Influence of temperature and time of storage on amount of vitamin C in strawberries

Abstract

Nutritional quality of strawberries comes mostly due to the large amount of containing vitamin C. There is a need of proper strawberry storage to preserve the high amount of vitamin C. Vitamin C was determined by the Tillmans method in strawberries stored at: room temperature, temperature of 4 0 C (cooling) and at temperature of-18 0 C (freezing). The storage was up to 11 days. Results were processed with t-test and after statistical processing we calculated the Pearson's correlation coefficient. In all three storage conditions there is a reduction of vitamin C amount. The amount of vitamin C in fresh strawberries is 60.85 mg/100 g and after 5 days it is only 7.57 mg/100 g for the strawberries stored at room temperature, 43.30 mg/100 g for the strawberries stored under cooling conditions and 44.54 mg/100 g for the frozen strawberries. After 11 days, there is almost no vitamin C (0.55 mg/100 g) in the strawberries stored at room temperature, the amount of vitamin C in the frozen strawberries was 28.21 mg/100 g, and in the cooling strawberries there was the highest amount of vitamin C that achieved 37.92 mg/100 g. For strawberries stored at room temperature and for freezing strawberries there is the highest value for Pearson's correlation coefficient and r = 0.93871402. This coefficient it is lower for cooled strawberries and freezing strawberries, r = 0.887226935. Vitamin C reduces differently in strawberries stored in different conditions. Until the fifth day, the tendency of vitamin C reduction is the smallest for the frozen strawberries , and after the fifth day it is the smallest for the cooling strawberries.

Full text

Journal of Hygienic Engineering and Design
15
Original scientic paper
UDC 634.75:577.164.2(497.776)”2014”
INFLUENCE OF TEMPERATURE AND TIME OF STORAGE
ON AMOUNT OF VITAMIN C IN STRAWBERRIES
Gorica Pavlovska
1*
, Emilija Dukovska
1
, Vesna Antoska Knights
1
, Vezirka Jankuloska
1
1
Faculty of Technology and Technical Sciences - Veles, University St. Kliment Ohridski - Bitola,
Dimitar Vlahov, 1400 Veles, Republic of Macedonia
*
e-mail: gorica.pavlovska@gmail.com
Abstract
Nutritional quality of strawberries comes mostly due
to the large amount of containing vitamin C. There is
a need of proper strawberry storage to preserve the
high amount of vitamin C.
Vitamin C was determined by the Tillmans method in
strawberries stored at: room temperature, temperature
of 4
0
C (cooling) and at temperature of -18
0
C (freezing).
The storage was up to 11 days. Results were processed
with t- test and after statistical processing we calculat-
ed the Pearson’s correlation coecient.
In all three storage conditions there is a reduction of
vitamin C amount. The amount of vitamin C in fresh
strawberries is 60.85 mg/100 g and after 5 days it is
only 7.57 mg/100 g for the strawberries stored at room
temperature, 43.30 mg/100 g for the strawberries
stored under cooling conditions and 44.54 mg/100 g
for the frozen strawberries. After 11 days, there is al-
most no vitamin C (0.55 mg/100 g) in the strawberries
stored at room temperature, the amount of vitamin C
in the frozen strawberries was 28.21 mg/100 g, and in
the cooling strawberries there was the highest amount
of vitamin C that achieved 37.92 mg/100 g. For straw-
berries stored at room temperature and for freezing
strawberries there is the highest value for Pearson’s
correlation coecient and r = 0.93871402. This coef-
cient it is lower for cooled strawberries and freezing
strawberries, r = 0.887226935.
Vitamin C reduces dierently in strawberries stored in
dierent conditions. Until the fth day, the tendency of
vitamin C reduction is the smallest for the frozen straw-
berries, and after the fth day it is the smallest for the
cooling strawberries.
Key words: Strawberries, Vitamin C, Storage, Cooling,
Freezing.
1. Introduction
Vitamin C (ascorbic acid, AA) is water-soluble, unstable
vitamin, which is a powerful antioxidant essential for
the human body (Anitra and Balz, [1]). The main sourc-
es of vitamin C are: citrus fruits, strawberries, peppers,
tomatoes, spinach and etc.
Strawberries are healthy, dietary and nutritionally rich
products because of well a balanced composition.
Large amounts of vitamin C, which they contain, deter-
mines their nutritional quality. It is recommended that
they should be eaten fresh, to use the most as their
nutritional components (Giampieri et al., [2]). Usually
strawberries can’t be eaten fresh as they are processed
or stored.
The storage conditions are very important for the
amount of vitamin C in them. If the fruit surface is dam-
aged, that causes major losses of ascorbic acid. Fruits
which have a low pH (citrus fruits) have smaller losses
of ascorbic acid and fruit with a soft consistency, such
as strawberries are more sensitive to external inuenc-
es. By reducing of the temperature, we reduced possi-
bility of losing vitamin C in fruits. Evaporation of straw-
berries is higher at the strawberries stored at a higher
temperature, and therefore the losses of water soluble
vitamin C are higher (Nunes et al., [3]).
Also high temperature adversely aects the content of
vitamin C, because it is unstable at the high tempera-
ture (Davey et al., [4]). To minimize the losses of vita-
min C in strawberries, it is recommended that they are
storage by cooling or freezing and not at room tem-
perature. Depending on storage temperatures, straw-
berries anti-oxidative ability is changes. Therefore, the
strawberries which are kept at a temperature between
10
0
C and 5
0
C have higher antioxidant capacity, and
higher concentration of phenols and anthocyanins
than those strawberries kept at a temperature of 0
0
C.

Journal of Hygienic Engineering and Design
16
Regarding dierent storage temperatures, the con-
centration of total phenols and avonoids is almost
the same (Zhao [5]). But the strawberries concentra-
tion of ascorbic acid is reduced, because it oxidizes to
dehydroascorbic acid, which still has vitamins proper-
ties. Storage at the long period, dehydroascorbic acid
oxidizes to dicetogulonic acid which has no vitamins
properties (Pavlovska and Tanevska, Bode et al., [6 - 7]).
The temperature and time of storage aect many
parameters of strawberries. This paper following the
amount of vitamin C in strawberries during dierent
storage conditions (room stored temperature, cooling
and freezing) for a period of 11 days.
2. Materials and Methods
Garden strawberries from the region of Demir Hisar,
Republic of Macedonia, are analyzed. Strawberries are
harvested in May, 2014. The samples are divided into
three parts and stored in various conditions. The rst
sample is strawberries left at room temperature (20 -
25
0
C), another at a cool temperature of 4
0
C and the
third samples frozen at a temperature of -18
0
C.
Determination of Vitamin C is performed in a homog-
enized strawberries with 2,6-dichlorofenolindofenol
using Tilman´s method according to the AOAC [8]. The
strawberries were pulverized in Ultraturex homogeniz-
er (Ika Labortechnik T25).
Numerical statistical analyses of data are made with
applying “Pearson’s correlation coecient” and the
Student t-test [9 - 10]. Since the results are obtained
under dierent storage conditions, a correlation coef-
cient between two sets ofdata results is calculated.
“Pearson’s correlation coecient”, or simply “the cor-
relation coecient” measures the relative strength of
the linear relationship between two variables, or two
sets of data obtained from dierent storage condi-
tions. The correlation coecientis obtained by divid-
ing thecovarianceof the two variables by the product
of theirstandard deviations.
Student’s t-test deals with the problems associated
with inference based on “small” samples. Two sets of
data can be used to determine if the averages of your
two samples are signicantly dierent.
3. Results and Discussion
3.1 Determining theamount of vitamin C
Garden strawberries are analyzed at room and fro-
zentemperaturesfor 1, 3, 4, 5, 7 and 11 days.
Over time the stored strawberries are succumbed towa-
terreduction, therefore they lose their shine and succulence
(juiciness) and their surfaces become wrinkled. Also they
get darker in color as a result of anthocyanins synthesis.
These changes occurred faster with room stored straw-
berries compared to samples stored cool.
Room stored strawberries became moldy after the
fourth day. The frozen storedsampledid not show the
same signs.Frozen sampleswhenmelting were char-
acterized by a diluted structure from disintegration
due to their structure.
In the Figure 1 is shown the amount of vitamin C
in room stored strawberries. Amount of Vitamin C is
60.85 mg/100 g in the fresh strawberries. The reduc-
tion of vitamin Ciscontinuous, 8 - 9 mgper day, or14 -
15%eachday. In terms of the third day, declining vitamin
C is very small after the fourth day and very big after the
fth day. After a week of storage at room temperature,
the vitamin C in strawberries is almost decomposed and
less than 2%, and after eleven days less than 1% relative
to the initial amount of vitamin C in fresh strawberries.
The amount of vitamin C in strawberries stored un-
derrefrigeration is shown in Figure 2. The reduction in
the content of vitamin C in strawberries stored cooling
is lenientlysignicantcompared with the decrease of
vitamin C in strawberries stored at room temperature.
The biggest jump in reduction of vitamin C has after
the rst day is storagethen it is smaller. After 11 days
of storage the amount of vitamin C in strawberries is
37.92mg/100 g, or it is reduced to less than 40% rel-
ative to the amount of vitamin C in fresh strawberries.
Figure 1.Amount of vitamin C inroomstoredstrawberries
Figure 2. Amount of vitamin C incoolstored strawberries

Journal of Hygienic Engineering and Design
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Frozen strawberriesorstoredstrawberriesat the low-
est temperature of -18
0
C have a dierent trendinthe
reduction of vitamin C (Figure 3). The amount of vita-
min C isquite well preservedinfrozenstrawberries in
the rst threestoreddays.Theamount of vitamin C in
strawberries hasbeen reduced to only 1 mg/100 gaf-
ter the rst day of storage,and 6 mg/100 g afterthe
third day. After one week of freezing, the amount of
vitamin C in strawberries hasreduced to less than 50%.
Strawberries contain 28.21 mg/100 g of vitamin Cafter
11 days of storing.
The comparison of the amount of vitamin C at three
dierent condition-stored strawberries is presented
in Figure 4 with the trend in reduction of vitamin C.
The highest amount of vitamin C ispresent inthe fro-
zenstoredstrawberries inthe rst ve days, but afterthe
seventhand after the eleventh day, highest amount of vi-
tamin C ispresentin the coolstored strawberries.Room
temperature storedstrawberries have thelowest quan-
tity ofvitamin Cin all the days of storage.
3.2 Determining the correlation coecient and Student’s
t-test
The correlation coecient measures the strength
andthe direction of a linear relationship between two
variables or two sets ofdata. It is calculated that the
correlation coecient between the data of strawber-
ries stored at room temperature and cooling stored
Figure 3. Amount of vitamin C infrozenstored strawberries Figure 4. Comparison of the amount of vitamin C
at the room stored strawberries, cool and frozen
strawberries, was r = 0.925769141.The correlation co-
ecient between the data of strawberries stored at
room temperature and frozen stored strawberriesis r
= 0.93871402. And r = 0.887226935 is the correlation
coecient between the data of strawberries cooling
stored and frozen stored strawberries. From the cor-
relation coecient result we can see that all groups are
in strong linear correlations.
But very importantly, is the content level of vitamin C
statistically signicant dierent in room stored straw-
berries than cooled stored strawberries? Alternatively,
is the content level of vitamin C statistically signicant-
ly dierent in room stored strawberries than frozen
stored strawberries?
In Table 1 the statistical parameters are presented
which have the following meaning:
- Theaverage value (mean)oftheamount of vitamin
Cinmg/100g;
sd - Thestandard deviation measures the amount of
variation or dispersion from the average. From the
statistical results we can see how much each mea-
surement deviates from the mean. It means in room
stored strawberries we have a highervariationin the
amount of vitamin C. For example, on the rst day the
amount of vitamin C is60.85mg/100g, on last day it is
0.55mg/100g, the average is 27.094mg/100g, so we
can see a highdispersionof 19.52fromtheaverage.In
Table 1. Representation of the parameters from Student’s t-test
Statistical parameters
Room stored
strawberries
Cooling
storedstrawberries
Frozen stored
strawberries
27,094 47,074 46,784
sd 19.52 4,65 10,91
cv 72 9,8 23,3
t-test - 3,39428 -4,97877
p-value (probability)
(statistical signicantly, p < 0.05)
0,00968 0,00209

Journal of Hygienic Engineering and Design
18
cooled (refrigerated) and frozen stored strawberries
we do not have so much variation in the amount of
vitamin C during the stored process. Cooled stored
strawberries have an sd of 4.65, and frozen stored
strawberries have an sd of 10.91.
cv -The variance measures how far a set of numbers
is spread out; a small variance indicates that the data
points tend to be very close to themeanand hence to
each other, while a high variance indicates that the data
points are very spread out around the mean and from
each other.The size of this dierence in comparison to
the variance (i.e. the range over which expression values
fall) will tell us whether this expression dierence is sig-
nicant or not. Therefore, if the dierence is large but the
variance is also large, then the dierence may not be sig-
nicant. On the other hand, a small dierence coupled
with a very small variance could be signicant.
We used the t-test for two groups to formalise this
calculation. The tests return a p-value that takes into
account the mean dierence and the variance and also
the sample size.
The p-value (probability) is a measure of how likely
you are to get this spot data if no real dierence exists.
Therefore, a small p-value indicates that there is a small
chance of getting this data if no real dierence exists
and therefore you decide that the dierence in the
group expression data is signicant. A p-value is said to
be signicant if it is less than the level of signicance,
which is commonly 5%(p < 0.05).
From the results of both t-tests there are statistical signi-
cantly dierences betweenthetwo groupsofsamples:
• Room stored strawberries andcooled storedstraw-
berries, t =-3.39428; p = 0.00968 < 0.05
• Room stored strawberries and frozen storedstraw-
berries, t =-4.97877; p = 0,00209< 0.05.
In the rst t-test(room stored andcool stored), levels
ofvitamin C are reduced in both casesbut there are
statistical signicant dierences in reducing the level
ofvitaminC in room stored then level ofvitaminC in
cooling stored strawberries. Cooled stored strawber-
ries keep a statistically signicant higher amount of
vitamin C than room stored strawberries.
In the second t-test(room stored and frozenstored),
the levelsof vitamin C are reduced in both casesbut
there are statistically signicant dierences in reduc-
ing the levelof vitamin Cin room stored rather than
the level of vitamin C in frozen stored strawberries.
Frozen stored strawberries are kept statistically with
a signicantly higher amount of C vitamin than room
stored strawberries.
From the statistical calculations and analysis above, we
prove that the best situation is that it is necessary for
strawberries to be kept for a longer period. The best
way is for strawberries to be stored under refrigeration,
or if it isnecessary for strawberries to be kept for the
longest period. Then we also get very good results for
the level of the amount of vitamin Cwhen strawberries
are stored frozen.
4. Conclusions
- It is determined the amount of vitamin C in straw-
berries which are stored at three dierent conditions
for a period of 11 days. Major losses of vitamin C occur
in the strawberries stored at room temperature. After
four days storage at room temperature, the amount of
vitamin C is almost halved, and after 7 and 11 days, al-
most no vitamin C. Strawberries stored by freezing the
highest values for vitamin C in the rst 5 days of stor-
age, and more storage for 7 and 11 days, it is best for
strawberries to be stored under refrigeration.
- The correlation coecient is stronger for straw-
berries stored at room temperature and freezing
(r = 0.93871402), but the correlation coecient is still
strong between strawberries stored at cooling and
freezing temperature (r = 0.887226935).
- From the results we can make the conclusion that
the samples are in a strong correlation, which means a
stronglineardependencebetween thetwo sets ofdata.
In our case a very strong linear dependenceofdata of
the amount of vitamin C inroom stored (as a rst vari-
able) andthedata of the amount of vitamin C incooled
stored strawberries (as a second variable). This means
that in both cases, the amount of vitamin C is reduced
by the same linear dependence. The same conclu-
sionappliesfor room stored (as a rst variable) andfro-
zen stored strawberries (as a second variable), where
bythe level of content of vitamin C is reduced also.
-But most importantly is that with the Student t-test
we statistically signicant proved the dierences be-
tween the samples.  This means with numerical and
statistical analyses we conrmed our experiment.
We have a highestvariation about the mean in room
stored strawberries and also variation relative to the
mean in the case of frozen and cooled stored straw-
berries. Following the t-test we calculated that the
statistically signicant dierences are so much lower
that0.05in both cases, in the rst case:room andcool
storedstrawberries, and in the other case, room and
frozenstoredstrawberries.
-With a strong correlation coecient, and when the
dierence is statisticallysignicant, we can state that
it is likely that the dierence is not caused by this ran-
dom uctuation. Therefore we conrm it is best for
strawberries be stored under refrigeration.

Journal of Hygienic Engineering and Design
19
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