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54 Journal of Food, Agriculture & Environment, Vol.8 (1), January 2010
www.world-food.net
Journal of Food, Agriculture & Environment Vol.8 (1) : 54-58. 2010
WFLPublisher
Science and Technology
Meri-Rastilantie 3 B, FI-00980
Helsinki, Finland
e-mail: info@world-food.net
The effect of long-term frozen storage on the nutraceutical compounds, antioxidant
properties and color indices of different kinds of berries
Mariana-Atena Poiana *, Diana Moigradean, Diana Raba, Liana-Maria Alda and Mirela Popa
Banat’s University of Agricultural Sciences and Veterinary Medicine, Food Technology Department, Calea Aradului 119,
300645, Timisoara, Romania. *e-mail: atenapoiana@yahoo.com
Received 23 September 2009, accepted 4 January 2010.
Abstract
The effects of the Individual Quick Freezing (IQF) process and frozen storage at -18°C up to 10 months, on the nutraceutical compounds, antioxidant
properties and color indices of various berries (blueberry, red raspberry and blackberry) have been evaluated. Samples were extracted and analyzed
for their total phenolics content, total monomeric anthocyanins, vitamin C, antioxidant activity and color indices. Total anthocyanins and color
indices were evaluated by using pH-differential method, total phenolics content was measured using Folin-Ciocalteu procedure, vitamin C content
using 2,6-dichlorophenolindophenol method and antioxidant activity using ferric-reducing antioxidant power (FRAP) assay. Blueberry contains the
highest amounts of polyphenols, anthocyanins and antioxidant activity among the berries studied. The highest content of vitamin C was found in
fresh raspberry. After freezing, no significant difference was observed for investigated nutraceuticals and color of berries, because the IQF is a rapid
and non-destroying preservation method. Results showed that the frozen storage up to 4 months did not significantly affect the bioactive compounds
and color indices of berries. The degradation of these characteristics was not recorded more than 23% during six months of storage. After 10 months,
the content of polyphenols decreased up to 28-47% of the initial value; the total anthocyanins was found in proportion of 80-91%, and the ascorbic
acid content was kept at 62-76%. After 10 months of storage the smallest loss of antioxidant activity was recorded for blueberries (approximately
23%) and the biggest loss for raspberries (approximately 37%). The results showed a positive correlation between antioxidant capacity and
polyphenols, vitamin C and anthocyanins content. The correlation coefficient between FRAP and the total phenolics was higher than the correlation
coefficient between FRAP and total anthocyanins or FRAP and vitamin C for all investigated berries.
Key words: Frozen berries, anthocyanins, polyphenols, vitamin C, antioxidant activity, color.
Introduction
Berries are known for their bioactive properties such as antioxidant
activity, cardiovascular protection, antidiabetic properties, vision
improvement properties, and inhibition of carcinogenesis and
mutagenesis 20, 23, 24. Blueberries, raspberries and blackberries are
excellent sources of phytochemicals that are believed to have
significant biological activity 14, 18, 20. During the last decade, much
interest has been focused on berries due to their high levels of
anthocyanins and antioxidant capacity. Prior et al.18 reported a
significant correlation between the antioxidant capacity and the
total content of anthocyanins and phenolics among blueberries.
Many studies 4, 12, 14, 15 evaluated the phenolic content of different
berries and found significant differences in the anthocyanins,
phenolics and antioxidant capacity of phenolic content among
the different species. Anthocyanins have been associated with
the antioxidant properties of many common small fruit crops and
have been characterized as having significant beneficial effects
on various diseases. Anthocyanins had the greatest
antiproliferation effect with an inhibition of greater than 50% as
opposed to the phenolic acids, flavonols and tannins 23. Phenolic
compounds from berry extracts have been reported to exhibit a
broad range of protective health benefits that may reduce the risk
factors associated with certain types of cancer, cardiovascular
disease as well as other degenerative diseases 3. Only a small
percentage of berries reaches the fresh market, and most berries
end up frozen or canned. Frozen berries can be then processed in
jams, purees, jellies and juices 13, 20. Changes in antioxidant content
and color take place in frozen berries as a result of oxidation-
reduction reactions occurring in fruits. These changes are
influenced by the initial quality of berries, raw material processing
prior to freezing, freezing methods, storage conditions (temperature
and relative humidity), storage time of frozen berries and quality
of container 16, 21. Due to the high antioxidant levels found in
blueberries, blueberry processors are seeking effective processing
techniques such as IQF (Individual Quick Freezing) to further
optimize the amount of antioxidants retained in the final product.
Freezing of berries will increase flexibility for consumers by
extending the length of time in which fruits are available. IQF is
one of the simplest and least time-consuming ways to preserve
berries, but the long-term frozen storage might affect
anthocyanins, polyphenols, vitamin C, color quality and
antioxidant effects of berries. The literature provides many studies
about the effects of freezing on the retention of antioxidants in
different berries 1, 11, 13, 16, 21. Little is known, however, about the
effect of long-term frozen storage on color indices, polyphenols
Journal of Food, Agriculture & Environment, Vol.8 (1), January 2010 55
and other antioxidants in different kinds of berries. Because during
the storage of frozen berries the levels of antioxidant compounds
may be altered resulting in a change in antioxidant properties, the
goal of this research was to investigate how freezing and long-
term storage can affect the retention of antioxidant properties and
bioactive compounds in berries.
Material and Methods
Blueberry (Vaccinium myrtillus), red raspberry (Rubus idaeus)
and blackberry (Rubus fruticosus) were harvested in Romania, at
the commercial maturity stage. After harvesting, berries were
refrigerated (3-5°C for 24 h) and frozen by IQF freezing techniques
(Individual Quick Freezing) by passage through a Frigoscandia
freezing tunnel. The frozen samples were stored in polyethylene
bags in freezing box at temperature of -18°C for 10 months. Fresh
and frozen berries were supplied by S.C. LEGOFRUCT S.R.L from
Timisoara (the western part of Romania). The samples were
analyzed fresh (FR), immediately after freezing (0-F) and during
storage in frozen state after 2, 4, 6, 8 and 10 months (2-F, 4-F, 6-F,
8-F and 10-F), respectively.
For total anthocyanins, total polyphenols, vitamin C and
antioxidant activity determination, three replicates of berry extracts
were prepared in accord with Kalt et al. 8. Before the analysis, the
frozen berries were thawed in refrigerator (3-5°C) for 4 h. Berry
extracts for anthocyanin analysis were obtained by grinding berries
(~5 g) in 95% (v/v) ethanol (20 mL) acidified with HCl (0.1%, v/v)
for 2 min. The mixture was stored at room temperature in the dark
for 16 h and then filtered 8. The extracts for phenolics and vitamin
C analysis were obtained by grinding berries (~5 g) in hot 95% (v/
v) ethanol (10 mL) for 2 min. The solution was filtered. The
extraction of the residue was repeated twice following above
mentioned procedure. Three extracts were combined 8.
The extracts for antioxidant activity determination were obtained
by grinding berries (~20 g) in 95% (v/v) ethanol (20 mL) acidified
with HCl (0.1%). After 60 min the solution was filtered. The residue
was extracted again. The extracts were combined and diluted to
volume of 50 ml with methanol acidified with HCl (0.1%) 8.
Determination of total phenolics (P): Total phenolic content was
analyzed spectrophotometrically using an adapted
Folin-Ciocalteu colorimetric method described by Singleton and
Rossi 22. Extract samples were diluted to fall within the range of
the calibration curve. The calibration curve was prepared using
0.05-0.6 mM•L-1 gallic acid equivalents (GAE). The samples were
incubated for 2 h in the dark at room temperature prior to measuring
the absorbance reading at 750 nm using the UV-VIS
spectrophotometer (Analytic Jena Specord 205). Quantification
of the data was calculated based on the calibration curve generated
using gallic acid as the standard and the results were expressed
as mg of gallic acid equivalents (GAE) per 100 g of berry.
Determination of total monomeric anthocyanins (A): Total
monomeric anthocyanins of berry extracts were determined by
the pH differential method 6. Samples were diluted to the
appropriate concentration with 0.025 M potassium chloride buffer
(pH 1.0) and 0.4 M sodium acetate buffer (pH 4.5). The absorbance
was measured with the UV-VIS spectrophotometer (Analytic Jena
Specord205) using 1 cm path length disposable cells at 520 nm
and 700 nm after 15 min of incubation at room temperature. The
content of total anthocyanins was expressed as mg of cyanidin-3-
glucoside equivalents per 100 g of berry. The total monomeric
anthocyanins content was calculated using cyanidin-3-glucoside
coefficients (molar extinction coefficient ε = 26,900 L•cm-1•mol-1
and molecular weight MW = 449.2 g•mol-1). Percent of polymeric
color contents of these samples were determined by the bisulfite
bleaching method 6. Berry extract samples were diluted using the
appropriate dilution factor and treated with potassium metabisulfite
solution or distilled water. The samples were incubated at room
temperature for 15 min prior to measuring absorbance at 420, 520
and 700 nm. The percent of polymeric color was based on the
color density of the control sample and the polymeric color of the
bisulfite bleached sample.
Determination of vitamin C (VC): Vitamin C was measured by
titration with a 2,6-dichlorophenolindophenol sodium salt solution,
and chloroform was used for intensely colored extracts 2.
Ferric reducing antioxidant power (FRAP) assay: The ability
to reduce ferric ions was measured using methods of Benzie and
Strain 5. An aliquot (200 µL) of the extract with appropriate dilution
was added to 3 mL of FRAP reagent (10 parts of 300 mM sodium
acetate buffer at pH 3.6, 1 part of 10 mM TPTZ solution and 1 part
of 20 mM FeCl3•6H2O solution), and the reaction mixture was
incubated in a water bath at 37°C. The increase in absorbance at
593 nm was measured after 30 minutes. The antioxidant capacity
based on the ability to reduce ferric ions of the extract was
expressed as mM Fe2+/kg fresh weight -1 was calculated.
All chemicals and solvents used in this study were of analytical
grade. All tests were run in triplicates. The concentrations of total
phenolics, anthocyanins, ascorbic acid and the results of
antioxidant activity determination were expressed as related to
the fresh weight (FW) basis. Each value is the mean of three (n =
3) independent determinations.
Results and Discussion
The antioxidant activity, vitamin C, total phenolic and total
anthocyanin contents in fresh berries, after freezing and during
the 10 months storage period are shown in Table 1. No significant
changes in vitamin C, total phenolic content and antioxidant
activity of all analyzed berries were found after freezing. In our
study only slight increases of anthocyanins were found
immediately after freezing. It is most probable that the anthocyanins
in frozen fruit become more easily extractable. This might be due
to degradation of cell structures in berries. The losses of bioactive
compounds and antioxidant activity during the storage time are
presented in Fig. 1. It was found that contents of the antioxidant
compounds, ascorbic acid, polyphenols and monomeric
anthocyanins, decreased in frozen storage products. During the
first 4 months there was a slow degradation of antioxidants in
frozen berries stored at -18°C. At a longer storage the bioactive
compounds degradation rate was accelerated. The value of
percentage degradation was depending on berry species and
storage life.
At the end of frozen storage period, total phenol content
decreased up to 28.37% from the 0-F values for blueberry, 42.41%
for blackberry and greatest losses (47.42%) were found for
raspberry. The long storage time affected the vitamin C content:
after 6 months the losses were 16-19% while after 10 months the
56 Journal of Food, Agriculture & Environment, Vol.8 (1), January 2010
losses reached 23-38%. The smallest loss was registered for
blueberry. It can be concluded that storage in frozen state for a
period of more than 8 months significantly affects ascorbic acid
concentration in fruits investigated 17. Probably, significant
decrease of investigated compounds was due to water content in
non-frozen state. Activity and enzymatic reaction rate reached
maximum values in the layers of liquid water in frozen fruits.
Perhaps, this phenomenon contributes to the modification of
chemical compounds, including biologically active substances.
In frozen products the enzymatic reactions are slow, but not
completely blocked. In general, the activity of enzymes in frozen
berries is linked to the presence of non-frozen water. At a
temperature of -18°C in frozen berries water content represents
approximately 89% of total water of berries. Liquid water in these
products will be 11%. At a temperature of -30°C frozen water in
berries will be 91% of total fruit water and liquid water content 9%.
It was found that storage of frozen fruit for 6 months slowed
down anthocyanin degradation (losses in these compounds after
6 months were below 5% for blackberry and blueberry and 10%
for raspberry). After 10 months there was an accelerated
degradation of anthocyanins, so that losses were 9% for
blackberry, 13% for blueberry and 20% for raspberry. These data
are in agreement with other studies 7, 9, 10, 13, 21 showing that a
period of 6 months of freezing does not cause significant loss of
anthocyanin content. This information is important because the
monomeric anthocyanins represent approximately 25% of the total
antioxidant capacity of berries 5. Antioxidant activity decreased
during the frozen storage of berries. In the first 4 months of storage
there was a relatively small decrease in antioxidant capacity, which
was followed by a significant decline in the following months.
During the storage for 10 months, antioxidant capacity decreased
up to 23% of 0-F value for blueberry and up to 34-37% for both
0
5
10
15
20
25
30
35
40
246810
storage time (months)
()
raspberry
blueberry
blackberry
a
0
10
20
30
40
50
246810
storage time (months)
(%)
raspberry
blueberry
blackberry
b
0
10
20
30
40
246810
raspberry
blueberry
blackberry
0
5
10
15
20
246810
tti(th)
(%)
raspberry
blueberry
blackberry
%%
%
%
ab
cd
Storage time (months) Storage time (months)
Storage time (months) Storage time (months)
Figure 1.The losses of vitamin C (a), total phenolics (b), antioxidant activity (c) and total
anthocyanins (d) during the long-term frozen storage of berries.
Frozen fruits
Berries FR
0-F 2-F 4-F 6-F 8-F 10-F
Vitamin C (mg/100 g FW)
Raspberry 31.55 31.41 29.91 27.15 26.22 25.15 22.13
Blueberry 8.20 8.15 7.92 7.68 6.61 6.43 6.22
Blackberry 6.63 6.46 5.81 5.46 5.28 4.39 3.97
Total phenolics (mg GAE/100 g FW)
Raspberry 197.79 197.14 182.23 169.45 153.21 129.75 103.65
Blueberry 641.53 640.11 611.43 589.31 550.4 511.22 458.54
Blackberry 333.60 331.87 322.47 279.07 242.79 224.27 191.12
Antioxidant activity (mM Fe
2+
/kg FW)
Raspberry 40.16 39.21 37.89 35.72 31.38 28.37 24.84
Blueberry 58.31 57.94 55.16 53.10 50.44 47.10 44.82
Blackberry 49.64 48.73 46.02 43.17 38.46 37.32 32.29
Total anthocyanins (mg/100 g FW)
Raspberry 39.71 41.67 39.95 37.85 37.56 34.85 33.51
Blueberry 205.48 207.12 205.14 202.67 198.0 185.12 180.31
Blackberry 193.72 195.89 192.08 191.75 188.4 182.55 178.62
Table 1. Effect of freezing and frozen storage time on the total phenolics, vitamin C, total
anthocyanins and antioxidant activity of different berries.
Journal of Food, Agriculture & Environment, Vol.8 (1), January 2010 57
R
Y=A+B•X raspberry blueberry blackberry
FRAP=f(VC) R=0.96891 R=0.96272 R=0.96239
FRAP=f(P) R=0.99225 R=0.99194 R=0.99146
FRAP=f(A) R=0.96846 R=0.96744 R=0.96317
Table 2. Correlation coefficients obtained after simple regression
model applied for antioxidant activity, vitamin C, total
phenolics and anthocyanins contents.
raspberry and blackberry. Also, at the end of six months of frozen
storage period the losses of antioxidant activity of blueberry were
less than 13% compared with the values measured just after
freezing process while the losses in antioxidant activity for
raspberry and blackberry were in the range of 20-21%.
In this study, the correlations established between total
antioxidant activity and investigated bioactive compounds were
evaluated. Simple regression models were applied using the Origin
4.1 software program. The values of correlation coefficients (R)
are presented in Table 2. It was found a linear dependence between
antioxidant activity and nutraceutical compounds. Antioxidant
activity was strongly correlated with total phenolics, vitamin C
and total anthocyanin contents (R>0.95). The linear correlations
FRAP = f (P), FRAP = f (VC) and FRAP = f (A) for raspberry are
shown in Figs 2-4. For all investigated berries, the best correlation
coefficient (R>0.99) was recorded for FRAP = f(P).
The storage time affects the color quality of berries. The effect
of frozen storage time on berries color was quantified by measuring
the color density, polymeric color and percent polymeric color
(Table 3). The percentage of polymeric color is a result of the
degree of anthocyanins polymerization 19, 25. The color density
showed a similar trend over storage time. The color density
decreased due to destroying of monomeric anthocyanins and
increase in polymeric color. The percentage of polymeric color
from the fresh berries was 9-11%. By freezing, the degree of
pigments degradation was affected in a very small extent (less
than 1%). In the time of long-term frozen storage of berries at -
18°C there was a relative increase in the percent of polymeric
color. For raspberry, the percentage of polymeric color becomes
significant after 10 storage months (22.22%). After 10 months of
frozen storage, the best color stability was registered for blueberries
and blackberries. The IQF process can be used to assurance the
retention of various nutrients that are naturally present in berries.
Because the antioxidant activity of berries is an appealing
characteristic to consumers, we appreciate that the IQF of berries
followed by a six months frozen storage is a very good preservative
process to support almost superior bioactive compounds while
minimizing loss of color.
22 24 26 28 30 32
24
26
28
30
32
34
36
38
40
100 120 140 160 180 200
24
27
30
33
36
39
42
F
RAP (mM Fe2+/kg FW)
34 36 38 40 42
24
27
30
33
36
39
42
FRAP (mM Fe2+/kg FW)
T t l th i ( /100 FW)
FRAP = -11.4980 + 1.6447 • VC
R = 0.96891 FRAP = 7.601 + 0.1623 • P
R = 0.99225 FRAP = -34.7272 + 1.8003 • A
R = 0.96846
Vitamin C
(mg/100 g FW)
FRAP (mM Fe2+/kg FW)
Total anthocyanins
(mg/100 g FW)
Total phenolics
(mg GAE/100 g FW)
FRAP (mM Fe2+/kg FW)
FRAP (mM Fe2+/kg FW)
Figure 2. Linear correlation between
FRAP = f(VC) for raspberry. Figure 3. Linear correlation
between FRAP= f(P) for raspberry. Figure 4. Linear correlation
between FRAP=f(A) for raspberry.
Frozen
Berries FR
0-F 2-F 4-F 6-F 8-F 10-F
Color density index
Raspberry 7.14 7.09 6.90 6.71 6.05 5.27 5.04
Blueberry 11.77 11.68 11.51 11.21 10.85 10.71 10.43
Blackberry 12.28 12.21 12.15 11.96 11.80 11.53 11.58
Polymeric color index
Raspberry 0.78 0.80 0.83 0.87 0.94 1.02 1.12
Blueberry 1.05 1.10 1.17 1.23 1.36 1.44 1.50
Blackberry 1.16 1.19 1.23 1.29 1.34 1.38 1.43
% polymeric color
Raspberry 10.92 11.28 12.03 12.97 15.54 19.35 22.22
Blueberry 8.92 9.42 10.17 10.97 12.53 13.45 14.38
Blackberry 9.45 9.75 10.12 10.79 11.36 11.97 12.35
Table 3. Effect of frozen storage time on the color indices of frozen berries.
58 Journal of Food, Agriculture & Environment, Vol.8 (1), January 2010
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Conclusions
Bioactive compounds content is not affected by the IQF process.
Long-term frozen storage had a significant impact on the
nutraceutical compounds and color stability of berries, depending
on the kind of berries. In the first 4 months of frozen berries storage
there was a slight decrease in content of bioactive compounds,
after which the rate of degradation increased. The highest degree
of bioactive compounds degradation was recorded after 10 months
of frozen storage. Investigated berries can be presented in the
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antioxidant compounds stability during frozen berries storage was
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10 months of storage the polyphenols content decreased up to
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was found between antioxidant activity and bioactive compounds.
The raspberry color is very sensitive to long-term frozen storage:
the content of anthocyanins decreased after 10 months up ≈20%
of initial value, while the percent of polymeric color reached 22%.
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