Physicochemical and Antioxidant Properties of Oils Used by Local Fried Food Vendors in D/Line-Port Harcourt, Rivers State

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© 2020 by the authors; licensee Asian Online Journal Publishing Group
Agriculture and Food Sciences Research
Vol. 7, No. 1, 89-96, 2020
ISSN(E) 2411-6653/ ISSN(P) 2411-6653
DOI: 10.20448/journal.512.2020.71.89.96
© 2020 by the authors; licensee Asian Online Journal Publishing Group
Physicochemical and Antioxidant Properties of Oils Used by Local Fried Food
Vendors in D/Line-Port Harcourt, Rivers State
Emelike, N.J.T1
Ujong A. E.2

Achinewu, S. C.3
(

Corresponding Author)
1,2,3Department of Food Science and Technology, Rivers State University, NkpoluOroworukwo, Port Harcourt,
Rivers State, Nigeria.
Abstract
Frying initiates different physical and chemical changes which can cause degradation of oil and
quality of the fried food. This study was conducted to investigate and monitor the
physicochemical and antioxidant properties of frying oils used by local fried food vendors in
D/line-port Harcourt, Rivers State. A total of five frying oils were collected randomly during
frying operations from the study area and analyzed at weekly intervals (3 weeks) for
physicochemical and antioxidant properties. The results showed that all the frying oils collected
were above the permissible limit for free fatty acid and saponfication value while peroxide values
were within the Codex regulatory limit after three weeks of collection. The results also revealed a
decrease in iodine values of the frying oils except for oil samples collected from Agudama Street
and Railway close. Moisture content of all the frying oils was below the 0.3% maximum limit
while smoke points were not in line with the recommended standard as all the oils had smoke
points <170oC even up to the third week of collection. Increased usage of the oils during frying
also resulted to a decrease in total phenolic content except for frying oil collected from Kaduna
Street while a reverse was observed for total flavonoid content. The degradation in the quality of
oils used by local fried food vendors in D/line, Port Harcourt during frying operations is an
important health issue which could cause damaging health effects due to the toxic substances
produced.
Keywords: Antioxidant, Physicochemical, Oil, Frying, D/line, Collection.
Citation | Emelike, N.J.T; Ujong A. E; Achinewu, S. C (2020).
Physicochemical and Antioxidant Properties of Oils Used by Local
Fried Food Vendors in D/Line-Port Harcourt, Rivers State.
Agriculture and Food Sciences Research, 7(1): 89-96.
History:
Received: 18 March 2020
Revised: 22 April 2020
Accepted: 25 May 2020
Published: 15 June 2020
Licensed: This work is licensed under a Creative Commons
Attribution 3.0 License
Publisher: Asian Online Journal Publishing Group
Acknowledgement: All authors contributed to the conception and design of
the study.
Funding: This study received no specific financial support.
Competing Interests: The authors declare that they have no conflict of
interests.
Transparency: The authors confirm that the manuscript is an honest,
accurate, and transparent account of the study was reported; that no vital
features of the study have been omitted; and that any discrepancies from the
study as planned have been explained.
Ethical: This study follows all ethical practices during writing.
Contents
1. Introduction ...................................................................................................................................................................................... 90
2. Materials and Methods ................................................................................................................................................................... 90
3. Results and Discussion ................................................................................................................................................................... 91
4. Conclusion ......................................................................................................................................................................................... 95
References .............................................................................................................................................................................................. 95

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Contribution of this paper to the literature
This study contributes to literature by evaluating the level of degradation in the quality of frying oils
used by local fried food vendors in D/line, Port Harcourt during frying operations.
1. Introduction
Frying is one of the most common and longstanding culinary techniques used for preparation of foods
throughout the world. It involves submerging the food in hot, liquid fat at high temperature of 150oC-190oC [1].
During this process, the fried food absorbs some amount of oil and as a result, certain proportions of degraded
products from the oil are accumulated by the fried food [2].
Frying also initiates different physical and chemical changes which results in extensive degradation. The frying
process is also open to atmospheric oxygen and high temperature which can however cause degradation in the oil
leading to unpleasant odour and flavor [3]. Several studies have shown that continuous use of vegetable oils for
frying results in degradation which affects the physical and chemical properties of the oil quality Idun-Acquah, et
al. [4]. Codex Alimentarius Commission/FAO/WHO Food Standards [5] Reported that frequent frying of
vegetable oils leads to deterioration and lipid oxidation results in the formation of peroxides which are responsible
for primary oxidation in the oil [6]. Degradation of vegetable oils during frying result in loss of nutritive value and
causes damaging health effects due to the toxic substances produced [6]. According to Codex Alimentarius
Commission/FAO/WHO Food Standards [5] continuous deep fat frying decreases the unsaturated fatty acids of
oil and increases foaming, colour, viscosity, density, specific heat and free fatty acid contents.
There has been an increase in the consumption of street fried foods in Port Harcourt metropolis resulting from
high demand for such foods and the changing lifestyle and growth in the number of working women. Food items
that are often cooked by frying include yam, potato, fish, meat, plantain etc. Quality control measures are lacking
specifically as related to the quality of oil used by these fried foods street vendors in frying. Most of them use
vegetable cooking oil numerous times for frying before discarding so as to reduce cost. The frying oil is also
infrequently discarded, with these vendors instead simply adding more as oil is absorbed by the fried food [7].
According to Flores, et al. [8] this practice decreases the rate of hydrolytic alterations and therefore mask or slow
down this type of deterioration. Mensah and Obeng [9] Reported that vegetable oils for cooking are to be used 3-6
times before being discarded as waste; however most of these fried food vendors use these oils even more than the
supposed number of times. Some use the oils for frying until the colour changes to dark while others use it until the
flavor of the product is unacceptable. Good [10] Also reported that repeated usage of vegetable oil for frying
lowers the smoke point which makes the oil to smoke on heating at a lower temperature.
The consumption of repeatedly heated vegetable oil during frying has been linked with increasing the risk of
developing atherosclerosis, total serum lipid and low density lipoprotein (LDL) levels [11, 12]. There is need to
investigate the effect of repeated frying on the quality of oils used by these fried food vendors in order to provide
basic data and knowledge of the quality of oils used for frying. Therefore, this study was aimed at determining the
physicochemical and antioxidant of vegetable oils used by local fried food vendors in D/line, Port Harcourt, Rivers
State.
2. Materials and Methods
2.1. Sample Collection
A total of five (5) oil samples (100 ml each) were collected at weekly intervals for period of three weeks from
five Streets, all situated in D/line, Port Harcourt. The streets covered were Agudama, Emekuku, Kaduna, Wogu,
and Railway close. These oil samples were repeatedly used vegetable oils collected during frying operations. Fresh
unused branded oil (Kings refined vegetable oil) and unbranded (local vegetable oil) were also collected and used as
control samples. The oil samples were transported to the biochemistry laboratory of the Department of Food
Science and Technology, Rivers State University for analysis within 1 hr after collection and checked for
physicochemical and antioxidant properties. All chemicals used for this study were of analytical grade and obtained
from the department of Food Science and Technology laboratory, Rivers State University, Port Harcourt.
2.2. Methods
2.2.1. Physico-Chemical Analysis
Iodine value, peroxide value, free fatty acid content, saponfication value, smoke point and moisture content of
the oils collected were all determined using the AOCS method [13].
2.2.2. Antioxidant Analysis
2.2.2.1. The Total Phenolic Content
Total phenolic of the oils was determined by the Folin–Ciocalteu reagent (FCR) according to the procedure
reported by Emelike, et al. [14]. The oil sample (0.3 ml) was weighed into a conical flask and 3.0ml of Folin–
Ciocalteu reagent added into it. After 5min, 6% sodium carbonate (3.0 ml) was added and the mixture allowed
standing at room temperature for 90mins. The absorbance of the mixture was measured at 725nm using uv/vis
Spectrophotometer (USA). The total phenolic content was calculated from the calibration curve and the results
were expressed as mg of gallic acid equivalent per gram of oil (mgGAE/g oil).
2.2.2.2. Total Flavonoid Content
Total flavonoid content was determined using the method of Boham and Kocipai-Abyazan [15]. The oil
samples (0.5 ml) was weighed into a conical flask and 20 ml of 80% aqueous methanol added and shake using
orbital shaker for 3 hrs. This was followed by filtering using Whatman filter paper thereafter the filterate was
transferred into a moisture can and evaporated for 1 hr at 105oC, cooled and weighed.

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2.2.3. Statistical Analysis
All analysis was performed in duplicate and data was subjected to analysis of variance (ANOVA) using SPSS
Version 23 for windows (IBM Corporation, New York, USA) according to the method of Wahua [16]. Variability
within the means was separated by using Duncan Multiple Range test (DMRT) which was defined at (p<0.05).
3. Results and Discussion
3.1. Physicochemical Properties of the Frying Oils
3.1.2. Free Fatty acid Composition
The result of free fatty acid values (%) of the frying oils collected is shown in Figure 1. The chief composition
of oils is fatty acids and the degree of unsaturation is the very first factor influencing the oxidative stability of
frying oils. In this study, the lowest FFA of 0.01% and 0.03% was recorded in samples collected at week 1 from
Wogu and Kaduna street respectively with no significant difference (p<0.05) from each other. The highest FFA of
6.90% was recorded in oil samples collected from Wogu Street at week 3. An increase in the FFA was observed
after third week of collection for Railway Street (1.46-2.92%), Kaduna Street (0.03-1.02%), Wogu Street (0.01-
6.90%) and Emekuku Street (2.34-3.77%) while that Agudama Street decreased from 2.78-1.01%.
Oil samples collected from Wogu and Kaduna Street at first week as well as the branded oil used as control had
FFA values (0.01-0.33%) within the maximum permissible FFA of 0.3% for edible oils while others were above the
range. At week 2 and 3, all the frying oils (except for branded) collected had FFA values above the specification. A
study conducted by Idun-Acquah, et al. [4] also revealed similar results with %FFA ranging from 0.62-2.41% after
5 days of frying. The differences observed in the FFA values of the frying oils are a consequence of new oil added
during frying process which dilutes these values. Blending of vegetable oils alter their fatty acid profiles [17] and
can steeply retard oxidation of oils during frying. Idun-Acquah, et al. [4] Also stated that deviation in the FFA
values may be attributed to the high temperatures attained by the oil as well as the water from the fresh food
product causing the natural fatty acids in the oil to get hydrolyzed into free fatty. According to Emelike, et al.
[14], low moisture in oil decreases the fatty acid content while high moisture leads to its increase. Increase in FFA
from this study can also be attributed to thermal and oxidative decomposition resulting to breakdown of long
carbon chains into shorter chains. Free fatty acids are also formed as a result of the cleavage and oxidation of
double bonds to form carbonyl compounds at elevated temperature heating Lalas [18]. McWilliams [19]
Reported that the release of free fatty acid results to the formation of acreolin and smoking of the oil. Acreolin is
visible as bluish and acrid smoke and its vapour may cause eye, nasal and respiratory tract irritations in low level
exposure. Acreolin also induces the respiratory, ocular and gastrointestinal irritations by inducing the release of
peptides in nerve terminals innervating these systems [20]. Therefore, the oils collected at the second and third
week may contain some amount of acreolin since their FFA exceeded the maximum specification.
This study agrees well with other studies that the FFA content of oils increases with the number of frying
cycles [21] as well as with the frying time [22]. According to Thomas [23], smoke point of oils depends greatly
on free fatty acid. The more FFA an oil contains, the quicker it will breakdown and start smoking. This result also
indicates that smoke points of the frying oils will be very low.
Figure-1. Free fatty acid values (%) of the oils collected at weekly intervals.
3.1.3. Peroxide Value of Frying Oils
The result of the peroxide values of frying oils is shown in Figure 2. For all the samples collected at week 1,
the unbranded oil used as control had the highest peroxide value (PV) of 9.47meqO2/kg while the lowest PV of
0.80meqO2/kg was observed for the branded vegetable oil used as control. The PV of the oils were significantly
(p<0.05) different from each other for all the weeks. It was observed that samples collected from Agudama,
Emekuku and Wogu streets had decreasing PV of 7.79-2.20meqO2/kg, 5.79-2.29meqO2/kg and 6.57-
3.90meqO2/kg after the 3 weeks of collection while all other samples (except the controls) increased. The decrease
in PV of these frying oils may be that the oils were changed to fresh oils at point of collection at week 2 and 3. The
increase in the PV of the other samples following frying is because of the oxidation of carbon atoms adjacent to the
double bonds in the triacylglyceride structure leading to the formation of hydroperoxides. PV is a useful biomarker

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of the preliminary stages of rancidity occurring under mild conditions and the freshness of the lipid matrix thus the
greater the PV, the faster the oxidation of the oil occurring [24]. The observed increase in PV during heating of
oils has been reported by other authors [25, 26]. A study conducted by Idun-Acquah, et al. [4] obtained PV
ranging 10.00-25.250 meq O2/kg after 5 days of frying. They reported the deviations of PV from the standard
value to be due to the continuous exposure of the oil to light, high temperatures and atmospheric oxygen, which
reacts with the oil to form peroxides. According to Codex regulatory, oils to be utilized in deep frying should have
a codex regulatory maximum PV of 10meqO2/kg [27]. In this study, PVs increased but all the oils were still
concordant with the maximum codex standard.
Figure-2. Peroxide values (meq02/kg) of the oils collected at weekly intervals.
3.1.4. Iodine Value of Frying Oils
The result of the iodine values of the frying oils collected and analyzed is presented in Figure 3. Iodine value
(IV) is chemically the mass of iodine in grams that is consumed by 100gram of a chemical substance by mass as
oleic acid. Iodine value is often used to determine the amount of unsaturation in fatty acids. The higher the iodine
index, the faster is the tendency of oil oxidation during heating at elevated temperatures as in deep frying [28].
The maximum iodine value observed was the control (branded) sample with 44.69gI2 while the control (unbranded)
sample had the least IV with gI2. IV for all the oil samples at weekly intervals were significantly (p<0.05) different
from one another. Samples collected from Agudama and Railway Streets showed an increase in the IV after three
weeks of collection and this could probably be due to a change in the used oil to fresh oil or the addition of fresh oil
into the used oil in the frying medium. On the other hand, oil samples collected from Emekuku, Wogu and Kaduna
streets showed a decrement in IV. The observed decrement of the iodine value is concordant with the decrement in
double bonds attributed to oxidation and thermal decomposition and has been reported by Omara and Kigenyi
[29]. Chebet, et al. [30] Reported that a decrease in iodine value is an indication of deterioration in the vegetable
oils. Iodine value is of major interest in regard to oxidative stability of oils after frying of food. It is therefore
established from the study that the decrease in the iodine value of the oils after frying shows relatively higher
oxidation.
Figure-3. Iodine values (gI2) of the oils collected at weekly intervals.

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3.1.5. Saponification Value of Frying Oils
Result of the saponification values of the frying oils collected is shown in Figure 4. Saponification value (SV) is
used to determine the saponification number of a fat or oil which is an index of the average molecular weight of the
triaclyglceride in the sample. Saponification number is a very important factor in soap production. From the results
obtained, the reference sample (unbranded oil) and samples collected from railway at second week had SV within
the standard 196-206mgKOH/g of oil by National Standardization body [31]. There was an increase in the
saponification values of all the oils (except for samples collected from Wogu and Railway) even after second and
third weeks of collection with values ranging between 238.99-353.37mgKOH/g, 231.13-360.96mgKOH/g, and
208.27-293.89mgKOH/g for samples collected from Agudama, Emekuku and Kaduna Streets, respectively. This
increase above the specification for vegetable oils indicates that the oil to be discarded after frying can be used for
soap production. This result correlates with the findings of Alajtal, et al. [32] who reported an increase in the
saponification values of sunflower, corn and olive oils after frying from 21.09-22.44mgKOH/g, 19.41-
21.15mgKOH/g and 21.54-22.27mgKOH/g oil respectively.
Figure-4. Saponification values (mgKOH/g) of the frying oils collected at weekly intervals.
3.1.6. Smoke Point of Frying Oils
Result of the smoke points of the frying oils collected is presented in Figure 5. The smoke point is the
temperature at which a fat or oil produces a continuous wisp of smoke when heated. The maximum smoke point
observed was for the reference samples (170oC and 160oC for branded and unbranded vegetable oils respectively).
At week 1, the smoke point of all the frying oils ranged between 123-155oC, week 2, 130-165oC and week 3, 110-
150oC. A decline in the smoke point of frying oils collected from Agudama, Emekuku and Wogu Street for all the
three weeks studied. For Kaduna and Railway, an increase was observed only at week 2, following this was a
decrease. This was consistent with the findings of Choudhary and Grover [33] that a decrease in smoke point was
observed after first, second and third frying of rice bran oil blends. According to Sarwar, et al. [34] the smoke
point of cooking oil must be at least 170oC. The findings of the present study were not in line with this statement
as all the frying oil samples collected (except for the branded control) had smoke point <170oC even up to the third
week of collection. This may be due to the level of impurities in the oil from the frying process.
Figure-5. Smoke points (oC) of the frying oils collected at weekly intervals.
3.1.7. Moisture Content of Frying Oils
Result of the moisture content of the frying oils collected is shown in Figure 6. For all the samples collected at
week 1, the moisture content of the frying oils were found to be in the range of 0.13-0.20% with samples collected
from Agudama street recording lowest while samples from Railway and the unbranded sample were highest. After
three weeks of collection, the moisture was found to be in a range of 0.10-0.20% with frying oils from railway

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recording lowest and unbranded samples recording highest. However, there was no significant (p>0.05) difference
in the moisture content of the frying oils studied. The frying oils collected had values below the 0.2% maximal limit
for volatile matters at 105oC in oil and fats [35].
Figure-6. Moisture Content (%) of the oils collected at weekly intervals.
3.2. Antioxidant Properties of the Frying Oils
3.2.1. Total Phenolic Content (TPC)
Result of the total phenolic content of the frying oils collected is presented in Figure 7. Total phenolic content
of the frying oils at week 1, 2 and 3 ranged from 0.01-0.37mgGAE/g, 0.02-0.37mgGAE/g and 0.00-
0.37mgGAE/g, respectively. There was a decrease in the TPC of frying oils collected from Agudama, Worgu,
Emekuku and Railway Streets at 3rd week of collection while those from Kaduna recorded an increase. Total
phenolic content of control (branded oil) was significantly higher (p<0.05) than all other frying oils while total
phenolic content of unbranded oil did not differ with oil collected from Kaduna at 3rd week of collection and from
Railway, Emekuku and Agudama at first week of collection. The decrease in total phenolic content of oil during
frying operations is as a result of the exposure of the frying oil to high temperature and long time during deep
frying. The total phenolic content of the oils are close to that of Güzel, et al. [36] for vegetable oils in Turkey
(0.133-1.596mgGAE/ml). The results are higher than that of Xuan, et al. [37] for commercial vegetable edible oils
marketed in Japan (1.76-39.16mgGAE/g). Results presented here are in correlation with that of Gómez-Alonso, et
al. [38] where the reduction of antioxidant activity of olive oil correlated well with the number of deep-frying
cycles. Phenolics are known for their role in the oxidative stability of oils. Their presence may also prevent
deterioration of oils through quenching of radical reactions responsible for rancidity [39, 40].
.
Figure-7.Total phenolic content (mgGAE/g) of the oils collected at weekly interval.
3.2.2. Flavonoid Content of the Oils
Result of the flavonoid content of the frying oils collected is shown in Figure 8. Initial flavonoid content of the
frying oils collected ranged from 0.77-2.20% with oil collected from Wogu Street observed to be the lowest while
the branded oil was highest. At week 2 and 3, oils collected from Agudama and Kaduna streets were lowest in
flavonoid content. Flavonoid content of branded oil was significantly higher (p<0.05) than the frying oils collected
while at week 3, it showed no significant difference (p>0.05) except for oil collected from Kaduna street and
unbranded oil. Flavonoids are known for their antioxidant, anti-inflammatory, anti-atherosclerotic and anti-

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carcinogenic properties [41]. They are among antioxidant defensive systems protecting vegetable oils against
oxidative damage.
Figure-8.Flavonoid content (%) of the oils collected at weekly interval.
4. Conclusion
The study results showed degradation in the physicochemical and antioxidant properties of the vegetable
cooking oil after repetitive use of frying. All oil samples collected at first week from Wogu and Kaduna Streets
were within an acceptable range for percentage free fatty acid and as the weeks progressed, they were above the
permissible limit. There was also an increase in peroxide value of some oils while others showed a decrease,
however all the frying oils collected were within the Codex regulatory limit. All the oils collected showed a
decrease in Iodine value except for samples collected from Agudama and Railway close while increase in
saponification value above the specification for the oils was observed. Smoke point of the oils not in line with
standard limits even up to third week of collection while moisture content was below the 0.3% maximum limit.
Total phenolic content of the oils were relatively low and this decreased as collection weeks progressed except for
oils from Kaduna Street while a reverse was observed for flavonoid content. This study therefore shows the level of
degradation in the quality of oils used by these fried food vendors during frying operations. Safety measure should
therefore be put in place as these degradations in oil quality could cause damaging health effects due to the toxic
substances produced. Degraded oils are not suitable for human consumption but can be used for alternate uses
including biodiesel and soap production.
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