Studies on Drying of Osmotically Dehydrated Apple Slices

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Int.J.Curr.Microbiol.App.Sci (2018) 7(11): 633-642
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Original Research Article https://doi.org/10.20546/ijcmas.2018.711.077
Studies on Drying of Osmotically Dehydrated Apple Slices
Vikas Paradkar* and Gourav Sahu
College of Agricultural Engineering, Bapatla, India
*Corresponding author
A B S T R A C T
Introduction
The apple (Malus domestica) is a nutrient rich
fruit, widely grown in the temperate regions
with varied texture, colour, shape and size.
India and China is the largest producer of
apple. Apple is consumed in raw as well as
processed fruits. Apple can be processed into
many products such as fruit leather, candy,
fruit bar, and dehydrated slices. Many
traditional techniques are used for preparation
of dehydrated products which are not much
satisfactory from quality point of view. In
India every year about 20–40% percent of the
fruit and vegetable are waste due to improper
transportation, inadequate processing and
handling and lack of adequate storage system.
Due to moisture losses and spoilage, harvested
apple fruits should be marketed, processed or
preserved as early as possible. Hence it is very
important to convert the apple into other form
such as dried product, or to develop a method
for preservation and processing, to increase
the self-life and quality. The preservation of
food product is done to enhance the self-life of
without affecting the appearance, colour and
maintaining the physical and chemical
properties. In fruit and vegetable preservation
is usually carried out to slowdown the growth
of the bacteria, fugues and other
microorganism which cause spoilage. The
International Journal of Current Microbiology and Applied Sciences
ISSN: 2319-7706 Volume 7 Number 11 (2018)
Journal homepage: http://www.ijcmas.com
The present study was carried out to investigate the effect of solution concentration,
immersion time, process temperature and slices thickness on osmotic dehydration of apple
slices. The osmotic dehydration characteristics such as water loss (WL), weight reduction
(WR) and solid gain (SG) were studied for sugar solution of (50 and 70 ºBrix) at a
temperature of (30 °C and 50 °C) for 8 hours of immersion time. After osmotic
dehydration, the apple slices were dried in tray dryer for 8 hours at 50 °C and 60 °C
temperature. During osmotic dehydration, the maximum value of WL, WR, and SG was
found to be 62.9%, 52.0 %, and 11.2 % for 70 ºB sugar solution whereas the minimum
value was 52.9 %, 41.21 % and 10.50 % for 50 ºB solution respectively at 50°C after 8 h
of osmosis. The drying rate was faster at 60 °C drying temperature for all the sizes
osmotically treated apple slices as compared to 50°C drying temperature. This study
concludes that several factor such as solution concentration, temperature and immersion
time affected affects the osmotic dehydration characteristics. By using the combination of
osmotic dehydration followed by drying the losses during storage and handling can be
minimized and self-life of the apple fruits could be increased.
Ke yword s
Osmotic dehydration,
Water loss, Solid gain,
Weight reduction,
Drying
Accepted:
07 October 2018
Available Online:
10 November 2018
Article Info

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many techniques are used for preservation of
fruits and vegetable which includes drying,
dehydration, combination of both and
chemical treatment methods.
Osmotic dehydration is one of the potential
preservation techniques which produce high
quality products. It is the phenomenon of
partial removal of water from cellular material
such as fruits and vegetables. In food
processing industries the osmotic dehydration
is used for removal of moisture from fruit and
vegetable because it maintained the flavor,
color, texture and enhance self-life of final
product (Gouravsahu et al., 2017, Sutar, and
Gupta 2007; Pokharkar, 2001; Karthanos et
al., 1995). The several process parameter
influences the removal of water and addition
of solid, which includes solution
concentration, temperature, immersion time,
type of osmotic agent, sample to solution ratio
and agitation rate (Pokharkar, 2001). Osmotic
dehydration method is used as a pre-treatment
to several processes such as freeze drying,
vacuum drying, air drying and freezing,
(Dixon et al., 1976). During osmotic
dehydration process, water flows from
material to the osmotic solution, whereas
osmotic solute is transferred from solution to
the material. Osmotic dehydration is preferred
because it retains color, aroma, nutritional
constituents and flavor compound.
Drying is the process of removal of the
moisture or water to desired and predefined
level without affecting the loss of taste, flavor,
colour and nutrients (Singh and Kumar, 1984).
Drying of apple extent the self-life, increase
market value, easy to store as well as
decreases losses during storage. The
microorganisms responsible for food spoilage
are prevent by moisture removal. The removal
of moisture from fruits and vegetable can be
accomplished by drying or dehydration
methods. Drying and dehydration are carried
out to reduce the moisture content of the
product certain level so that growth of food
spoilage microorganism can be prevent and at
the same time high nutritive value is
maintained. Apples are rich in water content,
approximately 85% (w/w) and it is possible to
dehydrate and dried apples to enhance self-life
by maintaining good texture, high sugar
content and acidity and nutritional value. The
direct drying of apples by conventional tray
drying, cabinet or vacuum drying method
cause change in texture, colour, flavour and
nutrition loss in product. Pre-drying
treatments, such as partial dehydration of fruit
are suggested to improve the quality of dried
product. Hence combination of osmosis and
drying could be used to enhance the self-life
and quality of dried apple fruit without
affecting the colour, texture, flavour and
nutritive value. This study was carried out to
investigate the effect of osmotic solute,
solution concentration on dehydration of apple
slices at different temperature and immersion
time further followed by drying. The drying
characteristics of osmotically treated apple
were also studied.
Materials and Methods
Preparation of sample
The fresh Maharaji apple fruit were procured
from the market of Bapatla, Andhra Pradesh,
India. The initial moisture content was
approximately 85%. The apples were washed
in running tap water and drained under shed.
The washing was carried out to remove
adhering dust, dirt, impurities and surface
microorganism as well as to remove fungus,
insects and other pest from fruit surface. The
apple slices were prepared by chopping the
fruit with the help of knife or slicer in a
different thickness of 5-15mm. The initial
moisture content of raw fresh apples were
determined by oven drying method. The
osmotic solutions of different concentration of
sugar (50, 60 and 70 ºBrix) were prepared by

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dissolving required and calculated quantity of
sugar with distilled water. The apple slices of
different thickness (5-15mm) were weight
approximately 100 g for each treatment and
then immersed into the osmotic solution of
different concentration. The ratio of osmotic
solute to apple slices was 2:1 on weight by
weight basis were maintained.
Experimental procedure for osmotic
dehydration and drying
Osmotic dehydration is a suitable step to
reduce the water content of food material such
as fruit and vegetable. The prepared apple
slices of different thickness (5-20 mm) of
approximately 100 g weight were immersed in
sugar osmotic solution of different
concentration (50 and 70 ºBrix) and were
placed at different temperature of (30ºC and
50ºC). The osmotic dehydration was done in
apples by using water bath arrangement at
different temperature and different
concentration of sugar syrup. Each sample
was taken out from the container at hourly
interval up to 8 hour and were immediately
rinsed with water and placed in tissue paper to
remove excess solution and moisture from
surface. Finally the sample were weight and
moisture content, water loss and solute gain
for all the samples were measured at every
hours. After osmotic dehydration the treated
samples were weighed. The weighed apple
samples were spread in the form of thin layer
on aluminum trays. These aluminum trays
were put in tray dryer at a temperature of 60oC
for 8 h. The process flow chart of drying of
osmotically dehydrated apple slices is shown
in Figure 1.
Determination of moisture content of apple
fruit slices
Moisture content of raw and osmotically
dehydrated apple slices was measured by
using oven dry at 65⁰C for 24 h (Ranganna,
2000). Moisture content of samples is
measured based on drop in weigh from initial
weigh of sample. It is expressed in wet basis
and dry basis by equation 1 and 2 respectively.
Moisture content (% WB)
= 100...eq. (1)
Moisture content on (% DB)
= 100...eq. (2)
Determination osmotic dehydration
characteristics of apple slices
The osmotic dehydration is characterized by
solid gain, water loss and the weight reduction
from solute to solution during osmosis. The
net exchange in solute and water between
product and osmotic solution indicate the total
mass transfer during osmosis.
Water loss (WL)
The amount of water removed by fruit slices
during osmosis is known as the total water
loss
It is estimated on the basis of net weight loss
from apple slices during osmosis (Hawkes and
Flink, 1978).
WL =
Solid gain (SG)
The diffusion of solute and solution is takes
place which cause the adding of solid and
removal of water by apple slices. The loss of
water from apple slices leads to increase in
solid content during transfer phase. Solid gain
is the net amount of solid uptake by apple
slices during osmotic dehydration and it is
expressed on initial weight basis.
SG =

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Weight reduction (WR)
Weight reduction is the total exchange of solid
and liquid from the sample during osmotic
dehydration and will affect the final weight of
the sample.
It is determine by the following formula
WR=
or
WR = WL – SG
Where,
WL = Water loss in percentage
SG = Solid gain in percentage
Wt = Mass of apple slices after time t, g
Xt= Water content as a fraction of mass of
apple slices at time t
Wi = Initial mass of apple slices, g
Xi= Water content as a fraction of initial mass
of apple slices.
Drying of osmotically dehydrated apple
slices
The fruit slices were dipped in the desired
syrup solution for 8 hours.
After that they were removed from the
solution and gently washed to remove the
adhering sugar syrup to the slice.
Osmotically dehydrated apple slices were
dried in dryer at different temperature (50and
60ºC) and the moisture content and reduction
in weight were recorded at hourly interval.
Results and Discussion
Effect of osmotic solutes concentrations on
osmotic dehydration of apple slices
The solution concentrations are the important
factors which affect SG, WR and WR by
apple slices. WL, WR and SG by the apple
slices with different sugar solution
concentration of (50 and 70 0Brix) with time
are shown in the Figure 2. It is observed from
the Figure 2 that WL, WR and SG by apple
slices were increases with increase in sugar
concentration with respective immersion time.
The maximum value of WL, WR and SG was
found to be 62.9%, 52.0 %, and 11.2 % for 70
ºB solute concentration whereas the minimum
value of was reported to 52.9 %, 41.21 % and
10.50 % for 50ºB solute concentration
respectively after 8 hour of immersion time.
With increase in immersion time during
osmosis at same sugar concentration the WL,
WR and SG were also increases this indicate
that immersion time have significant effect on
osmotic process. For first 1 hour, there was
not much difference in WL, WR and SG but
after 1 hour, the WR was found to increase
more rapidly in slices dipped in 70°B solution
as compared to those dipped in 50ºB.
Effects of immersion temperature on
osmotic dehydration of apple slices
The effect of temperature change on osmotic
dehydration is shown in Figure 3. From the
data obtained by study, it was found that
moisture content decreases as temperature of
solution increases with given osmotic time for
similar concentration. This conclude that the
change in temperature affect the mass transfer
rate. As the temperature increases the SG as
well as WL also increases for given osmotic
time and solution. The maximum WL and WR
was found to 38.07% and 29.1 % at 50 °C
whereas lowest value was 28.5 % and 15.5 %
respectively at 30 °C after 8 hour of osmotic
dehydration of apple slices.

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Fig.1 Process flowchart of drying of osmotically dehydrated apple slices
Raw Apple
Washing and cleaning
Slicing (5-15 mm)
Osmotic dehydration for 8 h
at different temperature
Drying
Quality evaluation
Fig.2 Effect of solute concentration on osmotic dehydration of apple

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Fig.3 Effect of process temperature on osmotic dehydration of apple
Fig.4 Effect of slice thickness on weight reduction during osmotic dehydration of apple

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Fig.5 Effect of slice thickness on solid gain during osmotic dehydration of apple
Fig.6 Effect of slice thickness on water loss during osmotic dehydration of apple

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Fig.7 Effect of temperature on moisture content of apple
The value of SG decreases as WR and WL
increases. Highest value was SG was 12.97%
at 30 °C and lowest value was 8.56 % at 50
°C. From the observation it was concluded
that, the osmotic dehydration is the two way
mass transfer processes and it is affected by
processed temperature and time of immersion
(Farkas and Lazar, 1969; Hope and Vital,
1972; Beristain et al., 1990 and Alam et al.,
(2013)). Temperature can be one of the
advantageous factors to complete osmotic
dehydration rapidly but same time higher
temperature affects colour and flavor of
product.
Effect of slices thickness on osmotic
dehydration characteristics of apple slices
The effect of slice thickness on WR, WL and
SG is shown in Figure 4, 5 and 6 respectively.
The WR was much faster in case of 1.6mm
slice and this trend was followed by 5 mm
thickness apple fruit slices. It was also
observed that slice with more thickness,
greatly reduced the SG, WR and WL. All
three parameters were least when 18 mm
slices were used for osmotic dehydration. It
was also observed that the dehydration rate
decreased with the increase in slice thickness.
Drying of osmotic dehydrated apple slices
The osmotically dehydrated apple slices for 8
h in sugar solution of 70° B at temperature of
50 °C were dried in tray dryer at 600C and 50
°C temperature for 8 h. The moisture content
was measured at different drying time
intervals and data were analyzed. The effect
of drying temperature on moisture content is
shown in Figure 7. It was observed that
drying rate was faster in case of 60 ̊C as
compare to 50 ̊C and after 5 h of drying the
moisture content is almost constant for both
60 ̊C and 50 ̊C temperature. The weight
reduction was more rapid in first 4 h after that
drying rate gradually decreased and almost
reached constant.
The drying of osmotic dehydrated apple slices
was done at different temperature of 50 °C
and 60°C using cabinet dryer for 8 h. Initially
the apple slices were dehydrated osmotically

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for 8 h at 30 °C and 50 °C by different
concentrations of sugar solution. The
osmotically treated apple slices were dried in
tray dryer upto the moisture level of 14%.
During osmotic dehydration the maximum
value of WL, WR and SG was found to be
62.9%, 52.0 %, and 11.2 % for 70 ºB with 1.6
mm thick apple slice whereas the minimum
value was 52.9 %, 41.21 % and 10.50 % for
50ºB with 18mmrespectively after 8 h of
osmosis. The maximum WL and WR was
found to 38.07% and 29.1 % at 50 °C whereas
lowest value was 28.5 % and 15.5 % at 30 °C
respectively after 8 hour of osmotic
dehydration of apple slices. The value of SG
decreases as WR and WL increases. Highest
value was SG was reported to 12.97% at 30
°C and lowest value was 8.56 % at 50 °C.
From the study it was concluded that solution
concentration, sample immersion time and
solution temperature and thickness of slices
were the most prominent factors which affects
the solid gain, water loss and moisture loss
during osmotic dehydration of apple slices.
By processing of apple fruit post-harvest
losses during handling and storage can be
reduced, its self-life, product quality and
market value can be maximized.
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How to cite this article:
Vikas Paradkar and Gourav Sahu. 2018. Studies on Drying of Osmotically Dehydrated Apple
Slices. Int.J.Curr.Microbiol.App.Sci. 7(11): 633-642.
doi: https://doi.org/10.20546/ijcmas.2018.711.077