Journal of Hygienic Engineering and Design
Original scientic paper
INFLUENCE OF TEMPERATURE AND TIME OF STORAGE
ON AMOUNT OF VITAMIN C IN STRAWBERRIES
, Emilija Dukovska
, Vesna Antoska Knights
, Vezirka Jankuloska
Faculty of Technology and Technical Sciences - Veles, University St. Kliment Ohridski - Bitola,
Dimitar Vlahov, 1400 Veles, Republic of Macedonia
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
C (cooling) and at temperature of -18
The storage was up to 11 days. Results were processed
with t- test and after statistical processing we calculat-
ed the Pearson’s correlation coecient.
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 coecient and r = 0.93871402. This coef-
cient it is lower for cooled strawberries and freezing
strawberries, r = 0.887226935.
Vitamin C reduces dierently in strawberries stored in
dierent 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
Key words: Strawberries, Vitamin C, Storage, Cooling,
Vitamin C (ascorbic acid, AA) is water-soluble, unstable
vitamin, which is a powerful antioxidant essential for
the human body (Anitra and Balz, ). 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., ). Usually
strawberries can’t be eaten fresh as they are processed
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 inuenc-
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., ).
Also high temperature adversely aects the content of
vitamin C, because it is unstable at the high tempera-
ture (Davey et al., ). 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
C and 5
C have higher antioxidant capacity, and
higher concentration of phenols and anthocyanins
than those strawberries kept at a temperature of 0
Journal of Hygienic Engineering and Design
Regarding dierent storage temperatures, the con-
centration of total phenols and avonoids is almost
the same (Zhao ). 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 aect many
parameters of strawberries. This paper following the
amount of vitamin C in strawberries during dierent
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 -
C), another at a cool temperature of 4
C and the
third samples frozen at a temperature of -18
Determination of Vitamin C is performed in a homog-
enized strawberries with 2,6-dichlorofenolindofenol
using Tilman´s method according to the AOAC . The
strawberries were pulverized in Ultraturex homogeniz-
er (Ika Labortechnik T25).
Numerical statistical analyses of data are made with
applying “Pearson’s correlation coecient” and the
Student t-test [9 - 10]. Since the results are obtained
under dierent storage conditions, a correlation coef-
cient between two sets ofdata results is calculated.
“Pearson’s correlation coecient”, or simply “the cor-
relation coecient” measures the relative strength of
the linear relationship between two variables, or two
sets of data obtained from dierent storage condi-
tions. The correlation coecientis obtained by divid-
ing thecovarianceof the two variables by the product
of theirstandard 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 signicantly dierent.
3. Results and Discussion
3.1 Determining theamount of vitamin C
Garden strawberries are analyzed at room and fro-
zentemperaturesfor 1, 3, 4, 5, 7 and 11 days.
Over time the stored strawberries are succumbed towa-
terreduction, 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 storedsampledid not show the
same signs.Frozen sampleswhenmelting 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 Ciscontinuous, 8 - 9 mgper day, or14 -
15%eachday. 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-
derrefrigeration is shown in Figure 2. The reduction in
the content of vitamin C in strawberries stored cooling
is lenientlysignicantcompared 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 storagethen it is smaller. After 11 days
of storage the amount of vitamin C in strawberries is
37.92mg/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 inroomstoredstrawberries
Figure 2. Amount of vitamin C incoolstored strawberries
Journal of Hygienic Engineering and Design
Frozen strawberriesorstoredstrawberriesat the low-
est temperature of -18
C have a dierent trendinthe
reduction of vitamin C (Figure 3). The amount of vita-
min C isquite well preservedinfrozenstrawberries in
the rst threestoreddays.Theamount of vitamin C in
strawberries hasbeen reduced to only 1 mg/100 gaf-
ter the rst day of storage,and 6 mg/100 g afterthe
third day. After one week of freezing, the amount of
vitamin C in strawberries hasreduced to less than 50%.
Strawberries contain 28.21 mg/100 g of vitamin Cafter
11 days of storing.
The comparison of the amount of vitamin C at three
dierent condition-stored strawberries is presented
in Figure 4 with the trend in reduction of vitamin C.
The highest amount of vitamin C ispresent inthe fro-
zenstoredstrawberries inthe rst ve days, but afterthe
seventhand after the eleventh day, highest amount of vi-
tamin C ispresentin the coolstored strawberries.Room
temperature storedstrawberries have thelowest quan-
tity ofvitamin Cin all the days of storage.
3.2 Determining the correlation coecient and Student’s
The correlation coecient measures the strength
andthe direction of a linear relationship between two
variables or two sets ofdata. It is calculated that the
correlation coecient between the data of strawber-
ries stored at room temperature and cooling stored
Figure 3. Amount of vitamin C infrozenstored 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-
ecient between the data of strawberries stored at
room temperature and frozen stored strawberriesis r
= 0.93871402. And r = 0.887226935 is the correlation
coecient between the data of strawberries cooling
stored and frozen stored strawberries. From the cor-
relation coecient result we can see that all groups are
in strong linear correlations.
But very importantly, is the content level of vitamin C
statistically signicant dierent in room stored straw-
berries than cooled stored strawberries? Alternatively,
is the content level of vitamin C statistically signicant-
ly dierent in room stored strawberries than frozen
In Table 1 the statistical parameters are presented
which have the following meaning:
- Theaverage value (mean)oftheamount of vitamin
sd - Thestandard 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 highervariationin the
amount of vitamin C. For example, on the rst day the
amount of vitamin C is60.85mg/100g, on last day it is
0.55mg/100g, the average is 27.094mg/100g, so we
can see a highdispersionof 19.52fromtheaverage.In
Table 1. Representation of the parameters from Student’s t-test
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
(statistical signicantly, p < 0.05)
Journal of Hygienic Engineering and Design
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 themeanand 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 dierence in comparison to
the variance (i.e. the range over which expression values
fall) will tell us whether this expression dierence is sig-
nicant or not. Therefore, if the dierence is large but the
variance is also large, then the dierence may not be sig-
nicant. On the other hand, a small dierence coupled
with a very small variance could be signicant.
We used the t-test for two groups to formalise this
calculation. The tests return a p-value that takes into
account the mean dierence 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 dierence exists.
Therefore, a small p-value indicates that there is a small
chance of getting this data if no real dierence exists
and therefore you decide that the dierence in the
group expression data is signicant. A p-value is said to
be signicant if it is less than the level of signicance,
which is commonly 5%(p < 0.05).
From the results of both t-tests there are statistical signi-
cantly dierences betweenthetwo groupsofsamples:
• Room stored strawberries andcooled storedstraw-
berries, t =-3.39428; p = 0.00968 < 0.05
• Room stored strawberries and frozen storedstraw-
berries, t =-4.97877; p = 0,00209< 0.05.
In the rst t-test(room stored andcool stored), levels
ofvitamin C are reduced in both casesbut there are
statistical signicant dierences in reducing the level
ofvitaminC in room stored then level ofvitaminC in
cooling stored strawberries. Cooled stored strawber-
ries keep a statistically signicant higher amount of
vitamin C than room stored strawberries.
In the second t-test(room stored and frozenstored),
the levelsof vitamin C are reduced in both casesbut
there are statistically signicant dierences in reduc-
ing the levelof vitamin Cin room stored rather than
the level of vitamin C in frozen stored strawberries.
Frozen stored strawberries are kept statistically with
a signicantly higher amount of C vitamin than room
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 isnecessary for strawberries to be kept for the
longest period. Then we also get very good results for
the level of the amount of vitamin Cwhen strawberries
are stored frozen.
- It is determined the amount of vitamin C in straw-
berries which are stored at three dierent 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 coecient is stronger for straw-
berries stored at room temperature and freezing
(r = 0.93871402), but the correlation coecient 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
stronglineardependencebetween thetwo sets ofdata.
In our case a very strong linear dependenceofdata of
the amount of vitamin C inroom stored (as a rst vari-
able) andthedata of the amount of vitamin C incooled
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-
sionappliesfor room stored (as a rst variable) andfro-
zen stored strawberries (as a second variable), where
bythe level of content of vitamin C is reduced also.
-But most importantly is that with the Student t-test
we statistically signicant proved the dierences be-
tween the samples. This means with numerical and
statistical analyses we conrmed our experiment.
We have a highestvariation 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 signicant dierences are so much lower
that0.05in both cases, in the rst case:room andcool
storedstrawberries, and in the other case, room and
-With a strong correlation coecient, and when the
dierence is statisticallysignicant, we can state that
it is likely that the dierence is not caused by this ran-
dom uctuation. Therefore we conrm it is best for
strawberries be stored under refrigeration.
Journal of Hygienic Engineering and Design
 Anitra C. C., Balz F. (1999).Toward a new recommended
dietary allowance for vitamin C based on antioxidant and
health eects in humans. Am. J. Clin. Nutr., 69, pp.1086-
 Giampieri F., Tulipani S., Alvarez-Suarez J.M., Quiles J. L.,
Mezzetti B., Battino M. (2012). The strawberry: Compo-
sition, nutritional quality, and impact on human health.
Nutrition, 28, pp. 9-19.
 Nunes M. C. N., Brecht J. K., Morais A. M. M. B., Sargent
S. A. (1998). Controlling temperature and water loss to
maintain ascorbic acid levels in strawberries during post-
harvest handling. Journal of Food Science, 63, (6), pp.
 Davey M. W., Montagu M. V., Inze D., Sanmartin M.,
Kanellis A., Smirno N., Benzie I., Strain J., Favell D.,
Fletcher J. (2000). Plant L-ascorbic acid: chemistry, func-
tion, metabolism, bioavailability and eects of processing.
J. Sci. Food Agric., 80, pp. 825-860.
 Zhao Y. (2007). Berry Fruit, Value-Added Products for
Health Promotion. CRC Press, Washington, DC, USA.
 Pavlovska G., Tanevska S. (2013). Inuence of tempera-
ture and humidity on the degradation process of ascorbic
acid in Vitamin C chewable tablets. J. Therm. Anal. Calo-
rim., 111, (3), pp. 1971-1977.
 Bode A. M., Cunningham L., Rose R. (1990). Spontaneous
decay of oxidized ascorbic acid (dehydro-L-ascorbic acid)
evaluated by high pressure liquid chromatography. Clin.
Chem., 36, pp. 1807-1809.
 AOAC International. (2002). Ocial Methods of Analysis
of AOAC International. AOAC International, Secs., 967.21,
 Newbold P., Carlson W., Thorne B. (2007). Statistic for
business and economist. Pearson Education, Inc., New
 Levine D., Stephan D., Krehbiel T., Berenson M. (2008).
Statistics for Managers Using Microsoft Excel, Pearson
Prentice-Hall, New Jersey, USA.