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International Journal of Refrigeration 88 (2018) 360–369
Contents lists available at ScienceDirect
International Journal of Refrigeration
journal homepage: www.elsevier.com/locate/ijrefrig
Effect of freezing rate and storage on the texture and quality
parameters of strawberry and green bean frozen in home type freezer
Merve Bulut
a
, Özgür Bayer
b
, Emrah Kırtıl
a
, Alev Bayındırlıa , ∗
a
Department of Food Engineering, Middle East Techni ca l University, 06800 Ankara, Turkey
b
Department of Mechanical Engineering, Middle East Techni ca l University, 06800 Ankara, Turkey
a r t i c l e i n f o
Article history:
Received 7 August 2017
Revised 20 December 2017
Accepted 2 February 2018
Available online 11 March 2018
Keywo rds:
Freezing time
Freezing rate
Cellular integrity
Phenolic compounds
Antioxidant activity
Ascorbic acid
a b s t r a c t
The focus of interest of this research was freezing preservation of green bean and strawberry in a home
type freezer. Freezing time and freezing rates were determined from the cooling curves obtained experi-
mentally. Home type freezing provided slow freezing rate. NMR Relaxometry measurements showed that
freezing did not keep the cellular integrity in strawberries and green beans due to structural damage
and extracellular ice formation. Packaged samples were stored at −27 °C for approximately 3 months.
Although total phenolic content, antioxidant activity and color changes of samples were not significant,
significant L-ascorbic acid losses were determined for 13 weeks’ frozen storage of strawberries and green
beans. Home freezing should be done at appropriate freezing rate to preserve the texture and the nutri-
tional content of fruits and vegetables.
© 2018 Elsevier Ltd and IIR. All rights reserved.
Incidence de la vitesse de congélation et de la durée d’entreposage sur les
paramètres de texture et de qualité de fraises et de haricots verts congelés dans
un dans un congélateur domestique
Mots-clés: Durée de congélation; Vitesse de congélation; Intégrité cellulaire; Composés phénoliques; Activité antioxydante; Acide ascorbique
1. Introduction
Freezing rate is the most important factor in freezing process to
prevent food tissue damage and drip loss in thawing. Faster freez-
ing results in small ice crystals and a better frozen food quality
( De Ancos et al., 2006; Hui et al., 2011; Alexandre et al., 2013 ).
Freezing time is defined as the time required to decrease the tem-
perature of the product from its initial value to a target value at
its thermal center or the time elapsed between the onset of the
freezing until whole product is frozen. Therefore, freezing rate can
be estimated as (i) the difference between initial and final value of
product temperature divided by freezing time ( °C h
−1
) or (ii) the
distance between the surface and thermal center divided by the
time elapsed to reach the thermal center temperature to −15 °C
∗Corresponding author.
E-mail address: alba@metu.edu.tr (A. Bayındırlı).
when the surface temperature is 0 °C (cm h
−1
). Freezing rate is
a generic term used to compare the freezing operation on a rel-
ative basis. Freezing rates differ in the range of 0.2 and 100 cm h
−1
in commercial applications. Slow freezing such as bulk freezing in
cold chambers, ranges between 0.2 and 0.5 cm h
−1
. Air blast and
contact plate freezers operate in quick freezing ranges (0.5–3 cm
h
−1
). Frozen storage temperature is also important and decrease in
storage temperature leads to increase in frozen food quality ( Sun,
2011 ).
Comprehensive information can be found in the literature about
the effects of freezing and storage temperature on the nutrient
content of fruits and vegetables ( Kyureghian et al., 2010 ). The
amount of data that directly compares fresh and frozen fruits and
vegetables is very limited with most of the data dating back at
least 20–30 years when some of the methodology for determina-
tion of vitamins and other nutritional compounds was also lim-
ited. The reports on nutrient retention during frozen storage may
https://doi.org/10.1016/j.ijrefrig.2018.02.030
0140-7007/© 2018 Elsevier Ltd and IIR. All rights reserved.
M. Bulut et al. / International Journal of Refrigeration 88 (2018) 360–369 361
Nomenclature
AA antioxidant activity [mg DPPH
•/g sample]
a
∗CIE color value for greenness to redness [-]
b
∗CIE color value for blueness to yellowness [-]
C
DPPH
•DPPH
•concentration [mg dm
−3
]
CIE Commission Internationale d’Eclairage
c
f specific heat of frozen food [J kg
−1 K
−1
]
c
u specific heat of unfrozen food [J kg
−1 K
−1
]
D decimal reduction time [week]
DPPH
•2,2-diphenyl-1-picrylhydrazyl
E
f freezing time shape factor [-]
h heat transfer coefficient [W m
−2 K
−1
]
h
fg latent heat of freezing per unit mass of food [J kg
−1
]
k thermal conductivity [W m
−1 K
−1
]
k
r first order reaction rate constant [week
−1
]
LDPE low density polyethylene
L
∗lightness [-]
L
c characteristic length [m]
R
1 temporal freezing rate with the units of °C h
−1
R
2 spatial freezing rate with the units of cm h
−1
r
2 coefficient of determination [-]
RA relative area [%]
T
a temperature of freezing or thawing medium [K]
T
fin final product sample temperature [K]
T
fm nominal mean freezing temperature in Pham’s
freezing time method [K]
T
ini initial food temperature [K]
T
2 spin-spin relaxation time (transverse relaxation
time) [ms]
t time [week]
t
f freezing time [s]
TPC total phenolic content [mg GAE/ 100 g fruit]
Y(t) overall signal for NMR relaxometry measurement [-]
Greek symbols
E total color change [-]
ρdensity [kg m
−3
]
also be contradictory. The most common physical changes during
food freezing are modification in cell volume, dislocation of wa-
ter, mechanical damage and freeze cracking, freezer burn, and ice.
recrystallization. Chemical changes are enzymatic reactions, pro-
tein denaturation, lipid oxidation and vitamin loss. The change in
solute concentration and decompartmentation of cell contents dur-
ing freezing may affect the rate of these reactions ( Sun, 2011 ).
Bioactive compounds in fruits are mainly vitamins A and C,
carotenoids, and phenolics. De Ancos et al. (20 0 0) found that freez-
ing process slightly affected total phenolics of raspberry during
freezing and long-term storage (1 year at −20 °C). Mullen et al.,
(2002) reported that the total flavonol content and antioxidant ca-
pacity of raspberries were not significantly different in the fresh
and frozen berries stored at −30 °C. Chaovanalikit and Wrolstad
(2004) determined 50% loss of total phenolics in cherries for 6
months of storage at −23 °C. According to the finding of Poiana
et al., (2010) , there was reduction in antioxidant capacity during
the frozen storage ( −18 °C) of different fruits. The reduction was
small during 4 months of storage and a significant reduction was
observed in the following months for sour cherries, sweet cher-
ries and strawberries. Frozen vegetables showed lower antioxidant
activity and remain unchanged during long term frozen storage
( Hunter and Fletcher, 2002; Rickman et al., 2007; Murcia et al.,
2009; Hui et al., 2011 ).
Water-soluble vitamins are lost at sub-freezing temperatures.
Storage temperature is more effective than freezing method on the
retention of ascorbic acid. After 90 days of storage, the decrease
in ascorbic acid content of frozen strawberry was 64.5, 10.7 and
8.9% at −12, −18 and −24 °C, respectively ( Sahari et al., 2004 ).
Bahçeci et al., (2005) showed that the ascorbic acid is very sen-
sitive to break down during frozen storage of green beans and
the retention of this quality parameter is guaranteed by blanching.
After 121 days storage, Gonçalves et al., (2011) found that the vita-
min C content of broccoli significantly decreased at storage tem-
peratures of −7, −15 and −25 °C. Favell (1998) determined that
ascorbic acid level in quick-frozen product was equal to or much
better than fresh product such as whole green bean. Baardseth et
al., (2010) found a higher level of ascorbic acid in blanched/frozen
green beans than raw green beans. The reason for lower content
in raw sample was attributed to oxidation of the ascorbic acid
prior to analysis causing higher level of dehydroascorbic acid in
raw sample. The initial data was taken at 1st day after freezing.
Martins and Silva (2003) reported a significant loss of ascorbic acid
while a negligible loss of dehydroascorbic acid was detected dur-
ing 250-day storage of green beans at −7 °C, −15 °C and −30 °C.
The average retention of ascorbic acid is expressed relatively to an
initial, average value of 1st week of the frozen storage.
During freezing, an expansion occurs with the formation of ice
crystals causing cell wall rupture. Therefore, the texture of frozen
fruits and vegetables is usually softer after thawing when com-
pared to unfrozen product. Slow freezing rates produced consid-
erable softening due to extracellular ice formation ( Lisiewska and
Kmiecik, 20 0 0; Cheftel et al., 20 0 0; Hui, 20 06; Sinha et al., 2012 ).
Roy et al., (2001) suggested high temperature short time blanch-
ing before of rapid freezing for a superior textural quality in frozen
products. In the study by Ferreira et al., (2006) , the favorable effect
of rapid freezing was proved for texture of green beans. The high-
est mechanical resistance of tissues of green beans was detected
in samples frozen at the fastest rate (5.4 °C min
−1
) and air-thawed
at the slowest rate (0.35 °C min
−1
). Slow freezing (1.5 °C min
−1
)
increases mechanical damage. The positive effect of rapid freezing
may be lost if the conditions during frozen storage are insufficient.
In recent years, a diverse range of studies have been reported
to expedite the freezing process and to form small and evenly dis-
tributed ice crystals throughout a frozen food product with the
effects of freezing and frozen storage on quality characteristics
of foods. Besides, the focus of interest related to home freezing
preservation of fruits and vegetables is also increasing (Poiana et
al.; 2010, Bekta ¸s et al., 2010 ). The aim of this study is to deter-
mine the freezing times and rates and also to analyze some qual-
ity parameters of green bean and strawberry during storage in a
home type freezer. The analyzed quality parameters are cell in-
tegrity, color, l -ascorbic acid content, total phenolic content and
antioxidant activity.
2. Materials and methods
2.1. Materials
Strawberry (Festival variety) and green bean (Sırık variety) were
used in this study. The stems of the strawberries were picked.
Green beans were cut (2.5 cm in length), blanched at 80–85 °C for
2 min and cooled by flowing tap water. Low density polyethylene
(LDPE) bags (24 cm x 28 cm) were used for packaging of approx-
imately 500 g of strawberries. 250 g of green beans were pack-
aged in LDPE bags (20 cm ×22 cm). Packaged samples were frozen
in home type freezer (Arçelik 2572D). The freezer was static type
(without fan) freezer with 0.366 m
3 storage capacity.
362 M. Bulut et al. / International Journal of Refrigeration 88 (2018) 360–369
Fig. 1. Schematic of experimental set up (a) and packaged frozen strawberry and green bean samples (b). (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
2.2. Measurement of the sample temperature and estimation of
freezing time and freezing rate
The time-temperature measurements were done in an ex-
perimental set up composed of well controlled freezer, data
acquisition system, precise thermocouples and computer as shown
in Fig. 1 . Temperature values were recorded by using 34,972 A LXI
data acquisition / data logger switch unit device (Agilent Technolo-
gies, Inc. Malaysia) with 60 2-wire channels. 30 gage (correspond-
ing to 0.025 cm in diameter) T type thermocouples were used in
measurements. Thermocouples were inserted into the unpackaged
whole samples and thermocouple junctions were located at the
vicinity of the surface and thermal center (geometric center). For
packaged samples, same measurements were done for strawber-
ries and green beans in the middle of bags. The packaged sam-
ples were hold as one layer on semi rigid plastic freezer baskets
( Fig. 1 ). The freezer temperature was also measured by using ther-
mocouples located into freezers at different points, some of which
were positioned near to the samples. Cooling curves were obtained
during freezing of unpackaged and packaged samples at freezer
temperature sets of −23 ±1.1 and −27 ±0.3 °C by using recorded
temperature data for the estimation of freezing times and rates.
Both temporal and spatial freezing rates were estimated. Tempo ral
freezing rate ( R
1
) is defined as the difference between initial and
final center temperature of product divided by freezing time ( °C
h
−1
), whereas spatial freezing rate ( R
2
) is calculated as the distance
between the surface and the sample center divided by the time
elapsed between the surface reaching 0 °C and the center temper-
ature reaching −15 °C (cm h
−1
).
Using the thermophysical properties of the samples presented
in Table 1 and assuming one dimensional heat transfer, the freez-
ing times determined experimentally in this study were compared
with relations available in literature such as empirical relation of
Pham (2014) .
t
f
=
ρL
c
E
f
h
x
c
u
T
ini
−T
fm
T
ini
+ T
fm
2
−T
a
+
h
fg
+ c
f
T
fm
−T
fin
T
fm
−T
a
x
1 +
h L
c
/k
2
(1)
where each term was given in nomenclature with its proper unit.
Since the freezer was static and natural convection was the domi-
nant heat transfer mechanism, h was assumed to be 6.5 Wm
−2
K
−1
as indicated in Pham (2014) .
In this aspect, freezing time related to R
1
, was defined as the
time passing in between the states where temperature values of
the sample center were 0 and −15 °C. Pham’s equation is modified
according to the freezing time definition presented in this study
Tabl e 1
Thermophysical properties of strawberry and green bean ( ASHRAE Handbook, 2014 ).
Density
(kg m
−3
)
Specific heat above
freezing (kJ kg
−1
K
−1
)
Specific Heat below
freezing (kJ kg
−1
K
−1
)
Thermal conductivity
(Wm
−1
K
−1
)
Latent heat of fusion
(kJ kg
−1
)
Strawberry 800 4.00 1.84 1.10 306
Green bean 750 3.99 1.8 5 0.40 302
M. Bulut et al. / International Journal of Refrigeration 88 (2018) 360–369 363
and the first term in the first parenthesis was omitted since it rep-
resents the precooling time.
2.3. Frozen storage
Packaged samples were stored at −27 °C after freezing. Samples
were left to thaw at room temperature (natural thawing) for about
1 h for quality control experiments. Three or four replicates were
used per treatment. Each measurement was duplicated.
2.4. Determination of total phenolic content (TPC) and antioxidant
activity (AA)
Fruit and vegetable samples were blended. 2 g of blended sam-
ple was extracted with 20 cm
3 of ethanol: water mixture (50:50
v/v, MERCK, Germany) for TPC estimation and with methanol: wa-
ter mixture (70:30 v/v, MERCK, Germany) for AA analysis. Sam-
ples were homogenized using a homogenizer (IKA T-18 Ultra-
Turrax Homogenizer) at 14,0 0 0 min
−1 for 1 minute and allowed
to stand for 45 min at 4 °C for complete solvent extraction. Fol-
lowing the extraction, samples were centrifuged at 15 ,0 0 0 rpm and
4 °C (Sigma 2–16 PK, Germany) for 10 min. TPC s of the sample ex-
tracts were determined by Folin–Ciocalteu method ( Singleton et al.,
1999 ). Results were expressed in terms of gallic acid equivalent
(mg of GAE/100 g of fresh weight).
AA was evaluated by using 2,2-diphenyl-1-picrylhydrazyl
(DPPH
•, SIGMA, Germany) radical according to the method of
Brand-Williams et al., (1995) . 2 cm
3 of DPPH
•radical solution
(25 mg DPPH
•/ dm
3 methanol) and 0.1 cm
3 of methanol were
mixed, and absorption at 517 nm (SHIMADZU UV–1700 spectropho-
tometer, Japan) was measured to find the amount of DPPH
•at the
beginning of reaction. Methanol was used as blank. The DPPH
•
concentration in the reaction medium was determined from the
calibration curve. AA was calculated as:
AA
mg DPP H
•
g sample =
C
DP P H
•, t=0 min
−C
DP P H
•, t=60 min
weight of sample
×(
volume of extract
)
×(
dilution rate
) (2)
2.5. Determination of L -ascorbic acid content
4.5% Metaphosphoric acid (Merck, Germany) was used for
extraction by homogenization (IKA T-18 Ultra-Turrax Homoge-
nizer) at 14, 0 0 0/min for 1 min. l -ascorbic acid analysis was per-
formed using a Thermo Scientific Finnigan Surveyor HPLC with an
Auto Sampler Plus, LC Pump Plus and UV-VIS Plus Detector (San
Diego, CA). Reverse-phase separation was attained using a VAR-
IAN Chromsper5 C18 HPLC column (150 mm ×4.6 mm I.D., 5 μm;
Merck KGaA, Darmstadt, Germany). A standard curve was prepared
by basing on the peak areas obtained for different concentrations
of the standard solution. The results were expressed as mg / 100 g
fruit.
2.6. Color measurement
Surface color measurements were performed according to CIE
L
∗a
∗b
∗color notation system by using color measuring device
DATACOLOR 110 spectrophotometer (Lawrenceville, NJ, USA). L
∗in-
dicates the luminescence (lightness), a
∗means the color axis from
green to red, and b
∗shows the color axis from blue to yellow. Color
was measured when the samples were partially thawed (approxi-
mately 15–20 min) to prevent drip loss and clear appearance. Total
color change ( E ) was estimated by using BaCl
2 as reference ( L
0
∗,
a
0
∗, b
0
∗) and calculated as:
E =
(
L
∗−L
0
∗)
2
+
(
a
∗−a
0
∗)
2
+
b
∗−b
0
∗2
1 / 2
(3)
2.7. NMR relaxometry measurements
NMR relaxometry experiments were based on measurement
of spin-spin relaxation times ( T
2
). The multi-exponential decay
of transversal magnetization signal was recorded and the data
were inverse Laplace transformed to distinguish components with
different magnetic decay behavior. Different proton components
that constitute the sample, display different magnetic relaxation
rates. For live tissues, the proton pools are commonly associated
with cellular compartments ( Ersus et al., 2010 ). T
2 measurements
were conducted for strawberries and green beans before and dur-
ing freezing (pre-freezing stage, freezing stage, sub-freezing stage,
post-freezing stage) at all freezing rates. Approximately 1 cm slice
from the center of the samples was cut to fit into the NMR tube. T
2
experiments were performed using a 0.367 T (15.635 MHz) system
(NMR Mobile Systems Inc., Russia) having a 16 mm radio frequency
coil. T
2
was measured using a Carr–Purcell–Meiboom–Gill pulse se-
quence with an echo time of 2.0 ms, 2500 echoes, and 8 scans.
Relaxation spectra was obtained by applying Non-Negative Least
Square analysis to the T
2 decay curves using the mathematical
analysis software of MATLAB. For the purpose of one dimensional-
non negative least square analysis, PROSPA (Magritek Inc., Welling-
ton, New Zealand) was used. Recycle delay was adjusted to 3 s for
the analysis. In 1H NMR Relaxometry measurements, all 1H pro-
tons of the sample are converted to an excited state via a mag-
netic field. Thus the signal recorded, in theory, belongs to whole
1H proton population ( Hashemi et al., 2010 ). However, in prac-
tice, by manipulation of NMR parameters, signal could be restricted
to more specific sample components. This eases the assignment
of spectrum peaks to particular compartments. The mathematical
transformation involves the fitting of T
2 relaxation data into the
signal equation ( Eq. (4) ) as a sum of Gaussian functions for the
solid (S)part and the sum of exponential functions for the liquid
(L) part ( Mariette, 2009 ).
Y
(
t
)
=
S
i
exp −t
T
2
2
+
L
i
exp −t
T
2
2
(4)
where S
i
and L
i
denote the signal intensity coming from each com-
partment, Y(t) is the overall signal and, t and T
2 are the echo time
and the spin-spin relaxation time of the individual compartments,
respectively.
2.8. Statistical analysis
Results were processed by one-way analysis of variance
(ANOVA). The statistical significance of results was determined by
using the package program MINITAB (Version 16.1.1, Minitab Inc.,
Coventry, United Kingdom). Significant difference between means
were tested using Tukey’s test with a probability level fixed at
p < 0.05. Differences at p < 0.05 were considered to be significant.
3. Results and discussion
3.1. Freezing time and freezing rate
A typical cooling curve was presented in Fig. 2 . Super cooling
was detected for strawberry and green bean samples as cooling
down to a temperature below the freezing point of the sam-
ple without formation of ice. Once the crystal embryos exceed
the critical radius for nucleation, the system nucleates at a point
lower than freezing point. The temperature then increased to its
freezing point. Freezing point depends particularly on the solid
content of the fruits and vegetables. For example, Haiying et al.,
(2007) showed that the degrees of super cooling were small in
the vegetables because of the existence of many solid particles.
364 M. Bulut et al. / International Journal of Refrigeration 88 (2018) 360–369
Fig. 2. Cooling curve of packaged strawberries frozen at - 27 °C (three replications).
Characteristic parameters and also R
1 and R
2 of packaged straw-
berries and green beans were presented in Table 2 at different
freezer temperatures. R
2
of unpackaged strawberries were 0.75 and
1.0 2 cm h
−1
at −23 and −27 °C, respectively. For unpackaged green
bean, R
2 were found as 0.36 and 0.57 cm h
−1 at −23 and −27 °C,
respectively. The calculated values of freezing rates showed slow
freezing rate in home type freezer. Since the two freezing rate
definitions given in this study and also used in literature are re-
ferring to different physical concepts. But still a linear correlation
was found between R
1 and R
2 as given in Table 2 . Packaging also
decreased the freezing rates ( Fig. 3 ). Therefore, the rate of heat
transfer through package should be considered during the design
of the home type freezing systems for packaged frozen fruit and
vegetables. Experimental freezing time values, especially for −27 °C
freezer air temperature cases, showed consistency with the cal-
culated ones from Pham’s equation ( Pham 2014 ) as presented in
Table 2 . It should be kept in mind that Pham equation has also
10% uncertainty.
3.2. Cell integrity
The fastest relaxing portion of the solid signal is attributed to
non-exchangeable protons from solid fat, ice, protein and polysac-
charides (such as the ones in non-exchangeable CH bonds of
solids). These do not contribute to the signal; owing to the mea-
surement delay limitation of low-field NMR systems. What remains
is the signal coming from liquids and the exchangeable protons in
solids (like the ones in ∼OH groups). The exponential liquid sig-
nal is attributed to liquid water, fat and exchangeable protons from
solids. The exchangeable solid protons have very fast proton re-
laxations compared to the liquids ( Mariette, 2009 ). With the set
echo delay of 10 0 0 μs, the signal was assured to only come from
Tabl e 2
Characteristic parameters, freezing times and freezing rates of packaged strawberries and green beans at different freezer temperatures.
Strawberry
Freezer
temperature
( °C)
Experiment
no
Initial sample
temperature
( °C)
t
0
°C and t-
15
°C
(min)
Freezing time
measured (h)
Freezing time
from Eq. (1 ) (h)
Equivalent
radius
∗
(cm)
Temporal
freezing rate R
1
( °C/h)
Spatial freezing
rate R
2
(cm/h)
−23 °C 1 18. 49 33.6 6 and
262.00
3.80 2.77 1.5 8 3.95 0.41
2 18. 77 27.33 and
211.66
3.08 2.85 1.6 0 4.87 0.52
3 18. 72 24.83 and
186.66
2.70 2.77 1.5 8 5.56 0.59
R
1ave
= 4.79 °C/h R
2ave
= 0.51 cm/h
−27 °C 1 16. 51 19. 66 and
157.83
2.30 2.28 1.5 5 6.52 0.67
2 17.0 5 18.16 and
147.33
2.15 2.13 1.45 6.98 0.67
3 15.64 18. 0 0 and
149 .6 6
2.19 2.29 1.5 7 6.85 0.72
R
1ave
= 6.79 °C/h R
2ave
= 0.69 cm/h
R
2
= 0.103 R
1
( r
2
= 0.95)
Green bean
Freezer
temperature
( °C)
Experiment
no
Initial sample
temperature
( °C)
t
0
°C and t-
15
°C
(min)
Freezing time
measured (h)
Freezing time
from Eq. (1 ) (h)
Characteristic
length
∗∗
(cm)
Temporal
freezing rate R
1
( °C/h)
Spatial freezing
rate R
2
(cm/h)
−23 °C 1 15.85 11. 0 0 and
144.50
2.22 1.9 4 0.37 6.76 0.17
2 17.2 8 16. 66 and
146.66
2.16 1.8 5 0.36 6.95 0.17
3 14.14 7.5 0 and 84.00 1. 27 1. 33 0.27 11. 8 2 0.22
4 15.36 7.6 6 and 69.66 1. 0 3 1. 8 6 0.42 14. 57 0.35
R
1ave
= 0.23 °C/h R
2ave
= 0.23 cm/h
−27 °C 1 14. 42 5.83 and 60.00 0.90 1.5 6 0.42 16. 67 0.47
2 14. 51 10 .0 0 and 90.83 1.3 4 1.35 0.31 11.2 0 0.24
3 14. 37 8.00 and 78.33 1.17 1.19 0.30 12.8 2 0.26
R
1ave
= 0.23 °C/h R
2ave
= 0.32 cm/h
R
2
= 0.024 R
1
( r
2
= 0.83)
∗Calculated from sphericity ( Sahin and Sumnu, 2006 ).
∗∗Characteristic length was half thickness of the vegetable ( Sahin and Sumnu, 2006 ).
M. Bulut et al. / International Journal of Refrigeration 88 (2018) 360–369 365
Fig. 3. Spatial freezing rates (cm h
−1
) of strawberries and green beans at different
freezer temperatures ( °C).
Fig. 4. A representative one dimensional NMR spectrum of a fresh green bean sam-
ple. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
liquid components in the sample. The liquid signal, for our case,
was dominated by water while ice will not generate any NMR sig-
nal. This way, freezing could be monitored as a decrease in NMR
signal or extinction of the particular peak in NMR Spectrum that
refers to the frozen compartment. In addition to freezing, changes
in peak areas and locations could as well give important infor-
mation about the microstructural changes during freezing, such
as disruption of cellular barriers accompanied by release of sub-
cellular water which is the dominant reason in quality degrada-
tion in frozen fruits and vegetables ( Kirtil et al., 2014 ). T
2 relax-
ometry was utilized in literature to find out the effect of freez-
ing, drying, ripening and high pressure on the physiological change
at sub-cellular level in apple, avocado, tomato, strawberry, banana,
and onion ( Ersus et al., 2010; Marigheto et al., 20 05, 20 09; Raffo
et al., 2005 ). A representative spectrum of fresh green bean sam-
ple was shown in Fig. 4 . Relaxation spectra differ with changes
in food systems. This divergence remarks the changes associated
with protons. For example, new proton pools may be revealed
due to proton exchange among the subcellular regions of the food,
change in quantity of water and physiological events taking place
in food. These proton pools are associated with water compart-
ments inside viable cells which were displayed by a total of 3
peaks in relaxation spectra. These peaks refer to different states of
water entrapped inside various subcellular compartments ( Belton
and Capozzi, 2011 ). Thus, peak assignment is the initial, most im-
portant step in NMR Relaxation spectrum analysis. A correct peak
assignment is essential to ensure accurate data interpretation.
T
2 and the percent relative areas ( RA ) of each peak in the re-
laxation spectrum were presented in Tables 3 and 4 . Peak area is
a measure of the signal intensity of that particular water compart-
ment which provides an estimate of the amount of water in that
environment. RA , on the other hand, could also be considered as
the contribution of that cellular component to the whole signal
( Oztop et al., 2010; Kirtil et al., 2014 ). The T
2
values of the 1st peak
ranges between 4.6–7.1 ms. This peak with a small RA and short
T
2 was assigned to the protons that connect with the hard con-
stituents of the cell wall. The water may be present in little pores
inside the cell wall. The surrounding matrix and water-holding
sites clutch this water leading to close proximity between water
and neighboring molecules and restricted water mobility. The re-
duction of intermolecular voids, increases the amount of interac-
tions between molecules which increases proton exchange leading
to decreased T
2 times. The 3rd peak with the longest T
2 times and
highest RA s was associated with water inside of vacuoles. Vacuoles,
being the water reservoir of plant cells, embody 50–80% of cellu-
lar water which explains the high RA s. Vacuole water is also not
highly concentrated with water binding molecules like sugars and
proteins, hence is in a very mobile state ( Zhang and McCarthy,
2013 ), that caused it to display peaks between 360 and 590 ms
( Table 3 ). Cytoplasm water, on the other hand, is more densely
populated with macromolecules which give cytosol matrix a more
gel-like structure. Cytoplasm water is expected to exhibit lower re-
laxation times than vacuoles ( Raffo et al., 2005 ). Therefore, Peak 2
with intermediate RA s and T
2
s were attributed to cytosol water.
For all freezing rates, three peaks were initially visible in the
relaxation spectra. However, Peak 1, associated with the cell wall,
disappeared after freeze-thawing. The decrease in the number of
peaks points out to a disruption of the original cell compartmen-
talization for all samples ( Ersus et al., 2010 ). The small amount of
water located inside capillaries seem to be liberated by the dam-
aged induced by freeze-thawing. In frozen-thawed green beans,
Peak 1 disappeared and T
2 value of peak 2 slightly decreased for
all freezing rates. RA of peak 2 slightly increased in freezing rate
at −23 °C. The increase in RA may indicate the growth in size for
that cellular compartment ( Kirtil et al., 2014 ). The increase in RA of
peak 2 in slowest freezing rate indicates an accumulation of water
within the cytoplasm which was probably introduced by the re-
lease of inner cell wall water. Besides, water soluble solids inside
the cell wall and membrane could be dissolved inside the cyto-
plasm leading to decreased T
2 times as was the case for all spec-
imen. Overall, cell wall appears to be damaged for both strawber-
ries and green beans and for both freezing rates. T
2
value of peak 3
decreased for all treatments. However, RAs were not different from
fresh sample. Still, out of all specimen, NMR relaxation spectrum
of green beans frozen-thawed at higher rates displayed the closest
similarity to fresh ones. However, strawberries displayed a much
higher decrease in T
2 times for peak 2 and peak 3, which could be
associated with a more complete dissociation of cellular barriers.
These results show that during freeze-thawing green beans pre-
served cell integrity better than strawberries. The decrease in both
T
2 value and RA in peaks 2 and 3 could also be an indication of
drip loss of water after thawing. The decreased moisture content
of the samples could be reflected to NMR relaxometry data as a
decrease in T
2 times and signal intensities. The fact that strawber-
ries are known to exhibit more drip loss compared to green beans
is supportive of NMR results that display a much more prominent
decrease in T
2 times and RA s in strawberries.
Whether cell organelles preserve their initial fresh texture af-
ter thawing or not, depends on the formation and distribution
of ice crystals in cellular tissue during freezing stage. A slower
366 M. Bulut et al. / International Journal of Refrigeration 88 (2018) 360–369
Tabl e 3
Mean spin-spin relaxation times ( T
2
) at freezing stages of strawberries and green beans.
Strawberry freezer temperature Green bean freezer temperature
−23 °C −27 °C −23 °C −27 °C
Sample
temperature ( °C) T
2
(ms)
Sample
temperature ( °C) T
2
(ms)
Sample
temperature ( °C) T
2
(ms)
Sample
temperature ( °C) T
2
(ms)
Fresh (Control) 22 581.78 22 553.29 22 332.12 22 367.7
Pre-freezing 2.9 466.33 5 535.86 2.5 345.96 3.5 391.17
Freezing −1.5 228.05 −1.6 476. 88 −3.8 206.48 −3.8 320.12
Subfreezing −12.5 123.31 −15 117.77 −12 64.29 −13 99.9
Frozen −23.2 85.22 −27 75.63 −23.2 62.33 −27 85.63
Thawed 18 214.85 18 220.78 20 284.91 20 204.29
Tabl e 4
Mean spin–spin relaxation times ( T
2
) and percent relative areas ( RA ) of strawberry
and green bean samples.
Freezer
temperature ( °C)
Fresh-cut Frozen-thawed (20 °C)
T
2
(ms) RA (%) T
2
(ms) RA (%)
Strawberry
−23 Peak 1 4.6 0.05
Peak 2 96 6.6 13 1. 2 3
Peak 3 590 90.08 210 92.93
−27 Peak 1 5.7 0.37
Peak 2 62 5.49 7. 6 3.2
Peak 3 550 89.18 210 90.2
Green bean
−23 Peak 1 6.1 3.3
Peak 2 96 12. 81 72 17.2 7
Peak
3 360 77.2 310 77.05
−27 Peak 1 7.1 2.42
Peak 2 62 13. 65 43 13. 89
Peak 3 380 77.34 210 78.47
freezing rate favors the formation of a lower number of ice nuclei
accompanied by a larger crystal growth phase, and it is the oppo-
site for a faster freezing rate. Formation of larger ice crystals during
slow freezing rate, most likely resulted in more damage to cellular
structure. However, freezing rate at −27 °C displayed higher RA val-
ues (peak 2 in Fig. 4 ) and lower decrease in T
2 times, indicating a
lower disruption in cellular integrity. There were slight increases
in RA s of peak 3 in Fig. 4 which suggests an increase in extracellu-
lar water content coming from disrupted cellular compartments.
Though water moves out from that cellular compartment (cyto-
plasm or vacuole), it is still present in extracellular space, and a
signal is still being received from the compartments. The extracel-
lular water signal could have overlapped with the vacuole signal,
due to both being highly mobile which explains the slight increase
in RA .
Cell turgor pressure is associated with the quantity of water
in vacuole and textural characteristics of the food suffer a change
due to loss of turgor pressure ( Waldron et al., 1997 ). However, cell
wall component is the most important parameter in assessing the
cell integrity. As a result of slow freezing, the degree of structural
changes such as membrane disruption, shrinkage of the cell and
loss of water holding capacity were higher in strawberries if com-
pared with that of green beans. NMR relaxometry provided a non-
destructive analysis of the effect of freezing-thawing on food cell
components. Therefore, NMR analysis could be used for further
studies to see the effect of freezing rates on cell integrity during
the design of home type freezers.
3.3. Change of quality parameters during frozen storage at −27 °C
Ascorbic acid is one of the most heat sensitive components
in fruits and vegetables and widely used as a nutritional quality
indicator during fruit and vegetable processing. Ascorbic acid is
readily oxidized by ascorbic acid oxidase or under strong oxida-
tion conditions. The reversible equilibrium occurs between ascor-
bic acid and dehydroascorbic acid in which dehydroascorbic acid
irreversibly oxidizes to diketogluconic acid. Ascorbic acid content
of the fresh strawberry was measured as 38 mg/100 g in this study.
The ascorbic acid contents were 23 and 16 mg/l00 g for fresh
and blanched green beans, respectively. Strawberries have a rel-
atively high content of vitamin C, which is around 65–84 mg/
100 g according to Turkish Food Composition Database (2014) . This
range for green beans is 12–18 mg/100 g. The variation in vita-
min content of fresh fruits and vegetables is very large in the
literature. Vitamin levels depend on the plant cultivar, growing
conditions, maturity of the edible portion, post-harvest handling
and storage conditions. Ascorbic acid retention for strawberry and
green bean are presented in Table 5 . Blanching of green bean re-
duced the ascorbic acid content of about 30%. The losses dur-
ing hot water blanching may be assumed to be due to leaching
and thermal degradation of ascorbic acid to dehydroascorbic acid
and further oxidation which was also stated in a study by Tosun
and Yücecan (2008) . The ascorbic acid level was higher in both
frozen green bean and strawberry at the end of 1st week. The
reason for the higher ascorbic acid level in frozen sample was
attributed to the easier extraction of ascorbic acid after freeze-
thawing. Since, ice crystals formed as a result of freezing disrupted
the cell wall thereby improving the extraction of ascorbic acid. At
the end of the frozen storage, the ascorbic acid content of both
strawberry and green bean maintained a level above that of fresh
samples. The same pattern was observed in the studies of Favell
(1998) who found that level of ascorbic acid in quick-frozen prod-
uct was equal to or much better than fresh product. The frozen
green beans possessed a mean L-ascorbic acid level significantly
greater than both the fresh and fresh-stored samples but not for
strawberries according the study of Li et al., (2017) . The deteri-
oration rate of ascorbic acid during food processing follows first
order kinetics. Ascorbic acid losses were approximately 42 and
27% for 13 weeks frozen storage of strawberry and green bean,
respectively. Ascorbic acid in green beans showed high stabil-
ity ( k
r
= 0.024 week
−1
, D = 95 week, r
2
= 0.97) compared to that of
strawberry ( k
r
= 0.049 week
−1
, D = 47 week, r
2
= 0.98). The amount
of ascorbic acid in strawberry and green bean was approximately
two fold of that of the fresh samples in the 1st week. According
to the study of Serpen et al., (2007) , the degradation of ascorbic
acid is related to the balance of oxidation and reduction capacities
in peas during frozen storage. It was reported that blanching has a
significant effect on regeneration of dehydroascorbic acid to ascor-
bic acid and the oxidation of dehydroascorbic acid to diketoglu-
conic acid. However, it does not have a leading effect on oxidation
of ascorbic acid to dehydroascorbic acid. Also, blanching signifi-
cantly minimized the oxidation rate of dehydroascorbic acid into
diketogluconic acid. Hence, retention of total vitamin C in peas in-
creased during frozen storage. This increase was supposed to be
M. Bulut et al. / International Journal of Refrigeration 88 (2018) 360–369 367
Tabl e 5
Quality parameters of strawberry and green bean samples during frozen storage at −27 °C
∗.
Strawberry Green bean
Storage time
(weeks)
Ascorbic acid
(mg/100 g fruit)
TPC (mg GAE/ 100 g
fruit)
AA ×10
4
(g DPPH
·/ g
fruit)
Ascorbic acid
(mg/100 g
vegetable)
TPC (mg GAE/ 100 g
vegetable)
AA ×10
4
(g DPPH
·/
g vegetable)
Fresh 38.78 ±6.47e 210.14 ±12.40 abc 4.77 ±0.09ab 23.57 ±1.00e 69.16 ±12.76a 1.63 ±0.01a
1 82.49 ±3.35a 213.15 ±6.53abc 4.30 ±0.25b 36.63 ±0.64a 67.95 ±1.5 4a 1. 85 ±0.20a
2 187.92 ±10.81bc 5.36 ±0.07a 63.31 ±1.52a 1.83 ±0.15a
3 77.90 ±3.56a 214.07 ±7.10abc 4.38 ±0.19b 34.48 ±0.70ab 74.45 ±4.19a 1.89 ±0.04a
4 183.25 ±8.26c 5.20 ±0.25a 63.15 ±3.54a 1. 49 ±0.22a
5 71.64 ±0.93ab 32.97 ±1.0 4ab
6 227.05 ±5.84a 4.27 ±0.19b 63.98 ±1.78a 1.5 0 ±0.23a
7 61.61 ±0.61bc 32.49 ±2.14b
8 208.10 ±12.10abc 5.34 ±0.26a 71.67 ±5.01a 1.5 9 ±0.10a
9 55.41 ±0.75cd 31.39 ±1.51bc
10 208.57 ±17.44abc 4.87 ±0.30ab 76.70 ±6.39a 1. 87 ±0.20a
11 51. 22 ±3.57cd 28.90 ±0.38cd
12 216.78 ±13 .51 ab 4.35 ±0.29b 70.49 ±12.24a 1.7 7 ±0.23a
13 47.85 ±0.16de 26.59 ±0.14de
14 226.09 ±6.59a 5.31 ±0.26a 72.38 ±1.47 a 1.82 ±0.10a
∗Different letters in the same column indicate significant difference ( p ≤0.05).
TPC : total phenolic content in terms of gallic acid equivalent.
AA : antioxidant activity.
due to elimination of oxidative enzymes due to blanching and di-
minishing of atmospheric oxygen.
TPC and AA of strawberries and green beans were given in
Table 5 during frozen storage. Strawberry TPC ranged between
168 and 244 mg GAE/ 100 g in different studies. This value for
green and yellow beans ranged between 35.5 and 55.7 mg GAE/
100 g ( Poiana et al., 2010; Marinova et al., 2005 ). Phenolic com-
pounds in strawberries are ellagic acid, ellagic acid glucoside,
quercetin 3-glucoside, quercetin 3-gluronide and kaempferol 3-
glucoside. Green beans contain high levels of flavonoids: quercetin,
kaempferol, catechins, epicatechins, and procyanidins. TPC of fresh
strawberry obtained in our study, was close to literature values.
However, fresh green bean variety (Sırık) showed relatively higher
TPC than literature values. The variety, climatic conditions and har-
vest time are important parameters for TPC . Increase in TPC and AA
is a well-known concept as observed in our study due to cellular
disruption caused by thawing of the fruit before analysis which im-
proved extraction of these compounds ( De Ancos et al., 20 0 0 ). Ac-
cording to the study of Poiana et al., (2010) , the storage at −18 °C
up to 4 months did not have a significant effect on bioactive com-
pounds of strawberry. The results obtained are also in accord with
the studies of Gonz’ alez et al. (2003) . They found that TPC and
antiradical efficiency of blackberries was stable during 3 months
of frozen storage at −24 °C. The results of authors also showed
that TPC and antiradical efficiency of raspberry varieties showed
no significant change during 12 months of frozen storage. Accord-
ing to the study of Gonçalves et al., (2017) , freezing by forced air
was more effective than static air in retaining antioxidant activ-
ity of the strawberry pulp and after 6 months of frozen storage,
strawberry pulp retains high antioxidant capacity independent of
freezing method.
Color is an important food quality parameter due to its effect on
consumers’ preference. Chromatic changes are shown in Table 6 for
frozen storage. ࢞E of strawberry showed a change at first week of
frozen storage when compared to fresh sample, and then it main-
tained at a stable value. ࢞E of fruits is generally because of the
conversion of monomeric anthocyanins into polymeric form. In the
study of Holzwarth et al., (2012) , conventionally frozen ( −20 °C)
and thawed strawberries exhibited higher a
∗and b
∗values and
lower L
∗values compared to cryogenically frozen berries. Higher
a
∗values were attributed to pigment diffusion from the center of
Tabl e 6
CIE L
∗a
∗b
∗and total color difference ( ࢞E ) values of samples during frozen storage
at −27 °C
∗.
Storage
time
(weeks)
L
∗a
∗b
∗࢞E
Strawberry
Fresh
(control)
41.11 ±2.49 21.04 ±1.2 9 8.55 ±1.4 4 64.39 ±0.50b
1 32.26 ±0.72 23.01 ±3.09 10.79 ±1.52 72.52 ±1.8 0a
2 36.68 ±0.66 22.12 ±1.0 0 8.73 ±0.78 67.32 ±0.45ab
3 34.47 ±4.66 22.57 ±3.78 9.76 ±4.89 73.05 ±1.98 a
4 35.58 ±3.24 22.34 ±3.85 9.25 ±5.01 73.42 ±0.49a
6 31.23 ±1.43 24.29 ±1.4 5 9.70 ±0.05 73.54 ±1.82a
8 33.80 ±6.07 24.51 ±0.77 10.56 ±6.26 73.26 ±3.96a
10 29.67 ±1.7 3 27.24 ±4.50 9.85 ±0.52 73.31 ±2.88a
12 28.45 ±1.15 22.03 ±3.15 8.50 ±3.93 75.57 ±0.81a
Green
bean
Blanched
(control)
54.63 ±1.77 −14 .47 ±0.99 30.92 ±2.16 56.81 ±1.96b
1 47.14 ±3.19 −15.37 ±1.7 6 25.22 ±5.84 60.75 ±0.09ab
2 47.99 ±2.01 −13.04 ±3.55 19.98 ±5.49 57.34 ±4.54b
3 39.92 ±1.65 −15 .14 ±1.30 23.43 ±4.52 66.12 ±0.17a
4 54.52 ±1.0 0 −17.2 2 ±0.55 33.88 ±2.16 59.63 ±0.52ab
6 48.07 ±1. 0 0 −15.43 ±0.16 26.30 ±1.0 0 60.25 ±0.35ab
8 52.00 ±1.15 −16 .21 ±2.78 31.69 ±3.05 62.59 ±0.83ab
10 45.19 ±1.12 −16 .4 6 ±2.02 26.76 ±2.15 62.03 ±0.04ab
12 46.36 ±1.0 0 −16.4 0 ±1.00 26.56 ±2.52 63.43 ±0.60ab
∗Different letters in the same column indicate significant difference ( p ≤0.05).
L
∗: lightness; a
∗: color axis value from green to red; b
∗: color axis value from blue
to yellow.
the fruit to the outmost cell layers because of disrupted cell walls.
For the green beans, the blanched green bean samples were taken
as control. In this study, ࢞E of green beans did not significantly
change during frozen storage. The chlorophylls are responsible for
the green color of many vegetables. Conversion of chlorophylls to
pheophytins and pyropheophytins cause color change. According to
the study of Bahçeci et al., (2005) , blanching at 70 °C for 2 min re-
sulted in a decrease, while blanching at 90 °C for 3 min an increase
in the half-life of chlorophyll during frozen storage. Another type
of color deterioration is the removal of the phytol chain and forma-
tion of chlorophyllide from chlorophyll. According to the study of
368 M. Bulut et al. / International Journal of Refrigeration 88 (2018) 360–369
Martins and Silva (2002) , chlorophyll and color loss at high storage
temperatures are mainly attributed to pheophytisation. At lower
storage temperatures, color is stabilized probably by the formation
of metal–chlorophyll compounds and chlorophyll content does not
give a reliable prediction of color retention.
4. Conclusion
Home freezing of strawberry and green bean provided slow
freezing rates according to this experimental study. Freezing times
obtained in this experimental study were validated by using
Pham’s equation. During a storage period of 90 days at −27 °C,
slight changes were obtained in the TPC, AA and E values with
respect to the results obtained in the 1st week of frozen storage.
But ascorbic acid losses were significant and approximately 42 and
27% for 13 weeks frozen storage of strawberry and green bean, re-
spectively. In all freezing rates studied, degradation in cell integrity
was observed especially in the fruit sample due to large ice crys-
tal formation as shown by NMR relaxometry analysis. Therefore,
freezing rate must be the most important parameter for the frozen
storage analysis and also for the design of home-type freezers. The
freezer with two compartments may be designed and produced:
in one compartment the quick freezing will be performed and
in the second compartment will be for the storage of the frozen
samples.
Acknowledgment
The authors thank Middle East Technical University for sup-
porting the project ( Research Projects Fund: Grant No: BAP-03-14-
2014-005 )
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