Abstract (English):
Introduction. There is a rising concern over food safety caused by an increasing trend towards adulterating fresh cooking oil with used cooking oil in Malaysia. Recent decades have seen more cases of high-quality edible cooking oil adulteration with reused oil, driven by high market demand and profit margins. In this study, we aimed to analyze the properties of vegetable oils and their effect on the quality of fried chicken nuggets. Study objects and methods. We determined free fatty acid contents and characterized the properties of fresh palm olein, used cooking oil, and adulterated oil. We also compared the sensory quality attributes of chicken nuggets fried in fresh and adulterated oils. Results and discussion. The content of free fatty acids consistently increased with rising adulteration levels. The FTIR spectral analyses revealed significant differences between fresh, used, and adulterated oils at 3006, 2922, 2853, 2680, 1744, 1654, 987, 968, and 722 cm–1. The oil samples with high adulterant concentrations demonstrated a linear increasing trend in K232 and K 270 values, where higher absorbance values indicated severe deterioration in the oil quality. The sensory evaluation showed no significant effect (P > 0.05) of adulteration with used cooking oil on the quality of fried chicken nuggets. Conclusion. Our findings filled in a gap in the previous studies which only focused on the effects of adulteration on the oil properties. The study also provides valuable information to regulatory authorities on the reliability of quality parameters and modern instruments in edible oil adulteration detection.

Keywords:
Adulteration, fresh palm olein, used cooking oil, food safety, sensory evaluation, frying, chicken

INTRODUCTION
Vegetable oils are a group of fats that are extracted
from different parts of a plant, such as seeds, nuts, cereal
grains, and fruits [1]. They play a significant role in our
diet as the main source of dietary fat and nutrients, as
well as a flavor enhancer. In Malaysia, palm oil has been
widely employed in the frying process, particularly in
deep frying, owing to its high stability [2, 3].
Deep-fried foods have become popular due to the
ease and speed of thermal treatment, as well as unique
flavor, taste, and texture induced during the frying
process [4]. The quality of oil has become a major
concern to the deep frying industry since it affects the
sensory quality of fried food, such as fried chicken
nuggets [5].
However, the authenticity of cooking oil has been
a serious issue since old times [6, 7]. According to
statistics, 26.5% of all food fraud incidents (n = 1648)
in 1980–2012 were associated with cooking oils [8].
Vegetable oil adulteration can be defined as an addition
of cheaper, inferior, harmful, or unnecessary substances
to oil that could affect its nature and quality [9]. High
profit often drives this kind of fraudulent practice. Lim
et al. and Alagesh reported a rising concern over food
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Foods and Raw Materials, 2022, vol. 10, no. 1
E-ISSN 2310-9599
ISSN 2308-4057
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Tan S.L. et al. Foods and Raw Materials, 2022, vol. 10, no. 1, pp. 106–116
safety in light of the increasing trend of fresh cooking
oil adulteration in Malaysia [10, 11].
Used cooking oil (UCO), also known as waste
cooking oil or yellow grease, is the oil that has already
been used in food preparation processes [10]. In order
to reduce expenses, most food business operators and
caterers tend to reuse the oils repeatedly, toping them
up with fresh oil to mask the effects of degraded oil.
Moreover, by using a series of simple and low-cost
processes, including preliminary filtration, boiling, and
refining, they are able to recover the quality of waste
cooking oil to a certain extent to make it resemble that
of fresh oil [12]. Since the past decade, cooking oil
adulteration with refined waste oil has been rampant
in Asian countries, particularly in mainland China,
followed by other countries [13–16]. The situation
is worsened by the low purchase cost of UCO, its
wide availability, and high profit gain over the price
difference.
Various analytical techniques and parameters have
been developed to determine cooking oil adulteration.
The most common of them is a free fatty acid (FFA)
test [3, 17]. However, this test only measures the overall
levels of titratable acids, without identifying the profiles
of FFAs. In recent years, many sophisticated analytical
methods have been studied intensively, including the
FTIR fingerprint spectroscopic method. They have
proven fit to unravel the menace of adulteration in high
quality fresh oil [10, 18–20]. According to Amereih et al.
and Hashem et al., the UV-Vis spectrophotometric
method is also effective enough in adulteration detection
and quantification [20, 21].
Therefore, we aimed to study the properties and
quality of palm cooking oil adulterated with used
cooking oil. Palm oil was chosen as the most common
frying medium in Malaysia. In addition, we determined
the effects of adulteration on the sensory quality of fried
chicken nuggets, adding to former studies that mainly
report its effects on the oil properties.
STUDY OBJECTS AND METHODS
Oil sample collection. Fresh palm olein (FPO) and
frozen chicken nuggets were purchased from the local
market in Pagoh Jaya, Johor, Malaysia. Pre-filtered used
cooking oil (UCO) was collected from a local feedstock
trading company.
Formulation and preparation of adulterated
oil (AO). Sets of pure FPO and UCO samples were
prepared without any adulteration. A set of AO samples
was prepared by mixing FPO with 20, 40, 60, and
80% (v/v) of UCO. The mixtures were vortexed to
ensure complete homogenization.
Determination of free fatty acid (FFA) content.
The FFA content was determined using a conventional
acid-base titration method developed by the Malaysian
Palm Oil Research Institute, as previously reported by
Abdul Wahab et al., with slight modification [22, 23].
A 500 mL volumetric flask was filled with 50 mL of
1.0M sodium hydroxide solution that was diluted with
distilled water to the graduation mark. The solution was
standardized by titrating with a standard KHP solution.
Then, 2-propanol solution was heated to approximately
80°C and mixed with 1 mL of phenolphthalein indicator.
The heated alcohol solution was then neutralized by
adding the 0.1M sodium hydroxide solution drop by
drop until the first permanent light pink color was
obtained. Subsequently, an oil sample was mixed with
the neutralized alcohol solution and shaken vigorously
to ensure an even mixture. Finally, the still hot mixture
was titrated against the 0.1M sodium hydroxide solution
until another permanent light pink color was obtained.
The amount of sodium hydroxide consumed during
titration was recorded and used to determine the FFA
content (Eq.(1)). The results were expressed in mean ±
standard deviation in triplicate.
(1)
where V is the volume of NaOH, mL; M is the molarity
of NaOH, M; W is the weight of the oil sample, g.
Measurement of ATR-FTIR spectra. The
procedure followed the method described by Poiana
et al. [19]. The ATR-FTIR spectra of each oil sample
were scanned and recorded using a Spectrum Two FT-IR
spectrometer (PerkinElmer, United States) equipped
with an ATR accessory. A drop of each oil sample was
placed on the crystal at room temperature (25°C). All
the spectra were measured at the mid-infrared region
ranging from 4000 to 650 cm–1 with a scanning time of
60 s and 4 cm–1 resolution. The ATR-FTIR spectra were
obtained against the air background spectrum. After
every scan, a new reference air background spectrum
was performed. The ATR plate surface was gently
wiped with a soft tissue soaked in acetone to remove
any residues of the previous oil sample before placing a
new one. The FTIR spectra of all the oil samples were
recorded as an absorbance value in triplicate.
Measurement of UV-Vis absorption at 232 and
270 nm (K232 and K270). The procedure was based on
the method reported by Amereih et al. and Chong, with
slight modification [21, 24]. The absorption spectra of
all the oil samples were obtained at 200 to 800 nm using
a U-3900H UV-Vis spectrophotometer (Hitachi High-
Tech Corp., Japan). A quartz cuvette (1 cm path) was
filled with 1% of an oil sample in isooctane solution. The
absorption was measured against a blank of isooctane.
The maximum absorption values obtained at 232 nm and
270 nm were subsequently used to determine the specific
extinction coefficients, K232 and K270 respectively, as
outlined in Eq. (2).
Kλ = (2)
where Kλ is the specific extinction coefficient at
wavelength λ; Aλ is the absorption measured at
wavelength λ; c is the concentration of the oil
sample in solvent, g/100 mL; L is the path length
of the cuvette, cm.
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Deep frying and sensory evaluation. The frozen
chicken nuggets were weighed and made into batches
of 300 g. FPO was added into an EF-102T electric dual
tank deep fryer (Wibur, China) to reach its minimum
capacity of about 4 kg of oil. The FPO was heated
for about 20 min up to 175 ± 5°C. After pre-heating,
the first batch of frozen chicken nuggets was deepfried
for 3 min until the nuggets turned golden brown.
The subsequent frying cycles started at an interval of
20 min. At the end of a frying cycle, the nuggets fried
in FPO were taken for sensory evaluation. Due to
health concerns regarding reused oil, the use of AO in
preparing fried chicken nuggets for sensory evaluation
was limited to 60%. It simulated the AO that could be
commonly found in the local night market. The new
frying cycles began by replacing the FPO with AO
containing 60% UCO (w/w). Similarly, the fried chicken
nuggets prepared in 60% AO were then evaluated for
sensory acceptance.
The fried chicken nuggets prepared in FPO and
60% AO were evaluated for sensory attributes such as
color, flavor, juiciness inside, crispiness outside, taste,
and overall acceptability. The 9-point hedonic scale
was employed differently for each attribute, namely for
flavor, taste, and overall quality: 1 = extremely dislike,
5 = neither like nor dislike, and 9 = extremely like; for
crispiness outside: 1 = soft and 9 = crispy; for juiciness
inside: 1 = dry and 9 = juicy; and for color: 1 = dark
brown and 9 = golden yellow.
Statistical analysis. All the tests were conducted
in triplicate. The volume of titrant and % FFA were
recorded and expressed in mean ± standard deviation
in triplicate. The functional groups and their vibration
modes, as shown in the IR spectra, were matched to the
respective characteristic bands in FPO, AO, and UCO.
The absorbance intensities of the bands were evaluated
by comparing the peak heights. Each spectrum and
maximum absorbance at 232 nm and 270 nm were
reported in mean ± standard deviation. An independent
t-test was performed using Microsoft Excel to examine
the differences between the oil samples (P < 0.05).
ANOVA was used to analyze the sensory evaluation
data to determine significant effects between the fried
chicken nuggets prepared in different oils.
RESULTS AND DISCUSSION
Effect of adulteration with reused oil on FFA
content. In all the oil samples under analysis, FFA
contents increased with higher adulterant concentrations
(Table 1). The FFA level was the lowest in FPO (0.90 ±
0.16) and the highest in UCO (3.25 ± 0.06). Banani et al.
and Alias et al. reported that used or waste cooking oil
had high FFA values, which subsequently led to high
acidity and viscosity values [25, 26]. This finding was
similar to those by Abdul Wahab et al. and Panadare
and Rathod, who found higher FFA contents (2.33–
6.42%) in waste cooking oil compared to fresh cooking
oil [23, 27].
The relatively higher FFA content in UCO, which
was attributed to darker oil color, might be due to the
exposure to prolonged heating and moisture from food,
which induced the hydrolysis reaction of triglycerides
[23]. Since the FFA content in UCO was less than
15%, it was classified as yellow grease. This finding was
similar to the results reported by Abdul Wahab et al.,
Panadare and Rathod, and Rosnelly et al. [23, 27, 28].
All the findings indicated the deteriorating quality of the
UCO subjected to repeated heating cycles. As a result, it
was no longer suitable for frying or human consumption
due to increased oil acidity, which is potentially harmful
to human health. This observation was in agreement
with the results reported by Ahmad Tarmizi et al.,
Maskan and Bagci, and Chong [3, 29, 30].
Used or waste frying oil is an end product of frying.
It is subjected to harsh frying conditions and prolonged
exposure to excessive heat and atmospheric air due to
repetitive use. The chemical changes induced by frying,
such as hydrolysis and oxidation, generate reactions
in its by-products, such increasing FFA values, which
gives rise to off flavors and odors. This justifies the high
FFA content in the UCO in our study. We also found
that increasing adulterant concentrations corresponded
to high FFA values, which makes the adulterated oils
unsafe for frying or human consumption.
Characterization of oil properties using FTIR
spectra. We found no significant differences between
the spectral features, despite slight changes in the
absorbance of some bands and a few shifts in their
exact position. Figure 1 shows the FTIR spectra of the
FPO and UCO samples at ambient temperature. Both
the FPO and UCO displayed some typical spectral
features associated with oils. Both spectra were similar
in terms of shape, position of the characteristic bands,
and the presence of peaks. These similarities can be
explained by the same origin of the oils and the presence
of identical principle components in their composition,
which are triglycerides [10, 31].
However, variation in the oil composition is an
important factor that influences the exact position of the
bands, as well as shifts in the spectra [19, 31–33]. The
variation in both spectra could be due to the quality
degradation caused by adulteration. Table 2 summarizes
the significant aberrations observed in the FTIR
Table 1 FFA content in oil samples with different adulteration
levels
Oil sample Adulterant
concentration, %
Free fatty
acid, %
Fresh palm olein
20% adulterated oil
40% adulterated oil
60% adulterated oil
80% adulterated oil
Used cooking oil
0
20
40
60
80
100
0.90 ± 0.16
1.34 ± 0.14
1.69 ± 0.00
2.12 ± 0.05
2.58 ± 0.04
3.25 ± 0.06
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spectra of all the oil samples in response to adulteration
with UCO.
The FTIR spectrum is divided into two distinctive
regions. They are functional group and fingerprint
regions corresponding to 4000–1650 and < 1650 cm–1,
respectively. The entire spectra that we obtained for all
the oil samples were seemingly identical because of their
similar fatty acid compositions.
Nevertheless, we found that the UCO and
AO samples with increasing adulteration levels
demonstrated a slight aberration at 3006, 2922, 2853,
2680, 1744, 1654, 987, 968, and 722 cm–1 in terms
of absorption bands and absorbance intensity. The
adulteration of FPO with UCO resulted in a shift of the
3006 cm–1 band (Fig. 2a). This finding was in agreement
with [19, 32, 34, 35] showing that the exact band position
was determined by oil composition and unsaturation
level. FPO recorded its highest absorbance at 3007 cm–1,
while AO reached its maximum at 3006 cm–1 due to
reduced unsaturation. This suggested that exposure of
frying oil to high heat had an effect on its unsaturation
degree. Compared to FPO, the AO and UCO samples,
Figure 1 FTIR spectra for fresh palm olein (FPO)
and used cooking oil (UCO) at 4000–650 cm–1
which had been heated repeatedly, showed higher
absorbance at 3006 cm–1. This observation was
consistent with a previous study by Alshuiael and
Al-Ghouti, which proved that high heat application
caused oil to become more unsaturated by losing
hydrogen atoms [35].
We also observed strong and sharp absorption bands
at 2922 and 2853 cm–1 due to the symmetric stretching
vibration of aliphatic groups (-CH). The bands are
attributed to the presence of aliphatic fatty acid chains.
The high absorption peak of FPO at around 2922 cm–1
was determined by its unique fatty acid composition.
Apart from that, we identified a weak absorption
band at around 2680 cm–1, which could be attributed
to carbonyl ester (-C=O) caused by Fermi resonance
(Fig. 2b). The absorption band at 2680 cm–1 indicated
the presence of aldehyde containing the O=C-H group.
The increment of carbonyl aldehyde correlated with the
adulteration incidence. As we can see in Fig. 2b, the
concentration of aldehydes in the UCO samples and
in most AO samples was far higher than that in FPO,
except for 80% AO.
The concentration of volatile aldehydes is
associated with a degree of oxidative degradation of
oil, as aldehydes are major volatile compounds emitted
upon heating as thermal degradation products [36,
37]. Volatile compounds such as aldehydes, ketones,
alcohols, and acids are generated during oil degradation.
They create unfavorable aroma and flavor, shorten the
oil’s shelf life, and may induce health problems [38].
We found that the UCO exposed to repeated frying gave
high absorbance intensity at 2680 cm–1, which might
be attributed to an increment of volatile aldehydes due
to lipid oxidation that consequently degrades the oil
quality.
In relation to that, we observed a strong and sharp
absorption band at 1744 cm–1 (Fig. 2c) due to the
presence of the C=O group of triglycerides caused by
stretching vibration. This was due to the decomposition
of unstable primary hydroperoxides, which formed upon
oxidation, into stable secondary oxidation products such
as aldehydes and ketones, which cause an absorbance
Absorbance
Wavenumber, cm–1
Table 2 Significant aberrations in the FTIR spectra for oil samples in response to adulteration
Description of spectra feature Significance
Slight shift of band near 3006 cm–1
Strong band at 2922 and 2853 cm–1
Weak band at 2680 cm–1
Strong band at 1744 cm–1
Increased absorbance of band at 1680–1630 cm–1
(or decreased absorbance of band at 1654 cm–1)
Maximum absorption at 987 and 968 cm–1
Appearance of band at 968 cm–1
Progressive decrease in absorbance of band
at 700–725 cm–1 (or at 722 cm–1)
Reduced degree of unsaturation caused by diminution of cis- olefinic double
bonds (=CH)
Presence of aliphatic methylene (-CH2) group indicative of saturation level
Presence of saturated aldehydes as a marker of advanced oxidation
Appearance of carbonyl compounds and other secondary oxidation products
Presence of trans- and cis- isomers due to cis-trans- isomerization upon
thermal stress
Formation of trans- isomers (conjugated trans and non-conjugated transrespectively)
induced by conjugation and cis-trans- isomerization due to heat
Possible presence of secondary oxidation products (aldehydes, ketones) with
isolated trans- double bond indicative of advanced oxidation
Disappearance of cis- double bonds indicative of reduced unsaturation
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near 1744 and 1728 cm–1. High absorbance of FPO at this
band could be explained by prolonged storage, which
intensified the oxidative reaction [39].
The absorption band near 1680–1620 cm–1 could
be assigned to the C=C stretch (Fig. 2d). The peak at
1654 cm–1 showed a general declining trend in
absorbance with increasing adulteration levels, implying
the disappearance of the cis- carbon-carbon double bond
within the molecular structure. This could be due to the
thermal and/or oxidative degradation of the oil samples.
The accumulation of trans- fatty acids in all the
samples was further evident through an increasing
trend in absorbance from the cis- C=C stretch band
region (1660–1630 cm–1) to the trans- C=C stretch
band region (1680–1660 cm–1). This might be explained
by the occurrence of cis-trans isomerization induced
Figure 2 Significant aberrations in fresh palm olein (FPO), used cooking oil (UCO), and adulterated oil (AO) spectra at (a) 3006,
(b) 2680, (c) 1744, (d) 1680–1620, (e) 987–968, and (f) 722 cm–1
Absorbance
Wavenumber, cm–1
Absorbance
Wavenumber, cm–1
Absorbance
Wavenumber, cm–1
Absorbance
Wavenumber, cm–1
Absorbance
Wavenumber, cm–1
Absorbance
Wavenumber, cm–1
a b
c d
e f
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by thermal stress, changing the initial cis- geometric
configuration into trans- and resulting in trans- fatty
acids accumulation [19, 40, 41]. This finding was
further reinforced by the absorption bands at 968
and 722 cm–1, corresponding to bending vibration
of -HC=CH- in trans- and cis- configuration,
respectively (Fig. 2e and 2f).
The UCO generally demonstrated a higher
absorbance intensity than the FPO at 968 cm–1.
This observation related to the increment of transcomposition
caused by cis-trans- isomerization,
resulting in deteriorated oil quality. Meanwhile, the
band near 722 cm–1 was responsible for the cis- double
bonds of disubstituted olefins. The UCO showed lower
absorbance values compared to the FPO, which was
probably due to the reduction of cis- C=C double
bonds of unsaturated fatty acids. This observation was
consistent with а previous study [19] that showed а
progressive decline in absorbance at 722 cm–1 indicative
of cis-trans- isomerization in unsaturated fatty acids,
which subsequently resulted in double bonds vanishing
from the cis- conformation.
In addition, we found a very weak absorption band
at 987 cm–1, which indicated the presence of trans-,
trans- and/or cis-, trans-conjugated diene groups
of hydroperoxides. Oil oxidation causes cis double
bonds to disappear, also leading to the isomerization
of cis- fatty acids to trans- isomers and hydroperoxide
(primary oxidation products) generation [42]. Unstable
hydroperoxides decompose into aldehydes, ketones, and
other secondary oxidation products, which are more
stable. These volatile compounds are responsible for the
off-odor of the oxidized oils.
According to Guillen and Cabo, the absorption
band at 967 cm–1 indicates the possible presence of
secondary oxidation products such as aldehydes and
ketones, which contain isolated trans- double bonds [43].
As can be seen in Fig. 2e, the UCO showed a relatively
higher absorbance at 987 and 967 cm–1 compared to the
FPO. This was due to the generation of trans- isomers
that contributed to conjugated trans- isomers caused
Figure 3 (a) UV-Vis spectra of fresh palm olein (FPO), adulterated oil (AO), and used cooking oil (UCO) at 200–800 nm;
(b) enlarged view of UV-Vis spectra of oil samples at 200–400 nm
by the exposure of UCO to harsh frying that advanced
oxidation.
This observation was in agreement with the studies
by Lim et al. and Poiana et al. [10, 19]. Nevertheless,
we also used the UV-Vis spectroscopy as an exceptional
alternative to the FTIR spectroscopy in detecting the
presence of primary (232 nm) and secondary (270 nm)
oxidation products, which will be discussed later.
Detection and quantification of FPO adulteration
using UV-Vis spectrophotometry. The UV-Vis
spectrophotometry is a simple analytical method to
detect and quantify oil adulteration incidence. This
method evaluates the authenticity of oils by measuring
absorption bands between 200 and 400 nm [21]. The
UV-Vis spectra from 200 to 400 nm are considered to be
directly related to oil quality [20, 21].
Figure 3a illustrates a significant peak that we
observed within this range, from 200 to 400 nm. This
finding was consistent with the previous studies [20, 21],
which detected oil adulteration incidence by observing
the molecular absorption of UV-Vis spectra within the
designated range.
Figure 3b shows an enlarged view of the UV-Vis
spectra. We found that the maximum absorption at 232
and 270 nm was related to the presence of conjugated
dienes and trienes, which served as the best indicator of
oil quality. This is because conjugated dienes and trienes
are substances that form at an advanced oxidation stage,
indicative of degraded oil quality.
Oxidation products absorption at 232 and 270 nm
(K232 and K270). As mentioned above, the maximum
absorption at 232 and 270 nm correlate with the
presence of oxidation products that are exceptionally
powerful in determining the adulteration incidence in
oil. These absorptions are typically expressed as specific
extinctions at 232 and 270 nm denoted by K232 and K270,
respectively [20].
Traditionally, the peroxide value and the anisidine
value are often used together to measure the oxidative
status of edible oils. They reflect the concentration of
primary (hydroperoxides) and secondary (aldehyde
a b
Absorbance
Wavelength, nm
Absorbance
Wavelength, nm
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and ketones) oxidation products, respectively [44, 45].
Repeated frying accelerates the accumulation of
oxidative products, thus contributing to higher
peroxide and anisidine values indicative of deteriorated
oil quality [46].
Xu et al. reported that palm olein exhibited a
significant increment in the peroxide and anisidine
values with increasing frying cycles [47]. Therefore,
in counterfeit oil, their significant increase could be
considered a result of quality degradation. Nevertheless,
these chemical analyses are lengthy, expensive, and
involve hazardous chemicals [48]. Thus, we preferred to
use spectrophotometry to determine oxidative products
in the oil samples.
Figure 4a shows maximum absorption of all the
oil samples at 232 and 270 nm, while Fig. 4b shows an
increasing trend of K232 and K270 values with adulterant
concentrations. We found the absorbances at 270 nm to
be significantly higher than those at 232 nm.
We found a linear relationship between adulterant
concentrations and absorbances at 232 and 270 nm. The
AO samples showed higher absorbances with increasing
adulterant concentrations, while the FPO and the UCO
samples had the lowest and highest values, respectively.
We also observed a tendency for all the samples to
show much higher absorption at 270 nm, compared
to 232 nm. This was due to the formation of relatively
unstable hydroperoxides (primary oxidation products),
which directly correlated with absorbance at 232 nm
and could decrease in number over time [10]. They
tended to decompose into more stable, complex forms
of secondary oxidation products including aldehydes,
ketones, and alcohols, which corresponded to the
absorption at 270 nm.
This observation was in agreement with that
reported by Lim et al., Maskan and Bagci, and
Jolayemi et al. [10, 29, 49]. This experimental finding
also supported the application of specific absorbances
in the ultraviolet region at 232 and 270 nm to detect
adulteration. They can serve as an oil quality indicator
Figure 5 Comparison of sensory attributes for chicken nuggets
fried in different sets of oil samples
through the measurement of primary and secondary
oxidation indicative of oxidative deterioration [50]. This
finding was consistent with that reported by Amereih
et al., where high absorbance at these particular
wavelengths indicated oil adulteration [21]. Thus, high
quality oil shows low absorbances at 232 and 270 nm
and vice versa.
Effect of AO on the quality of fried chicken
nuggets. In this study, chicken nuggets were fried in
two sets of oil samples, FPO and 60% AO. Motivated by
health concern, we only used 60% AO to simulate the
adulterated palm cooking oil that was commonly found
in the night market. Figure 5 compares the average
scores of sensory attributes for the chicken nuggets fried
in FPO (code 831) and 60% AO (code 524).
We observed no significant difference (P > 0.05)
between the chicken nuggets fried in FPO and those
fried in 60% AO in terms of sensory attributes including
flavor, color, juiciness, taste, and overall acceptability.
However, there was a significant difference (P < 0.05) in
crispness. These observations concluded that adulterated
oil with 60% UCO did not have a significant effect
(P > 0.05) on the sensory perception of chicken nuggets
Figure 4 (a) Maximum absorption of fresh palm olein (FPO), used cooking oil (UCO), and adulterated oil (AO) with increasing
adulterant concentrations at 232 and 270 nm, (b) Direct relationship between adulterant concentrations and UV absorbances
at 232 and 270 nm
Absorbance
Wavelength, nm
K extinction
Adulterated oil, %
a b
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fried in it, compared to FPO. Our findings were similar
to those by Enriquez-Fernandez et al., who reported
an insignificant difference (P > 0.05) between the
foods fried in used oil and fresh oil in terms of sensory
evaluation [51].
Color differences were insignificant (P > 0.05)
between the nuggets fried in FPO and those fried in 60%
AO, both having a golden brown color. However, we
observed that the 60% AO-fried nuggets were cooked
faster and therefore turned golden brown in a shorter
time than those fried in FPO. 60% AO was much darker
and intense in color compared to FPO. Thus, our study
showed a negligible effect of frying oil on the color of
chicken nuggets. This finding was in line with the results
by Ahmad, but opposite to those reported by Li, who
emphasized that the color of frying oil influenced the
color of fried foods [52, 53].
Although taste differences were insignificant
(P > 0.05) between the chicken nuggets fried in FPO and
those fried in 60% AO, we observed an appreciable gap
in the scores. Some panelists mentioned an unpleasant
rancid taste of the samples coded 524, which were fried
in 60% AO. This rancid taste became more obvious
and intense over time. This observation was further
enhanced by Okparanta et al., who reported that rancid
oil led to abnormal rancid taste in fried foods [54].
However, there was a significant difference
(P < 0.05) in crispiness, a desirable textural quality
of fried foods. The chicken nuggets fried in 60% AO
tended to be perceived with increased crispiness,
compared to those fried in FPO. This observation might
be due to a considerable time gap between the frying
process of the samples and their sensory evaluation.
The prolonged exposure to atmospheric air could have a
noticeable influence on the sensory crispiness of both the
FPO- and 60% AO-fried nuggets.
This finding was consistent with those by Antonova
and Sung [55, 56]. In particular, Antonova reported
a correlation between increased holding time under
ambient conditions and decreased crispiness perceived
by the panelists [55]. Holding time, which is defined
as the minimum and maximum time after frying that
a product can be used for sensory evaluation, should
be determined for fried chicken nuggets to minimize
variation in the test results. The previous studies
suggested that breaded fried chicken nuggets should be
served for sensory evaluation within 10 min after frying,
under ambient conditions, to avoid variation in the test
results [55]. Any longer than the suggested holding time
can have an impact on the panelists’ sensory perception.
However, it is worth noting that the chicken nuggets
fried in adulterated oil with 60% used oil (7.53 ± 1.28)
were found to be preferred in terms of overall quality,
compared to those fried in fresh palm olein (7.33 ± 1.15).
This finding can be supported by Bluementhal and
Bordin et al., suggesting that the optimum quality of
fried food can be achieved with moderately altered
and reused frying oil, instead of fresh oil [57, 58].
This is because of the role of surfactant compounds
in the frying process. These compounds accumulate
in increasingly abused oils and facilitate the contact
between foods and oil, thus contributing to better
characteristics of fried food products.
CONCLUSION
In conclusion, our study showed the effects of
adulteration with used cooking oil on both the oil
properties and the quality of fried chicken nuggets. We
observed higher FFA contents in the oils as adulterant
concentrations increased. Pure UCO recorded the
highest FFA value and reached the discard point set by
legislation.
The chemical characterization of oil properties by
using the FTIR spectral analyses determined some
differences between FPO, UCO, and AO in terms of
the exact position of band appearance and absorbance
intensities. Significant aberrations in the FTIR spectra
were observed at 3006, 2922, 2853, 2680, 1744, 1654,
987, 968, and 722 cm–1.
The UV-Vis spectral analysis used absorbances
at 232 and 270 nm (K232 and K270, respectively) as
an indicator of oil adulteration. We found a linear
increasing relationship between the adulterant
concentrations and the K extinction values, which
enabled the detection and quantification of adulteration
with UCO.
The sensory evaluation of the chicken nuggets
fried in FPO and AO showed no significant effects of
adulteration with UCO on their quality.
CONTRIBUTION
The authors were equally involved in writing the
manuscript and are equally responsible for plagiarism.
CONFLICT OF INTEREST
The authors declare that there is no conflict of
interest.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the technical
assistance from the laboratory staff with operating
the instruments. The authors would also like to thank
the Faculty of Applied Sciences and Technology,
Universiti Tun Hussein Onn Malaysia, for its support
throughout the study, as well as CS Oil & Fats Sdn. Bhd.
for supplying the pre-filtered UCO to be used as raw
material in this project.

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