguaiacol W1, W2 1.28–8.59 0.35–27.9
Aromatic compounds
Toluene W2 2.53–22.4 nd
Naphthalene W1, W2 3.65–10.1 nd
2-Methyl-naphthalene W1, W2 1.06–2.36 nd
R: Reported in
[115]
for the study of sun-dried sultanas.
B: Reported in
[116]
for the study of sun-dried Thompson seedless raisins.
W1: Reported in
[117]
for the study of sun-dried raisins (Thompson Seedless, Flame Seedless and Crimson Seedless)
cultivated in China.
Bh: Reported in
[119]
for the study of sun-dried raisins (cultivars: Chriha, Raseki, Assli, and Meski) cultivated in Tunisia.
W2: Reported in
[118]
for the study of sun-dried raisins (Thompson Seedless, Crimson Seedless and Zixiang Seedless)
cultivated in China.
1:
Min-Max concentration values reported for the free and bound-form volatile compounds as reported by.
[118,119]
tr: trace.
nd: not detected in raisins.
34 R. KHIARI ET AL.
enhancing the floral, fruity and fatty aroma. According to this study, the process of
dehydration promotes the occurrence of glycosidically bound volatiles as a result of the
elevation of temperature and the natural hydrolysis of acids, which, in turn, characterized
the outstanding floral, fruity and fatty fragrances of raisins.
The aromatic composition of four Tunisian raisin varieties (Chriha, Razeki, Assli and
Meski) has been analyzed by Bhouri et al.
[119]
A total of 80 compounds were identified
that were present at different levels among varieties. Nonterpene hydrocarbon derivatives
were the main components of the Chriha and Assli cultivars (35.8% and 26.3%, respec-
tively). The Razeki variety was marked by apocarotenoids (25.5%). Oxygenated nonter-
pene derivatives, including esters, alcohols, aldehydes, acids and ketones were the most
remarkable odoriferous metabolites (57.5%) in the Assli variety. According to these
authors, the difference between the aroma profiles of the studied cultivars may be related
to genetic factors and environmental conditions which could be responsible, in part, for
the aroma and taste of the raisins produced.
Wang et al.
[118]
studied the aroma profiles of Thompson Seedless raisins (TSR) and
Centennial Seedless raisins (CSR) and Zixiang Seedless raisins (ZSR). These authors
identified a total of 91 compounds of which 72, 26 and 8 derived from fresh grapes,
UFAO and the Maillard reaction, respectively. TSR and ZSR were mostly characterized by
components that came from grapes and UFAO, while CSR was distinguished by the
volatiles generated from grapes and the Maillard reaction. The aroma characters of TSR
and CSR were similar, while the floral, fruity, green and roasted aromas of CSR were
higher than those of TSR due to the contributions of benzene-acetaldehyde, 2-pentylfuran,
(E)-2-nonenal and 3-ethyl-2,5-dimethyl pyrazine. The floral and fruity aromas of ZSR
were attributed to the components decanal, rose oxide, geraniol, linalool and β-damasce-
none, which were found in much greater amounts than in TSR and CSR. In contrast, the
green and roasted aroma intensities of ZSR were lower than the other two varieties. These
results indicated that the aromatic profile of Zixiang Seedless grapes was well retained in
their raisins, while the process of sun drying ostensibly made the differences between
Thompson Seedless grapes and Centennial Seedless grapes larger.
Effect of drying on the nutritional characteristics
In addition to their appealing appearance, delicious taste and outstanding aroma, raisins
among other dried fruits add important vitamins, minerals and other bioactive com-
pounds to the human diet. According to the latest report of the United States Department
of Agriculture
[129]
, 100 g of raisins provide 299 kcal of energy, 59.19 g of sugars, 3.07 g of
proteins, 0.46 g of lipids, 3.70 g of dietary fibers, 749 mg of potassium, 101 mg of
phosphorus, 50 mg of calcium, 32 mg of magnesium, 11 mg of sodium, 1.88 mg of
iron, 2.3 mg of vitamin B6, 0.17 mg of vitamin C, 0.12 mg of vitamin E as well as non
negligible amounts of phenolic compounds. Additionally, based on a 2000-calorie daily
diet, merely 40 g (1/4 cup) of raisins are required to provide 125 kcal of energy and their
intake will cater for the need of the most essential nutrients.
[4]
It is well known that high temperatures and long operation time during the process of
dehydration may affect the nutritional quality of dried fruits. While the amount of some
FOOD REVIEWS INTERNATIONAL 35
elements depleted during drying, the content of others, in contrast, will be raised.
[133]
Research that has examined the effect of drying on the changes of nutrients in raisins is
previously reported
[3–5,9,13,15,60,134–138]
, the main results of some of which are outlined in
Tables 5 and 6.
Proximate chemical composition
The nutritional quality depends commonly on the drying procedure. However, regardless
of the type of technology or pretreatment employed for grape drying, each method
involves heat and air, which would influence the nutritive value of raisins, in general.
As Clary et al.
[60]
reported, microwave and sun drying substantially affected the
nutritional proprieties of Thompson seedless grapes since potassium, carbohydrate, cal-
ories, ash, fiber as well as minerals including calcium, iron and sodium were about 4.5
times greater in the dried fruit samples compared to the fresh grapes (Table 5). Similarly,
to other dried fruits, raisins’fiber and minerals are generally concentrated in response to
the process of drying and water evaporation.
[142]
The proximate composition of two varieties of raisins (Thompson seedless and
Imperial seedless) was evaluated by Carranza-Concha et al.
[13]
This study focused on
the estimation of sucrose, fructose, glucose and some minerals (K, Mg, Ca and P) as
affected by microwave and hot air drying with or without pretreatment (NaOH). Based on
their results, both drying methods as well as pretreatments affected the retention of the
studied constituents. In general, all the dehydrating treatments caused a reduction in
sucrose content (not detected in dried grapes) presumably due to its susceptibility to
hydrolysis favored by the high temperatures. In comparison to fresh grapes, glucose and
fructose levels had been raised due to the removing of moisture, which concentrates these
constituents. However, when the berries were pretreated with NaOH, some losses in
glucose and fructose content were detected, but the latter carbohydrate was more affected.
This may be attributed to the interconvertible character of fructose in a basic medium due
to the equilibrium between its keto and enol forms (keto-enol tautomerism); that is, the
conversion of fructose into its enediol, which is, in turn, converted into a glucose
molecule.
[13]
Important levels of minerals were also found in raisins including potassium
(129–228 mg/100 g DW), calcium (3.8–61 mg/100 g DW) and magnesium (4.1–7.2 mg/
100 g DW). According to this investigation, convection drying combined with pretreat-
ment with NaOH was effective in increasing the levels of calcium and potassium sub-
stantially compared with microwave and hot air drying without pretreatment.
Zemni et al.
[3]
studied the changes of nutrients in Italia Muscat raisins subjected to
oven and greenhouse drying as well as two chemical pretreatments as described above.
Both methods of drying and pretreatment significantly affected the retention of nutritional
elements; however, each treatment influenced the proximate attributes differently. For
instance, grapes that were dried in the oven and dipped in olive oil and K
2
CO
3
solution
(OI treatment) had the highest contents of pH and sugar, while those dried in the
greenhouse with NaOH dipping (GII treatment) were found to have the highest values
of acidity and protein (Table 5). The analysis of minerals disclosed that, in general,
greenhouse-dried raisins had high amounts of calcium (83.69 mg/100 g DW), magnesium
(54.79 mg/100 g DW) and sodium (44.77 mg/100 g DW). The difference between the
levels of these macro- as well as micronutrients is suggested to be caused by chemical
reactions (such as diffusion into the intercellular spaces and Maillard reaction products
36 R. KHIARI ET AL.
Table 5. Nutrient changes during drying reported in raisins.
Clary et al.
[60]
Carranza-Concha et al.
[13]
Zemni et al.
[3]
Nutrient
(content per 100 g)
Fresh
fruit
Microwave
vacuum Sun drying MWD HAD MWD + NaOH HAD + NaOH Sun drying OI OII GI GII
Proximate composition
Water (g/g) 2.74 0.02 0.06 0.24 0.31 0.37 0.26 0.25 0.27 0.19 0.23 0.29
Protein (g) 1.08 3.63 3.10 nr nr nr nr 1.25 1.21 1.07 1.01 1.25
Ash (g) 0.56 2.70 2.28 nr nr nr nr 6.17 1.50 4.00 5.33 4.50
Carbohydrate (g) 24.91 90.99 89.02 79.3 54 60.4 72 31.50 49.70 42.00 47.37 47.37
Total lipid (g) 0.18 0.00 0.11 nr nr nr nr nr nr nr nr nr
Fiber (g) 1.60 3.90 6.30 nr nr nr nr nr nr nr nr nr
Minerals
Calcium (mg) 21.10 54.40 54.30 3.8 61 5.7 21.44 40.84 36.31 43.98 83.96 73.88
Potassium (mg) 200.00 900.00 870.00 193 228 129 167 348.40 208.00 280.80 196.30 178.10
Sodium (mg) 3.60 3.90 3.60 nr nr nr nr 44.31 9.63 9.88 10.36 44.77
Magnesium (mg) Nr nr nr 4.1 6.2 7.2 4.8 35.99 42.30 8.15 15.10 54.79
Iron (mg) 1.02 1.38 3.74 nr nr nr nr 2.44 0.04 0.75 1.21 1.15
Zinc (mg) Nr nr nr nr nr nr nr 0.27 0.07 0.22 0.25 0.20
Copper (mg) Nr nr nr nr nr nr nr 0.73 0.11 0.28 0.08 0.08
nr: not reported; MWD: Microwave drying; HAD: Hot air drying; OI: grape samples dried in the oven with pretreatment I (dipping in olive oil and K
2
CO
3
solution); GI: grape samples dried in
the greenhouse with pretreatment I (dipping in olive oil and K
2
CO
3
solution); OII: grape samples dried in the oven with pretreatment II (dipping in NaOH solution) and GII: grape samples
dried in the greenhouse with pretreatment II (dipping in NaOH solution).
FOOD REVIEWS INTERNATIONAL 37
which are able to chelate minerals interfering, therefore, with their solubility) that may
occur during each processing method and operating condition employed.
[19]
Fabani et al.
[141]
analyzed and quantified 29 mineral elements in the grape varieties
(Arizul, Sultanina, Superior and Flame) after sun drying in order to characterize their
Table 6. Phytochemicals reported in raisins.
Karadeniz et al.
[
139
]
Parker et al.
[140]
Fabani et al.
[141]
Content in mg/kg sample mg/kg sample µg/100 g DW
Compounds
Fresh
grapes
Sun-
dried
raisins
Dipped
raisins
Golden
raisins
Fresh
grapes
Sun-
dried
raisins
Golden
raisins
Fresh
grapes
Sun-
dried
raisins
PHENOLIC COMPOUNDS
Phenolic acids
Gallic acid nr nr nr nr nr nr nr 7–31 0.12–
0.50
Caftaric acid 100.7 39.6 45.2 84.3 7.9 31.4 130.4 41.6–
171
3.26–19
Fertaric acid nr nr nr nr nr nr nr 151–
333
0.53–1.2
Coutaric acid 31.8 6.7 7.7 27.3 14.8 nd 34.1 nd-
20.1
0.64–3.5
Oxidized cinnamic A nd 3.7 2.9 nd nr nr nr nr nr
Oxidized cinnamic B nd 6.1 5.1 nd nr nr nr nr nr
2-S-glutathionyl caftaric acid nd nd 8.1 nd 8.8 nd nd nr nr
Protocatechuic acid nd 6.8 2.8 nd nd 4.4 nd nr nr
Stilbenes
trans-resveratrol nr nr nr nr nr nr nr 2.6–3.8 nd-27
FLAVONOIDS
Flavonols
Quercetin nr nr nr nr nr nr nr 0.8–3.3 12.9–98
Quercetin glycoside A 21.9 34.7 20.6 37.1 15.2 15.6 65.7 nr nr
Quercetin glycoside B 3.9 7.3 39 41.5 25.6 6.5 43.4 nr nr
Quercetin-3-O- Rutinoside 0.9 5.2 6.5 3.5 tr 8.3 14.4 75–
140
109–260
Quercetin-3-O- Glucuronide nr nr nr nr nr nr nr 4.5–32 3.4–41.9
Isoquercitrin nr nr nr nr nr nr nr 31–40 39–121
Kaempferol nr nr nr nr nr nr nr 0.6–3.3 26.6–49
Kaempferol glycoside A nd 11.2 16.7 6.5 tr 7 9.8 nr nr
Kaempferol glycoside B 19.4 23.7 29.5 7.6 tr 9.3 14.3 nr nr
Kaempferol hexoside nr nr nr nr nr nr nr 35–55 81–213
Isorhamnetin nr nr nr nr nr nr nr 0.72–
1.2
3.2–17
Isorhamnetin-hexoside nr nr nr nr nr nr nr 1.8–9 2.4–43
Flavan-3-ols
(+)-catechin nr nr nr nr nr nr nr 20–38 15–158
(-)-epicatechin nr nr nr nr nr nr nr 1.6–14 15–27
Procyanidin dimer nr nr nr nr nr nr nr 4–6nd
Flavones
Astilbin nr nr nr nr nr nr nr 0.3–3 0.8–27
nd: not detected; nr: not reported; tr: trace.
38 R. KHIARI ET AL.
multielemental composition. The raisins presented high levels of K (639–883 mg/100 g),
Ca (51–121 mg/100 g) and Mg (28–42 mg/100 g). Conversely, lower values of Na (0.8–
21 mg/100 g) were registered. Boron (B) was also measured at considerable amounts (2.0–
5.4 mg/100 g). This micronutrient is known to have an important role in bone health.
Regarding heavy metals that are considered as toxic elements and serve as indicators of
environmental contamination such as Aluminum (Al), Cadmium (Cd) and Plomb (Pb),
the multielemental analysis revealed that ten elements were below the detection limit
(< LOD). Only Al was above the quantification limit (LOQ) (from 0.6 to 2.9 mg/100 g),
but within the mean dietary intakes recommended by the World Health Organization
(2.5–6.3 mg/day), which means that the investigated raisins were safe for consumption.
Vitamins
Although the recommended daily allowance of vitamins is required in trace amounts for a
healthy and balanced diet, being deficient in these micro-constituents may increase the
risk of many health problems including some serious diseases.
[133]
To date, vitamins from
raisins constitute the less studied attribute. The little information available on the dried
grapes vitamins’composition as affected by dehydrating operations informs on their
content in vitamins A and C, thiamine (vitamin B1), riboflavin (vitamin B2) and niacin
(vitamin B3). Indeed, it is generally admitted that the process of drying entails loss of
vitamins in most dried fruits, mainly vitamin C (ascorbic acid), which is considered as one
of the most vulnerable vitamins to heat, oxygen and light exposure.
[5,60]
Nevertheless, as
far as raisins are concerned, the content of such micro-elements has been reported to be
highly influenced by the drying technique and pretreatment applied. Some vitamins are
degraded at high dehydration temperature, conversely others are concentrated in raisins
with an increase of their bioavailability.
[135]
As Clary et al.
[60]
reported, the vitamin contents of some raisins have been raised in
response to sun drying and MWVD. According to this study, MWVD exhibited better
preservation of vitamins compared to sun drying. The most conspicuous difference was
vitamin A, which was found at 378 I.U./100 g in MWVD, at 80 I.U. in the fresh berries but
was absent in sun-dried grapes. The content of vitamin C, thiamine and riboflavin were
also higher in the MWVD samples than in the fresh or sun-dried ones. The level of
vitamin C was about 41% higher in raisins produced by MWVD (12.50 mg/100 g),
whereas those obtained by sun drying (8.83 mg/100 g) were about 29% richer in this
micronutrient than the fresh grapes (0.30 mg/100 g). Thiamine and riboflavin were also
detected at much higher rates in the MWVD samples (0.29 mg/100 g and 0.31 mg/100 g,
respectively) than in the fresh (0.04 and 0.06 mg/100 g, respectively) or sun-dried ones
(0.17 and 0.15 mg/100 g, respectively). It seems that the relatively long drying time
associated with the sun-drying method contributed to the severe loss of vitamins, espe-
cially those sensitive to heat and oxidation.
Likewise, Carranza-Concha et al.
[13]
indicated that both methods of dehydration (in hot
air (HAD) and microwave (MWD)) as well as pretreatment (with/without dipping in
NaOH solution) contributed to the increment of vitamin C levels in the dried studied
grapes. For instance, the values of ascorbic acid registered in MWD and HAD samples (8
and 11.54 mg/100 g, respectively) were about 3.2 times greater than those detected in the
fresh fruits (2.31 and 3.59 mg/100 g, respectively). Additionally, the use of NaOH as
predrying treatment increased even more the quantities of vitamin C (13.34 and 9.84 mg/
FOOD REVIEWS INTERNATIONAL 39
100 g in MWD + NaOH and HAD + NaOH samples versus 3.8 and 2.92 mg/100 g in the
fresh grapes, respectively). This might be explained by the role of the pretreatments such
as blanching and dipping in alkaline and other solutions, which is stated to reduce the loss
of vitamins during drying through the inhibition of their degradation.
[143]
Phytochemicals
Recently, phytochemicals as bioactive compounds of dried fruits have gained importance
because they contribute to a plethora of health benefits. Owing to their strong antioxidant
properties, phytochemicals are suggested to be involved in lowering the incidence of many
diseases such as cancer, diabetes type 2, obesity, stroke, cardiovascular disease and other
chronic disorders.
[144]
There are a number of review articles reporting the phytochemicals (phenolic acids,
flavonoids, stilbenes and anthocyanins) as well as their bioavailability in raisins.
[9,135,138,145]
Here, we will rather emphasize their changes as affected by the drying process, with an update
of the most recent researches.
Table 6 summarizes the most ascertained phytochemicals in raisins under different
drying operations. As can be clearly seen, dehydration strongly influenced the content of
polyphenols, which showed various behaviors in response to each processing condition.
Although both Karadeniz et al.
[139]
and Parker et al.
[140]
identified the same polyphenolic
compounds, in the former study generally a loss of these bioactive constituents due to
dehydration has been recorded, whereas in the latter their accumulation has been notice-
able. The difference between the component profiles from the two investigations may be
attributed to the cultivar analyzed, the drying operations as well as the extraction condi-
tions of these phytochemicals.
Karadeniz et al.
[139]
stated that when compared with fresh grapes (on a dry weight basis), a
loss of around 90% of caftaric and coutaric acids has been observed. The substantial degrada-
tion of hydroxycinnamic acids was more accentuated than that for the total flavonols (rutin,
quercetin glycosides A and B, and kaempferol glycosides A and B), which were lost at
respective percentages of 67%, 56% and 62% for sun-dried, dipped and golden raisins. This
was explained by the fact that flavonol glycosides were not as susceptibleas hydroxycinnamics
to enzymatic oxidation and that these latter components are rapidly converted to reaction
products upon processing. Alternatively, complete degradation of procyanidins and flavan-3-
ols was marked since they were absent in all the raisin samples.
On the other hand, Parker et al.
[140]
noted the presence of considerable amounts of
caftaric acid in sun-dried and dipped raisins (41.4 and 130.4 mg/kg, respectively) in
comparison to the fresh fruits (7.8 mg/kg). Consequently, the metabolite 2-S-glutathionyl
caftaric acid was not detected in the sun-dried and golden samples since it is normally
formed due to the decompartmentalization of caftaric acids and glutathione through
dehydration. Additionally, flavonols (especially kaempferol glycosides A and B) were
identified in higher levels in the sun-dried and golden raisins than in the fresh grapes.
The predominance of these compounds in raisins may be the result of flavonol glycosyla-
tion due to the presence of higher concentrations of glucose during dehydration, which
reacts with a flavonol specific glycosyl-transferase to form flavonol glycosides.
[146,147]
Fabani et al.
[141]
analyzed the changes of phenolic profiles in fresh and sun-dried grapes, 17
phytochemicals belonging to the classes of phenolic acids, flavonoids and stilbenes have been
characterized but their distribution was variable and dependent on the variety analyzed.
40 R. KHIARI ET AL.
Similarly to the report of Karadeniz et al.,
[139]
phenolic acids were the most affected constituents
by drying. The value of their losses ranged from 82.58% (coutaric acid) to 99.63% (fertaric acid).
Quite the reverse, flavonols were present at higher concentrations in raisins in comparison to
fresh berries. Among the nine flavonols identified, rutin (quercetin-3-O-rutinoside) was the
most abundant and was found at concentrations ranging from 109 to 260 µg/100 g DW in the
sun-dried samples versus 75–140 µg/100 g DW in the fresh grapes. Quercetin, kaempferol
hexoside as well as isoquercitrin were also detected at greater amounts in raisins than in fresh
fruits with an increase in the order of 97%, 74% and 67%, respectively. Flavones were repre-
sented by only one chemical, Astilbin, which had lower levels in fresh grapes (0.8–27 µg/100 g
DW)thaninraisins(0.3–3 µg/100 g DW). Flavan-3-ols were marked by three representative
components: (-)-epicatechin and (+)-catechin found at higher concentrations in raisins than in
grapes (2 to 4 times greater, respectively), while procyanidin dimer was found only in fresh
grapes (4–6 µg/100 g DW). Interestingly, this study showed the presence of stilbenes, which were
distinguished by the typical trans-resveratrol. Its occurrence was favored by sun drying, namely,
in the variety Sultanina, which contained a concentration of 27 µg/100 g DW in raisins against
2.6 µg/100 g DW in fresh grapes. Since grapes are known to produce stilbenes, mainly
resveratrol, in response to mold infections, UV irradiation and other physiological stresses
[148]
,
the production of this compound in raisins could probably be attributed to their exposure to the
high temperature (40–45°C) during the natural sun-drying process.
Another group of flavonoids, anthocyanins, have been also measured in raisins.
Anthocyanins are characterized as colored phenolic compounds because they are exclusively
accumulated in the grape skin and provide the red, purple and blue color of the berries.
[149]
There are some reports that pointed out the presence of such pigments in raisins.
[137,149,150]
However, only the study of Fabani et al.
[141]
examined the anthocyanin changes during drying.
According to their results, 5 anthocyanins were identified and quantified in the red variety
Flame, but uniquely two of which, peonidin-3-glucoside and cyanidin-3-glucoside, were
identified in much lower contents in the dried samples, compared to fresh ones. The
degradation of these compounds could be ascribed to the high temperatures applied through-
out the process of drying or probably to the action of some enzymes, such as peroxidase,
which use anthocyanins as substrates.
The inconsistency between the different phenolic profiles reported in the previous data
may be related to the pattern of each phytochemical, which vary significantly upon
processing operations and with respect to the grape variety, as well. Some phytochemicals
were concentrated in raisins and increased their contents, while others undergo chemical
reactions and would degrade partially or entirely during the process of dehydration.
[141]
All the identified phenols in raisins have been reported to possess a wide range of
benefits for food and pharmaceutical industries because of their antioxidant, antimicro-
bial, anti-inflammatory, antiatherogenic and anticarcinogenic proprieties as well as their
contribution in the prevention of many other chronic diseases.
[4,138,144]
However, the
thorough investigation of the complete phytochemical profiles and their retention in
dried grapes as well as the relation to their bioactivities (namely their strong antioxidant
activity) are still unclear and need to be explored.
FOOD REVIEWS INTERNATIONAL 41
Effect of drying on the microbiological characteristics
As stated above, because of the low moisture content and reduced water activity (a
w
)of
raisins, most of the biochemical reactions (especially the activity of enzymes) are inacti-
vated and hence the growth of microorganisms is inhibited.
[113]
Nonetheless, if the water
activity of the dried products surpasses the recommended level (a
w
= 0.7) for safe storage,
potential microbial spoilage may occur, which, in turn, constitutes a health hazard due to
the possible contamination of the dried fruits with their toxins.
[5]
Despite certain reports
that have examined the incidence of mycotoxins in dried grapes (e.g., aflatoxins and
ochratoxins),
[3,93,151–161]
only a few papers have studied the influence of dehydrating
process on the microbial quality of raisins.
[3,46,161]
In McCoy et al., the effect of processing on the microbiological properties of unpro-
cessed (given from farmers) and processed (procured from manufacturer) Afghan raisins
was investigated. The results of this study showed that the number of total aerobic bacteria
(9.69 × 10
3
CFU/g), yeasts (from 2 × 10
1
to 4 × 10
1
CFU/g) and molds (from 1.29 × 10
2
to
5.29 × 10
3
CFU/g) were within the tolerable limits for the processed samples, whereas the
unprocessed dried grapes were suggested to be unacceptable. The values registered for
these latter samples were as follows: 3.09 × 10
4
CFU/g of total aerobic bacteria, yeasts
varying from 3 × 10
3
to 1.1 × 10
4
CFU/g and molds from 9.2 × 10
3
to 5.8 × 10
5
CFU/g. In
addition, processing was found to reduce the total coliforms from an average of 4.89 × 10
3
CFU/g (unprocessed raisins) to zero (processed raisins). The authors concluded that the
cleaning operation employed by the manufacturer prior to processing resulted in more
salubrious raisins.
The microbial quality of dried grapes produced by two methods of dehydration (shade
and solar drying) was assessed by Farahbakhsh et al.
[46]
Coliforms were counted in
samples obtained by both drying procedures at an average of 2 Log CFU/g. However,
neither Salmonella nor E. coli was detected. Solar drying permitted the growth of higher
levels of fungi in comparison with the shade one. This was attributed to the lower duration
of processing in addition to the use of SO
2
as pretreatment for the shade dehydration.
In the evaluation of dipping pretreatment (potassium carbonate-ethyl oleate) influence
on the occurrence of ochratoxin A
[97]
in sultanas and currants raisins, Zhang et al.
[161]
observed that OTA was not detected in the fresh and dried samples (with or without
pretreatment) of both varieties. However, after storage, dipped samples were found to be
more contaminated with OTA than the control, to a greater extent. These results indicated
that the pretreatment could enhance mycotoxin contamination of raisins as a consequence
of the wax removal through the chemical predehydrating treatment. According to the
authors of this study, when the wax is detached from their surface, the grapes would be
more prone to fungi sporulation and/or mycotoxin infection.
Zemni et al.
[3]
analyzed the mycological characteristics of Italia Muscat raisins as
affected by different drying methods and pretreatments. Their findings showed a remark-
able discrepancy in the fungal flora distribution. The common genera of fungi discerned
were Aspergillus, Penicillium, Cladosporium and Fusarium. Ochratoxinogenic species such
as Aspergillus ochraceus and Aspergillus carbonarius were the most detected molds being
found at levels of 15.56% and 10.41% in greenhouse and oven dried grapes pretreated with
olive oil and K
2
CO
3
solution, respectively. These authors argued that none of the drying
techniques/pretreatments completely hindered the spoilage of filamentous fungi. All the
42 R. KHIARI ET AL.
obtained raisins were contaminated with two potential ochratoxinogenic species, at least
with an ability of OTA production varying from 0.02 to 38.87 μg OTA/g CYA medium.
Future trends
Although great progress has been achieved in drying technology, there is a need for
further development of cost effective and energy efficient methods in the food industry.
In raisin processing in particular, many aspects should be considered for both pretreat-
ment and drying methods.
For pretreatments, given that the raisin industry commonly use chemical pretreatment in
practice, residual alkali solution, sulfite/sulfur dioxide in dried grapes raises food safety
concerns.
[19]
In addition, with the consumer awareness of food additives and the increasing
demand for healthy products, alternative predrying techniques should be established. For
example, screening and selection of natural solutions or phytochemicals that might play the
same role as the widely employed chemicals in grape drying should be developed. Novel thermal
pretreatments such as high-humidity hot air impingement blanching
[21]
,microwave
[39]
and
ohmic heating
[40]
have successfully showed their applicability in the laboratory scale. However,
it is difficult to scale-up these equipment to industry size due to the complexity of reproducing
the same operating conditions and results.
[19]
Therefore, future investigations are required to
design and optimize the extension of their application in large scale.
As for raisin processing, there has been much work on innovative dehydration techni-
ques, which led to good and effective results in terms of drying enhancement as well as
energy efficiency. Nevertheless, since high temperature and long drying time affect nega-
tively the quality of dried foods in general, further research is needed to produce better-
quality raisins. For instance, combination of drying methods offers the possibility to
obtain more appreciated dried products by consumers.
[63]
Data on the variability of the
physicochemical, nutritional (namely, vitamins, phytochemicals and aroma) and mycolo-
gical quality throughout raisin processing are still lacking, which makes it a potential area
of research. Additionally, the use of modern process systems engineering including
computer technology, sensor technology and online detection technology may help to
design smart control systems. These process-engineering instruments are based on math-
ematical modeling, which can provide a powerful approach of designing, optimizing and
operating food processes.
Conclusion
Based on their convenience, availability, nutrients and price, raisins are a great fruit choice
to be included in the human diet. Processing of such foodstuffcome to the forefront as it
has a great effect on the quality of the final product. Although many reviews have dealt
with the description of grape dehydrating processes, most of them focused on highlighting
the drying kinetics of these methods in view of their optimization, in terms of reducing
the energy costs and minimizing the time of dehydration, rather than enhancing the
quality and stability of the dried fruits. In this context, the influence of drying on the
physicochemical, nutritional and microbiological quality attributes has been scrutinized in
the present review. Nevertheless, future studies are needed in order to meet stringent
raisin quality. Since grape pretreatment is a crucial drying operation, innovative predrying
FOOD REVIEWS INTERNATIONAL 43
techniques such as the combination of physical and chemical treatments should be
established. Additionally, drying conditions should focus on gaining a better understand-
ing of the variability of the nutritional and mycological features throughout the raisins
processing by using automatic and direct online measurements. Furthermore, an
improved comprehensive knowledge of the nutritional (vitamins and phytochemicals) as
well as aromatic raisin profiles as affected by drying procedures may help to identify the
use of such components more effectively in the food and pharmaceutical industries.
Finally, appropriate assortment of drying methods/pretreatments for the preservation of
grapes is therefore required in order to produce healthy raisins with higher nutritive value.
Abbreviation
AEEO alkaline emulsion of ethyl oleate solution
ANN artificial neural network
CM carbonic maceration
DOV dry-on-vine
DW dry weight
EO ethyl oleate
HAD hot air dehydration
HHAIB high-humidity hot air impingement blanching
HW hot water
ID infrared drying
MFNN multilayer feed forward neural network
MV Machine vision
MW microwave
MWD microwave drying
MWVD microwave vacuum drying
OD oven drying
OTA ochratoxin A
PEF pulsed electric fields
PPO polyphenol oxidase
PVD pulsed vacuum drying
SPME solid phase micro-extraction
TD tray-dry technique
VD vacuum drying
UFAO unsaturated fatty acid auto-oxidation
Acknowledgments
The authors would like to thank Professor Nourhène BOUDHRIOUA for her assistance with the
preparation of this manuscript. The authors have no conflicts of interest to declare.
Funding
This work received no funding.
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