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A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and Grape Seed Oil against Domestic Deep Frying

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The aim of this study was to evaluate the relative stability of extra virgin olive oil (EVOO), virgin coconut oil (VCO) and grape seed oil (GSO) against domestic deep frying. Oil samples were subjected to deep frying at 190 °C for 30, 60, and 90 min and then compared with fresh oil samples in terms of fatty acid composition, peroxide value (PV), p-anisidine value (p-AV), total oxidation value (TOTOX), iodine value (IV), free fatty acid content (%FFA) and total phenolic content (TPC). Experimental results showed that the changes in the fatty acid composition, p-AV and TOTOX were in the order, GSO > EVOO > VCO throughout the experiment, while PV was in the order, VCO > EVOO > GSO. Meanwhile, the reduction in the IV was in the order, GSO > VCO > EVOO throughout the experiment. On the other hand, the changes in the %FFA were in the order, VCO > GSO > EVOO throughout the experiment. VCO had the greatest stability against domestic deep frying, followed by EVOO and GSO had the least stability against domestics deep frying.
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Journal of Food Science and Engineering 4 (2014) 71-81
A Comparative Study of Stability of Extra Virgin Olive Oil,
Virgin Coconut Oil and Grape Seed Oil against Domestic
Deep Frying
Nyam Kar Lin and Chew Kin Ken
Department of Food Science and Nutrition, Faculty of Applied Sciences, UCSI University, Kuala Lumpur 56000, Malaysia
Received: November 8, 2013 / Published: February 20, 2014.
Abstract: The aim of this study was to evaluate the relative stability of extra virgin olive oil (EVOO), virgin coconut oil (VCO) and
grape seed oil (GSO) against domestic deep frying. Oil samples were subjected to deep frying at 190 °C for 30, 60, and 90 min and then
compared with fresh oil samples in terms of fatty acid composition, peroxide value (PV), p-anisidine value (p-AV), total oxidation
value (TOTOX), iodine value (IV), free fatty acid content (%FFA) and total phenolic content (TPC). Experimental results showed that
the changes in the fatty acid composition, p-AV and TOTOX were in the order, GSO > EVOO > VCO throughout the experiment,
while PV was in the order, VCO > EVOO > GSO. Meanwhile, the reduction in the IV was in the order, GSO > VCO > EVOO
throughout the experiment. On the other hand, the changes in the %FFA were in the order, VCO > GSO > EVOO throughout the
experiment. VCO had the greatest stability against domestic deep frying, followed by EVOO and GSO had the least stability against
domestics deep frying.
Key words: EVOO, VCO, GSO, oxidative stability, domestic deep frying.
1. Introduction
Vegetable oils become dominant in market partly
due to increasing of public awareness about the
negative health effects of the consumption of saturated
fat and cholesterol. Unlike animal fats, vegetable oils
contain no cholesterol, generally low in saturated fat,
and are a good source of Vitamin E. With such
composition, the consumption of vegetable oils has
beneficial effects on human health, such as reducing
the risk of cardiovascular disease and lowering blood
low density lipoprotein (LDL) cholesterol level [1].
However, an important issue regarding the stability
of vegetable oils under high temperature is raised when
they are served as frying oils in the domestic deep
frying process. Under these conditions, vegetable oils
are difficult to be stabilized because of their high
Corresponding author: Nyam Kar Lin, Ph.D., assistant
professor, research field: fats and oils chemistry. E-mail:
nyamkl@ucsiuniversity.edu.my.
content of unsaturated fatty acids [2]. Unsaturated fatty
acids are vulnerable to oxidation because this type of
fatty acids contains at least one double bond where
chemical reactions can be easily taken place [3]. As a
result, various chemical reactions such as oxidation and
hydrolysis of the components in vegetable oils will be
performed rapidly during domestic deep frying [4].
Oxidation and degradation of different components in
vegetable oils can result a variety of substances to be
formed, including volatile carbonyls, hydroxyl acids,
keto acids and epoxy acids [5]. Some of these degraded
products have harmful effects on human health as they
can destroy vitamins, inhibit enzymes and potentially
cause mutations or gastrointestinal irritations [6]. Apart
from that, some of the degraded products can also cause
unpleasant odour and flavour in the deteriorated
vegetable oils [7]. Although frying oils are not the main
products in the domestic deep frying process, the quality
and composition of frying oils will definitely affect the
D
DAVID PUBLISHING
A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and
Grape Seed Oil against Domestic Deep Frying
72
fried foods. It is because frying oils will migrate into
foods during domestic deep frying and their content in
fried foods can be ranged from 5% to 40% [8]. Hence,
the stability of vegetable oils upon domestic deep frying
is a key concern when it comes into the criteria of
selecting of vegetable oils for domestic deep frying.
To date, many studies have been carried out to
evaluate the stability of various vegetable oils (such as
olive oil, sunflower oil, soybean oil and corn oil)
against deep frying [9]. However, there is no much
studies which have been done on the stability of virgin
coconut oil (VCO), extra virgin olive oil (EVOO) and
grape seed oil (GSO) against deep frying and thermal
treatment. Besides that, the comparative study of the
stability of these vegetable oils against domestic deep
frying also has not been done in any study.
2. Materials and Methods
2.1 Materials
EVOO, VCO and GSO were purchased from local
supermarket, Cheras, Malaysia.
2.2 Methods
2.2.1 Deep Frying Experiment
All deep frying experiments were performed using
the same electrical deep fryer equipped with thermostat
(Philips, Cucina DH 6151, China). Appropriate amount
of French fries were removed from freezer and
defrosted in chiller (3 °C) for 2 h before the deep frying
experiment was carried out. For each deep frying
experiment, 500 ± 1 mL of vegetable oil was preheated
for about 15 min to reach 190 °C. Then, 30 ± 1 g of
French fries were fried for 5 min and removed.
Subsequently, the oil sample was heated for another 5
min without French fries. The procedures of frying
French fries for 5 min and heating vegetable oil without
French fries for 5 min were referred as one frying cycle.
Every type of vegetable oils was subjected to deep
frying for three different total frying cycles, which
were three, six and nine total frying cycles, equivalent
to the periods of 30, 60 and 90 min, respectively.
Throughout the frying experiment, deep fryer lid was
opened to induce oxidative deterioration of oil sample
by allowing it to react with the atmospheric oxygen. All
the deep frying experiments were done in replicate.
2.2.2 Analyses
Iodine value (IV), free fatty acid content (%FFA),
peroxide value (PV) and p-anisidine value (p-AV)
were determined according to AOAC [10]. While total
oxidation value (TOTOX) was determined according
to the following equation:
TOTOX = 2 × PV + p-AV (1)
2.2.3 Fatty Acid Composition
The fatty acid composition was determined by
conversion of oil to fatty acid methyl esters prepared by
adding 950 μL of n-hexane to 50 mg of oil followed by
50 μL of 30 mL/100 mL sodium methoxide in
methanol [11]. The mixtures were vortexed for 5 s and
allowed to settle for 5 min. The top layer (1 μL) was
injected into a gas chromatograph (Hewlett-Packard
Model 5890 instrument (Palo Alto, CA, USA)),
equipped with a flame-ionization detector (FID) and a
Hewlett-Packard Model 3392A integrator. A polar
capillary column BPX70 (0.32 mm internal diameter,
30 m length and 0.25 m film thickness; SGE
International Pty. Ltd., Victoria, Australia) was used at
a column head pressure of 10 psi. Helium (99.995%) at
approximately 23 mL/min (measured at oven
temperature 150 C) was used as the carrier gas, and
nitrogen (99.999%) at 20 mL/min was used as the
makeup gas. The FID and injector temperatures were
both maintained at 220 C. The initial column oven
temperature was 115 C, temperature programmed to
180 C at 8 C/min and held at this temperature until
the analysis was completed. FAME peaks were
identified by comparison of retention times to a
Supelco 37 component FAME mix (obtained from
Sigma-Aldrich, St. Louis, US). The peak areas were
computed, and percentages of FAME were obtained as
area percentages by direct normalization (the data are
expressed as normalized percent of all identified
A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and
Grape Seed Oil against Domestic Deep Frying
73
FAME). Only the more abundant FA (> 0.2%) was
considered. All analyses were carried out in triplicate.
2.2.4 Total Phenolic Content (TPC)
Extraction of phenolic compounds from oil samples
was done according to the method described by
Rotondi et al. [12] with some modification. 5.0 ± 0.1 g
of oil sample was weighted into 50 mL Falcon tube. 5
mL of hexane was then added to dissolve oil sample by
means of vortex. The mixture was extracted with 10
mL of methanol:water (60:40, v/v). The mixture was
vortexed for 5 min and subsequently centrifuged at
4,500 rpm for 5 min. The methanolic phase was
collected and the hexane phase was re-extracted twice
with 10 mL of methanol:water (60:40, v/v) each time.
The combined methanolic fractions from three
extractions were subjected to final washing with 10 mL
of hexane to eliminate residual oil sample in a
separating funnel. The methanolic fractions were
evaporated under reduced pressure at 40 °C until
dryness using rotary evaporator. The residue was then
reconstituted with 8 mL of methanol:water (60:40, v/v)
for EVOO sample and 2 mL of methanol:water (60:40,
v/v) for GSO and VCO sample.
2.2.5 TPC Assay of Methanolic Extract
TPC of methanolic extract was determined in the
same way as gallic acid standards by replacing the
gallic acid standard with methanolic extract. The result
was expressed as mg of gallic acid equivalent per 100 g
of oil sample (mg GAE/100 g of oil) and calculated
based on the equation below:
100
17.18
Ab V
TPC Wt

(2)
where, Ab = absorbance of methanolic extract at 765
nm; wt = weight of oil sample (g) and V = volume of
methanol:water (60:40, v/v) used to reconstitute the
extract after evaporating until dryness (mL).
2.3 Statistical Analysis
All the deep frying experiments and preparation of
fresh oil samples were done in replicate sets while
analyses were performed in triplicate. The
experimental results were recorded as mean ± standard
deviation. The results were analysed using one-way
analysis of variance (ANOVA) by MINITAB software
(Version 14.1.1.0), in which frying period was the
factor while the values of experimental results were
chosen as responses. The significant differences (P <
0.05) among the results were determined along with the
Tukey’s test in this study.
3. Results and Discussion
3.1 Changes in IV of Oil Samples during Deep Frying
Experiment
The changes in the IV of EVOO, VCO and GSO
throughout the deep frying experiment are presented in
Fig. 1. Oxidation and degradation of PUFA and MUFA
would be taken place when oil samples were subjected
to thermal treatment [9]. The loss of PUFA and MUFA
in oil samples caused a reduction in the level of
unsaturated double bonds.
Among these three different types of oil samples,
GSO experienced the greatest loss of IV throughout the
deep frying experiment, followed by VCO. EVOO
experienced the least loss of IV during the deep frying
experiment, in which its IV had no significant decrease
(P < 0.05) in the first 60 min of deep frying experiment.
EVOO only experienced a significant loss (P < 0.05) of
IV after subjecting it to deep frying for 90 min, in
which it lost a total IV of 4.1 g I2/100 g of oil. These
results also indicated that GSO experienced the
greatest loss of unsaturated double bonds, followed by
VCO, and EVOO experienced the least loss of
unsaturated double bonds during the deep frying
experiment.
GSO experienced the greatest loss in its IV during
the deep frying experiments due to its consisting of
higher percentage of PUFA but lower percentage of
MUFA and SFA than EVOO and GSO. Oils with high
IV are prone to oxidise and degrade more rapidly than
those with low IV [13]. In other word, the loss of
A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and
Grape Seed Oil against Domestic Deep Frying
74
Fig. 1 IV of EVOO, VCO and GSO before and after deep frying for 30, 60 and 90 min.
( EVOO; GSO; VCO)
Each value from the figure represents the mean (n = 3), whereas the error bar represents the standard deviation. Means within each type
of vegetable oils bearing with different superscripts show significantly different (P < 0.05).
unsaturated double bonds in oils with high IV would be
performed more rapidly than those with low IV during
the deep frying experiment. However, the changes in
the IV of VCO and EVOO during the deep frying
experiment showed contradiction to this concept.
EVOO (which had higher IV than VCO) experienced
lesser loss of IV than VCO throughout the deep frying
experiment. This circumstance could be due to VCO
had lower phenolic content and higher level of free
fatty acid than EVOO, as evidenced by oil samples’
TPCs that are shown in Table 1 and oil
samples’ %FFAs that are shown in Fig. 2. With low
level of phenolic compounds in VCO, the antioxidant
activity within VCO may not be effective to protect
PUFA and MUFA from oxidation and degradation
during the deep frying experiment. In addition, high
content of free fatty acids in VCO could also accelerate
the oxidation and degradation of PUFA and MUFA
during deep frying experiment. This is due to the
carbonyl group on free fatty acid had catalytic effect on
the formation of free radicals by decomposition of
hydroperoxides [14]. All these factors might work
together to reduce the stability of VCO and resulted in
a greater reduction in its degree of unsaturation as
compared to EVOO during the deep experiment.
Additionally, a reduction in the rate of the loss of IV
was found in VCO as the deep frying experiment was
progressed to 90 min. This phenomenon could be due
to the availability of unsaturated fatty acids in VCO
become limited as the deep frying experiment was
performed to 90 min. Based on the fatty acid
composition of fresh VCO that is shown in Table 2,
VCO contained limited unsaturated fatty acids, in
which these fatty acids only accounted 5.63% of the
total fatty acids content.
3.2 Changes in %FFA of Oil Samples during Deep
Frying
The changes in the %FFA of EVOO, VCO and GSO
throughout the deep frying experiment are showed in
Fig. 2. Generally, GSO experienced gradual increase in
its %FFA while EVOO experienced no significant
change (P < 0.05) in its %FFA as the progression of
deep frying experiment. The %FFA of VCO was
observed to be substantial decrease in the first 30 min
of the deep frying experiment and the decreasing
of %FFA in VCO became gradual after 30 min of the
deep frying experiment.
A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and
Grape Seed Oil against Domestic Deep Frying
75
Table 1 TPC of EVOO, GSO and VCO before and after deep frying for 30, 60 and 90 min (n = 3)*.
Time of deep frying (min) Total phenolic content (mg GAE/100 g of oil)**
EVOO GSO VCO
0 (Fresh) 15.84 ± 0.56a 2.40 ± 0.04a0.66 ± 0.02a
30 7.54 ± 0.32
b
1.14 ± 0.05
b
0.51 ± 0.03
b
60 5.44 ± 0.16c 0.85 ± 0.14c0.49 ± 0.02
b
90 3.97 ± 0.14
d
0.60 ± 0.13
d
0.31 ± 0.03c
Total loss (%) 74.94 75.00 53.03
*Replicate.
**Each value from the table represents the mean ± standard deviation. Means within each column with different superscripts are
significantly different (P < 0.05).
Fig. 2 %FFA of EVOO, VCO and GSO before and after deep frying for 30, 60 and 90 min.
( EVOO; GSO; VCO)
Each value from the figure represents the mean (n = 3), whereas the error bar represents the standard deviation. Means within each type
of vegetable oils bearing with different superscripts show significantly different (P < 0.05).
The %FFA of EVOO was observed to be no
significant change (P < 0.05) throughout the deep
frying experiment. One possible reason that EVOO
experienced no significant change (P < 0.05) in
its %FFA throughout the deep frying experiment could
be due to it had high antioxidant content, particularly
phenolic compounds, as evidenced by its TPC that is
shown Table 1. Therefore, the presence of high
phenolic compounds in EVOO might reduce the
formation rate of FFA from the hydrolysis of TG
and/or decomposition of aldehydes into certain
threshold, in which no significant increase (P < 0.05)
in %FFA was observed throughout the 90 min deep
frying experiment.
When vegetable oils are subjected to thermal
treatment in the presence of moisture from foods,
hydrolysis must be taken place [15]. Therefore,
experiencing a reduction in %FFA in VCO throughout
the deep frying experiment suggested that the
degradation of FFA occurred at the same time of the
formation of FFA from hydrolysis. Moreover, the
degradation of FFA performed faster than the
formation of FFA from hydrolysis. This circumstance
could be due to VCO had an unusual high initial %FFA.
Conversely, VCO had a %FFA of 1.03% oleic acid
equivalent. High %FFA in VCO may accelerate the
A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and
Grape Seed Oil against Domestic Deep Frying
76
Table 2 Fatty acid composition of (a) EVOO, (b) VCO, (c) GSO before and after deep frying for 30, 60 and 90 min (n = 3)*.
(a)
Fatty acid Percentage of fatty acid (%)**
Fresh oil sample Deep frying for 30 min Deep frying for 60 min Deep frying for 90 min
Palmitic acid (C16:0) 14.09 ± 0.28
b
14.44 ± 0.13
b
14.90 ± 0.77ab 15.76 ± 0.27a
Palmitoleic acid (C16:1) 1.26 ± 0.07
b
1.54 ± 0.02a1.61 ± 0.11a1.41 ± 0.19ab
Stearic acid (C18:0) 2.14 ± 0.10a 2.32 ± 0.29a2.59 ± 0.20a2.33 ± 0.19a
Oleic acid (C18:1) 71.20 ± 4.38a 72.02 ± 3.39a71.67 ± 3.74a71.82 ± 3.83a
Linoleic acid (C18:2) 11.31 ± 1.14a 9.68 ± 0.78
b
9.24 ± 0.24
b
8.68 ± 0.15
b
SFA 16.23 ± 0.26c 16.78 ± 0.41
b
c17.49 ± 0.73ab 18.09 ± 0.21a
MUFA 72.42 ± 1.19a 73.50 ± 0.81a73.27 ± 0.49a73.23 ± 0.11a
PUFA 11.35 ± 1.14a 9.71 ± 0.78
b
9.24 ± 0.24
b
8.68 ± 0.15
b
(b)
Fatty acid Percentage of fatty acid (%)**
Fresh oil sample Deep frying for 30 min Deep frying for 60 min Deep frying for 90 min
Caprylic acid (C8:0) 8.82 ± 0.05a 8.46 ± 0.13
b
8.77 ± 0.11a8.65 ± 0.14a
Capric acid (C10:0) 6.97 ± 0.10a 6.88 ± 0.06a6.90 ± 0.07a6.90 ± 0.03a
Lauric acid (C12:0) 50.27 ± 0.31a 50.19 ± 0.41a50.31 ± 0.33a49.47 ±0.20a
Myristic acid (C14:0) 17.88 ± 0.03a 17.99 ± 0.16a18.01 ± 0.08a17.77 ± 0.19a
Palmitic acid (C16:0) 7.82 ± 0.12
b
8.32 ± 0.07
b
8.40 ± 0.15
b
9.07 ± 0.54a
Stearic acid (C18:0) 2.61 ± 0.22a 2.89 ± 0.21a2.80 ± 0.23a2.99 ± 0.15a
Oleic acid (C18:1) 4.40 ± 0.33a 4.29 ± 0.18a4.00 ± 0.29a4.25 ± 0.29a
Linoleic acid (C18:2) 1.23 ± 0.07a 0.98 ± 0.04
b
0.81 ± 0.08c0.60 ± 0.11
d
SFA 94.36 ± 0.38
b
94.73 ± 0.21ab 95.19 ± 0.31a95.36 ± 0.22a
MUFA 4.41 ± 0.33a 4.29 ± 0.18a4.00 ± 0.29a4.03 ± 0.29a
PUFA 1.23 ± 0.07a 0.98 ± 0.04
b
0.81 ± 0.08c0.61 ± 0.11
d
(c)
Fatty acid Percentage of fatty acid (%)**
Fresh oil sample Deep frying for 30 min Deep frying for 60 min Deep frying for 90 min
Palmitic acid (C16:0) 7.08 ± 0.20
b
7.64 ± 0.17
b
7.77 ± 0.28
b
9.34 ± 0.83a
Stearic acid (C18:0) 3.57 ± 0.65a 4.11 ± 0.04a4.10 ± 0.05a4.24 ± 0.13a
Oleic acid (C18:1) 18.65 ± 0.87a 19.62 ± 0.36a21.49 ± 1.48a21.09 ± 1.93a
Linoleic acid (C18:2) 70.70 ± 0.60a 68.63 ± 0.56ab 66.64 ± 3.25ab 65.33 ± 2.77
b
SFA 10.65 ± 0.52c 11.75 ± 0.19
b
11.86 ± 0.23
b
13.58 ± 0.71a
MUFA 18.68 ± 0.87a 19.62 ± 0.32a21.51 ± 1.58a21.08 ± 1.60a
PUFA 70.67 ± 0.60a 68.63 ± 0.51ab 66.63 ± 1.25
b
65.34 ± 2.30
b
*Replicate;
**Each value from the table represents the mean ± standard deviation. Means within each row with different superscripts are
significantly different (P < 0.05).
oxidation and degradation of different component
within it, including FFA themselves (14). Although the
formation of FFA from hydrolysis was taken place, the
degradation of FFA by oxidation and other degradation
reactions in VCO might be performed more rapidly.
Hence, a substantial reduction in the %FFA of VCO
was observed in the first 30 min of deep frying
experiment. However, as the deep frying experiment
was progressed to 90 min, the rate of the reduction
of %FFA in VCO was observed to be generally
reduced. This trend could be due to the catalytic effects
of FFA on the oxidation and degradation of FFA was
reduced as the level of FFA in VCO was reducing with
periods of deep frying. As a result, the loss of %FFA in
VCO became gradual as the progression of deep frying
experiment.
A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and
Grape Seed Oil against Domestic Deep Frying
77
3.3 Changes in PV and p-AV of Oil Samples during
Deep Frying
The changes in the PV and p-AV of EVOO, GSO
and VCO throughout the deep frying experiment are
shown in Figs. 3 and 4, respectively. In general, both
PVs and p-AVs of all oil samples were increased with
periods of deep frying experiment.
VCO was observed to have the highest PV, followed
by EVOO and GSO had the lowest PV throughout the
deep frying experiment (Fig. 3). In terms of p-AV, all
oil samples experienced substantial increases in p-AV
after using for deep frying for 30 min (Fig. 4). In the
subsequent 60 min of the deep frying experiment, the
p-AVs of both EVOO and GSO were increased with
periods of deep frying; whereas the p-AV of VCO was
observed to be no significant change (P < 0.05), in
which its p-AV was remained at a value around 5.50.
Apart from that, VCO was observed to have the lowest
p-AV, followed by EVOO and GSO had the highest
p-AV among all oil samples throughout the deep frying
experiment.
In most situations, vegetable oils with high PV are
accompanied with high p-AV and this condition
indicates that the vegetable oils are highly oxidised
[16]. However, the PV and p-AV of VCO showed a
contradiction to their findings when its was compared
to the PVs and p-AVs of EVOO and GSO during the
deep frying experiment. Throughout the deep frying
experiment, VCO had higher PV than EVOO and GSO,
but its p-AV was lower than EVOO and GSO. Similar
situation was also observed in the case of EVOO and
GSO. EVOO had higher PV than GSO but its p-AV
was lower than GSO throughout the deep frying
experiment.
Based on the fatty acid compositions of oil samples
that are shown in Table 2, VCO had the lowest
percentage of unsaturated fatty acids (5.64%), followed
by EVOO (83.77%), and GSO had the highest
percentage of unsaturated fatty acids (89.35%).
Therefore, during the deep frying experiment, VCO
might experience the lowest decomposition rate of
hydroperoxides, followed by EVOO and GSO. When
Fig. 3 PV of EVOO, VCO and GSO before and after deep frying for 30, 60 and 90 min.
( Fresh; 30 min; 60 min; 90 min)
Each value from the figure represents the mean (n = 3), whereas the error bar represents the standard deviation. Means within each type
of vegetable oils bearing with different superscripts show significantly different (P < 0.05).
A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and
Grape Seed Oil against Domestic Deep Frying
78
Fig. 4 p-AV of EVOO, VCO and GSO before and after deep frying for 30, 60 and 90 min.
( Fresh; 30 min; 60 min; 90 min)
Each value from the figure represents the mean (n = 3), whereas the error bar represents the standard deviation. Means within each type
of vegetable oils bearing with different superscripts show significantly different (P < 0.05).
the decomposition of hydroperoxides was performed
slowly in oil samples, high level of hydroperoxides
would be retained while low level of aldehydes would
be formed from the decomposition of hydroperoxides
or vice versa. As a result, VCO had the highest level of
hydroperoxides but the lowest level of aldehydes,
followed by EVOO, and GSO had the lowest level of
hydroperoxides but the highest level of aldehyes during
the deep frying experiment.
3.4 Changes in TOTOX of Oil Samples during Deep
Frying
The changes in the TOTOX of EVOO, GSO and
VCO throughout the deep frying experiment are
presented in Fig. 5. Generally, the TOTOXs of all oil
samples were increased with periods of deep frying.
Among three different oil samples, VCO had the
lowest TOTOX throughout the deep frying
experiment, followed by EVOO and GSO. These
results indicated that VCO experienced the least
degree of oxidation, followed by EVOO and GSO
experienced the greatest degree of oxidation during
the deep frying experiment.
The phenomena of having different TOTOX in
different oil samples during the deep frying experiment
could be explained based on oil samples’ degree of
unsaturation. GSO had the highest degree of
unsaturation, followed by EVOO and VCO had the
lowest degree of unsaturation (Fig. 1). Therefore,
during the deep frying experiment, the oxidation
reaction in GSO would be performed most rapidly,
followed by EVOO and the oxidation reaction in VCO
would be performed in the lowest rate.
3.5 Changes in the TPC of Oil Samples during Deep
Frying Experiment
The TPCs of EVOO, GSO and VCO at different
intervals of the deep frying experiment are recorded in
Table 1. Basically, the TPCs of all oil samples were
decreased with periods of deep frying. This
circumstance was possible due to the phenolic
compounds in oil samples were destroyed by thermal
destruction and/or contributing antioxidant activity to
the oil samples for purpose of preventing and/or
slowing down the oxidative reactions that were carried
out in oil samples.
A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and
Grape Seed Oil against Domestic Deep Frying
79
Fig. 5 Total oxidation values of EVOO, VCO and GSO before and after deep frying for 30, 60 and 90 min.
( Fresh; 30 min; 60 min; 90 min)
Each value from the figure represents the mean (n = 3), whereas the error bar represents the standard deviation. Means within each type
of vegetable oils bearing with different superscripts show significantly different (P < 0.05).
The percentage losses of TPC in EVOO, GSO and
VCO at different intervals of deep frying experiment
were calculated in this study in order to determine the
relative losses of TPC among three different oil
samples during the deep frying experiment. As
observed, the percentage losses of TPC in EVOO and
GSO were almost the same and greater than VCO
throughout the deep frying experiment. Among all oil
samples, VCO experienced the least percentage loss of
TPC at the end of deep frying experiment.
Experiencing a great percentage loss in the TPC of
EVOO could be due to phenolic compounds which
were the major antioxidants that contributed to the
oxidative stability of EVOO during deep frying
experiment. Phenolic compounds and tocopherols are
the main antioxidants that are presented in virgin olive
oil [17]. However, Baldioli et al. [18] revealed that the
level of phenolic compounds in virgin olive oil was
correlated (r = 0.97) with its oxidative stability but the
level of tocopherol showed a low correlation (r = 0.05)
with its oxidative stability. In other words, phenolic
compounds might be the main natural antioxidants that
contributed to the stability of virgin olive oil. Therefore,
during the deep frying experiment, phenolic
compounds might be decomposed rapidly by
contributing antioxidant activity to EVOO for the
purpose of preventing and delaying the oxidative
deterioration of EVOO. As a result, the percentage loss
of TPC in EVOO was very high during the deep frying
experiment.
On the other hand, experiencing different percentage
losses of TPC in GSO and VCO during the deep frying
experiment could be due to GSO and VCO experienced
different degree of oxidation during the deep frying
experiment. When the oxidation reactions were
performed rapidly in oil samples, the rate of the
formation of free radicals in oil samples would also be
increased or vice versa [19]. Therefore, the formation
of free radicals in GSO would be performed most
rapidly while the formation of free radicals in VCO
would be performed most slowly during the deep
frying experiment. Since phenolic compounds can act
as primary antioxidants by scavenging the free radicals,
experiencing high formation rate of free radicals in oil
A Comparative Study of Stability of Extra Virgin Olive Oil, Virgin Coconut Oil and
Grape Seed Oil against Domestic Deep Frying
80
samples would destroyed the phenolic compounds in
oil samples rapidly or vice versa. As a result, GSO
experienced a great percentage loss in TPC while VCO
experienced less percentage loss in TPC during the
deep frying experiment.
3.6 Changes in the Fatty Acid Composition of Oil
Samples during Deep Frying Experiment
The changes in the fatty acid composition of EVOO,
VCO and GSO during deep frying experiment are
shown in Table 2. Overall, the fatty acid compositions
of all oil samples were observed to be change after
subjecting oil samples to deep frying for different
periods.
VCO was found to experience the least change in
term of fatty acid composition, while GSO generally
experienced the greatest change in its fatty acid
composition throughout the deep frying experiment.
The change in the fatty acid composition of EVOO was
generally observed to be greater than VCO but smaller
than GSO throughout the deep frying experiment.
The phenomena of experiencing different degree of
changes in the fatty acid compositions in different oil
samples during the deep frying experiment could be
due to oil samples had different percentage of heat
sensitive fatty acids (unsaturated fatty acids) and
different percentage of heat stable fatty acids (SFA).
VCO was found to have the lowest percentages of
unsaturated fatty acids (5.64%), followed by EVOO
(83.77% of unsaturated fatty acids) and GSO was
found to have the highest percentages of unsaturated
fatty acids (89.35%). Therefore, during the deep frying
experiment, having the lowest percentage of
unsaturated fatty acids in VCO might prevent it to
experience the most drastic change in its fatty acid
composition, followed by EVOO and GSO, which had
the highest percentage of unsaturated fatty acids, might
allow it to experience the greatest change in its fatty
acids composition. Additionally, although VCO had
higher percentage of SFA than EVOO and GSO, while
EVOO had higher percentage of SFA than GSO, SFA
are more resistant to oxidation and degrade less readily
under the conditions of deep frying. Therefore, during
the deep frying experiment, the loss of SFA in VCO
might not sufficient to allow it to experience a great
change in its fatty acid composition as compared to
EVOO and GSO, while the loss of SFA in EVOO
might also not enough to produce a drastic change in its
fatty acid composition as compared to GSO.
Among these three different types of vegetable oils,
VCO is the most suitable for the use of domestic deep
frying, followed by EVOO, and GSO is the most
unsuitable for the use of domestic deep frying.
4. Conclusions
In conclusion, VCO had the greatest stability against
domestic deep frying, followed by EVOO, and GSO
had the least stability against domestics deep frying. In
other words, among these three different types of
vegetable oils, VCO is the most suitable for the use of
domestic deep frying, followed by EVOO, and GSO is
the most unsuitable for the use of domestic deep frying.
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