INTRODUCTIONVegetable oils are a group of fats that are extractedfrom different parts of a plant, such as seeds, nuts, cerealgrains, and fruits [1]. They play a significant role in ourdiet as the main source of dietary fat and nutrients, aswell as a flavor enhancer. In Malaysia, palm oil has beenwidely employed in the frying process, particularly indeep frying, owing to its high stability [2, 3].Deep-fried foods have become popular due to theease and speed of thermal treatment, as well as uniqueflavor, taste, and texture induced during the fryingprocess [4]. The quality of oil has become a majorconcern to the deep frying industry since it affects thesensory quality of fried food, such as fried chickennuggets [5].However, the authenticity of cooking oil has beena serious issue since old times [6, 7]. According tostatistics, 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 additionof cheaper, inferior, harmful, or unnecessary substancesto oil that could affect its nature and quality [9]. Highprofit often drives this kind of fraudulent practice. Limet al. and Alagesh reported a rising concern over foodCopyright © 2022, Tan et al. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 InternationalLicense (http://creativecommons.org/licenses/by/4.0/), allowing third parties to copy and redistribute the material in any medium or format and to remix,transform, and build upon the material for any purpose, even commercially, provided the original work is properly cited and states its license.Foods and Raw Materials, 2022, vol. 10, no. 1E-ISSN 2310-9599ISSN 2308-4057107Tan S.L. et al. Foods and Raw Materials, 2022, vol. 10, no. 1, pp. 106–116safety in light of the increasing trend of fresh cookingoil adulteration in Malaysia [10, 11].Used cooking oil (UCO), also known as wastecooking oil or yellow grease, is the oil that has alreadybeen used in food preparation processes [10]. In orderto reduce expenses, most food business operators andcaterers tend to reuse the oils repeatedly, toping themup with fresh oil to mask the effects of degraded oil.Moreover, by using a series of simple and low-costprocesses, including preliminary filtration, boiling, andrefining, they are able to recover the quality of wastecooking oil to a certain extent to make it resemble thatof fresh oil [12]. Since the past decade, cooking oiladulteration with refined waste oil has been rampantin Asian countries, particularly in mainland China,followed by other countries [13–16]. The situationis worsened by the low purchase cost of UCO, itswide availability, and high profit gain over the pricedifference.Various analytical techniques and parameters havebeen 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 overalllevels of titratable acids, without identifying the profilesof FFAs. In recent years, many sophisticated analyticalmethods have been studied intensively, including theFTIR fingerprint spectroscopic method. They haveproven fit to unravel the menace of adulteration in highquality fresh oil [10, 18–20]. According to Amereih et al.and Hashem et al., the UV-Vis spectrophotometricmethod is also effective enough in adulteration detectionand quantification [20, 21].Therefore, we aimed to study the properties andquality of palm cooking oil adulterated with usedcooking oil. Palm oil was chosen as the most commonfrying medium in Malaysia. In addition, we determinedthe effects of adulteration on the sensory quality of friedchicken nuggets, adding to former studies that mainlyreport its effects on the oil properties.STUDY OBJECTS AND METHODSOil sample collection. Fresh palm olein (FPO) andfrozen chicken nuggets were purchased from the localmarket in Pagoh Jaya, Johor, Malaysia. Pre-filtered usedcooking oil (UCO) was collected from a local feedstocktrading company.Formulation and preparation of adulteratedoil (AO). Sets of pure FPO and UCO samples wereprepared without any adulteration. A set of AO sampleswas prepared by mixing FPO with 20, 40, 60, and80% (v/v) of UCO. The mixtures were vortexed toensure complete homogenization.Determination of free fatty acid (FFA) content.The FFA content was determined using a conventionalacid-base titration method developed by the MalaysianPalm Oil Research Institute, as previously reported byAbdul Wahab et al., with slight modification [22, 23].A 500 mL volumetric flask was filled with 50 mL of1.0M sodium hydroxide solution that was diluted withdistilled water to the graduation mark. The solution wasstandardized by titrating with a standard KHP solution.Then, 2-propanol solution was heated to approximately80°C and mixed with 1 mL of phenolphthalein indicator.The heated alcohol solution was then neutralized byadding the 0.1M sodium hydroxide solution drop bydrop until the first permanent light pink color wasobtained. Subsequently, an oil sample was mixed withthe neutralized alcohol solution and shaken vigorouslyto ensure an even mixture. Finally, the still hot mixturewas titrated against the 0.1M sodium hydroxide solutionuntil another permanent light pink color was obtained.The amount of sodium hydroxide consumed duringtitration was recorded and used to determine the FFAcontent (Eq.(1)). The results were expressed in mean ±standard deviation in triplicate.(1)where V is the volume of NaOH, mL; M is the molarityof NaOH, M; W is the weight of the oil sample, g.Measurement of ATR-FTIR spectra. Theprocedure followed the method described by Poianaet al. [19]. The ATR-FTIR spectra of each oil samplewere scanned and recorded using a Spectrum Two FT-IRspectrometer (PerkinElmer, United States) equippedwith an ATR accessory. A drop of each oil sample wasplaced on the crystal at room temperature (25°C). Allthe spectra were measured at the mid-infrared regionranging from 4000 to 650 cm–1 with a scanning time of60 s and 4 cm–1 resolution. The ATR-FTIR spectra wereobtained against the air background spectrum. Afterevery scan, a new reference air background spectrumwas performed. The ATR plate surface was gentlywiped with a soft tissue soaked in acetone to removeany residues of the previous oil sample before placing anew one. The FTIR spectra of all the oil samples wererecorded as an absorbance value in triplicate.Measurement of UV-Vis absorption at 232 and270 nm (K232 and K270). The procedure was based onthe method reported by Amereih et al. and Chong, withslight modification [21, 24]. The absorption spectra ofall the oil samples were obtained at 200 to 800 nm usinga U-3900H UV-Vis spectrophotometer (Hitachi High-Tech Corp., Japan). A quartz cuvette (1 cm path) wasfilled with 1% of an oil sample in isooctane solution. Theabsorption was measured against a blank of isooctane.The maximum absorption values obtained at 232 nm and270 nm were subsequently used to determine the specificextinction coefficients, K232 and K270 respectively, asoutlined in Eq. (2).Kλ = (2)where Kλ is the specific extinction coefficient atwavelength λ; Aλ is the absorption measured atwavelength λ; c is the concentration of the oilsample in solvent, g/100 mL; L is the path lengthof the cuvette, cm.108Tan S.L. et al. Foods and Raw Materials, 2022, vol. 10, no. 1, pp. 106–116Deep frying and sensory evaluation. The frozenchicken nuggets were weighed and made into batchesof 300 g. FPO was added into an EF-102T electric dualtank deep fryer (Wibur, China) to reach its minimumcapacity of about 4 kg of oil. The FPO was heatedfor about 20 min up to 175 ± 5°C. After pre-heating,the first batch of frozen chicken nuggets was deepfriedfor 3 min until the nuggets turned golden brown.The subsequent frying cycles started at an interval of20 min. At the end of a frying cycle, the nuggets friedin FPO were taken for sensory evaluation. Due tohealth concerns regarding reused oil, the use of AO inpreparing fried chicken nuggets for sensory evaluationwas limited to 60%. It simulated the AO that could becommonly found in the local night market. The newfrying cycles began by replacing the FPO with AOcontaining 60% UCO (w/w). Similarly, the fried chickennuggets prepared in 60% AO were then evaluated forsensory acceptance.The fried chicken nuggets prepared in FPO and60% AO were evaluated for sensory attributes such ascolor, flavor, juiciness inside, crispiness outside, taste,and overall acceptability. The 9-point hedonic scalewas employed differently for each attribute, namely forflavor, taste, and overall quality: 1 = extremely dislike,5 = neither like nor dislike, and 9 = extremely like; forcrispiness outside: 1 = soft and 9 = crispy; for juicinessinside: 1 = dry and 9 = juicy; and for color: 1 = darkbrown and 9 = golden yellow.Statistical analysis. All the tests were conductedin triplicate. The volume of titrant and % FFA wererecorded and expressed in mean ± standard deviationin triplicate. The functional groups and their vibrationmodes, as shown in the IR spectra, were matched to therespective characteristic bands in FPO, AO, and UCO.The absorbance intensities of the bands were evaluatedby comparing the peak heights. Each spectrum andmaximum absorbance at 232 nm and 270 nm werereported in mean ± standard deviation. An independentt-test was performed using Microsoft Excel to examinethe differences between the oil samples (P < 0.05).ANOVA was used to analyze the sensory evaluationdata to determine significant effects between the friedchicken nuggets prepared in different oils.RESULTS AND DISCUSSIONEffect of adulteration with reused oil on FFAcontent. In all the oil samples under analysis, FFAcontents 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 oilhad high FFA values, which subsequently led to highacidity and viscosity values [25, 26]. This finding wassimilar to those by Abdul Wahab et al. and Panadareand Rathod, who found higher FFA contents (2.33–6.42%) in waste cooking oil compared to fresh cookingoil [23, 27].The relatively higher FFA content in UCO, whichwas attributed to darker oil color, might be due to theexposure to prolonged heating and moisture from food,which induced the hydrolysis reaction of triglycerides[23]. Since the FFA content in UCO was less than15%, it was classified as yellow grease. This finding wassimilar 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 theUCO subjected to repeated heating cycles. As a result, itwas no longer suitable for frying or human consumptiondue to increased oil acidity, which is potentially harmfulto human health. This observation was in agreementwith 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 prolongedexposure to excessive heat and atmospheric air due torepetitive use. The chemical changes induced by frying,such as hydrolysis and oxidation, generate reactionsin its by-products, such increasing FFA values, whichgives rise to off flavors and odors. This justifies the highFFA content in the UCO in our study. We also foundthat increasing adulterant concentrations correspondedto high FFA values, which makes the adulterated oilsunsafe for frying or human consumption.Characterization of oil properties using FTIRspectra. We found no significant differences betweenthe spectral features, despite slight changes in theabsorbance of some bands and a few shifts in theirexact position. Figure 1 shows the FTIR spectra of theFPO and UCO samples at ambient temperature. Boththe FPO and UCO displayed some typical spectralfeatures associated with oils. Both spectra were similarin terms of shape, position of the characteristic bands,and the presence of peaks. These similarities can beexplained by the same origin of the oils and the presenceof identical principle components in their composition,which are triglycerides [10, 31].However, variation in the oil composition is animportant factor that influences the exact position of thebands, as well as shifts in the spectra [19, 31–33]. Thevariation in both spectra could be due to the qualitydegradation caused by adulteration. Table 2 summarizesthe significant aberrations observed in the FTIRTable 1 FFA content in oil samples with different adulterationlevelsOil sample Adulterantconcentration, %Free fattyacid, %Fresh palm olein20% adulterated oil40% adulterated oil60% adulterated oil80% adulterated oilUsed cooking oil0204060801000.90 ± 0.161.34 ± 0.141.69 ± 0.002.12 ± 0.052.58 ± 0.043.25 ± 0.06109Tan S.L. et al. Foods and Raw Materials, 2022, vol. 10, no. 1, pp. 106–116spectra of all the oil samples in response to adulterationwith UCO.The FTIR spectrum is divided into two distinctiveregions. They are functional group and fingerprintregions corresponding to 4000–1650 and < 1650 cm–1,respectively. The entire spectra that we obtained for allthe oil samples were seemingly identical because of theirsimilar fatty acid compositions.Nevertheless, we found that the UCO andAO samples with increasing adulteration levelsdemonstrated a slight aberration at 3006, 2922, 2853,2680, 1744, 1654, 987, 968, and 722 cm–1 in termsof absorption bands and absorbance intensity. Theadulteration of FPO with UCO resulted in a shift of the3006 cm–1 band (Fig. 2a). This finding was in agreementwith [19, 32, 34, 35] showing that the exact band positionwas determined by oil composition and unsaturationlevel. FPO recorded its highest absorbance at 3007 cm–1,while AO reached its maximum at 3006 cm–1 due toreduced unsaturation. This suggested that exposure offrying oil to high heat had an effect on its unsaturationdegree. 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–1which had been heated repeatedly, showed higherabsorbance at 3006 cm–1. This observation wasconsistent with a previous study by Alshuiael andAl-Ghouti, which proved that high heat applicationcaused oil to become more unsaturated by losinghydrogen atoms [35].We also observed strong and sharp absorption bandsat 2922 and 2853 cm–1 due to the symmetric stretchingvibration of aliphatic groups (-CH). The bands areattributed to the presence of aliphatic fatty acid chains.The high absorption peak of FPO at around 2922 cm–1was determined by its unique fatty acid composition.Apart from that, we identified a weak absorptionband at around 2680 cm–1, which could be attributedto carbonyl ester (-C=O) caused by Fermi resonance(Fig. 2b). The absorption band at 2680 cm–1 indicatedthe presence of aldehyde containing the O=C-H group.The increment of carbonyl aldehyde correlated with theadulteration incidence. As we can see in Fig. 2b, theconcentration of aldehydes in the UCO samples andin most AO samples was far higher than that in FPO,except for 80% AO.The concentration of volatile aldehydes isassociated with a degree of oxidative degradation ofoil, as aldehydes are major volatile compounds emittedupon 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 theoil’s shelf life, and may induce health problems [38].We found that the UCO exposed to repeated frying gavehigh absorbance intensity at 2680 cm–1, which mightbe attributed to an increment of volatile aldehydes dueto lipid oxidation that consequently degrades the oilquality.In relation to that, we observed a strong and sharpabsorption band at 1744 cm–1 (Fig. 2c) due to thepresence of the C=O group of triglycerides caused bystretching vibration. This was due to the decompositionof unstable primary hydroperoxides, which formed uponoxidation, into stable secondary oxidation products suchas aldehydes and ketones, which cause an absorbanceAbsorbanceWavenumber, cm–1Table 2 Significant aberrations in the FTIR spectra for oil samples in response to adulterationDescription of spectra feature SignificanceSlight shift of band near 3006 cm–1Strong band at 2922 and 2853 cm–1Weak band at 2680 cm–1Strong band at 1744 cm–1Increased absorbance of band at 1680–1630 cm–1(or decreased absorbance of band at 1654 cm–1)Maximum absorption at 987 and 968 cm–1Appearance of band at 968 cm–1Progressive decrease in absorbance of bandat 700–725 cm–1 (or at 722 cm–1)Reduced degree of unsaturation caused by diminution of cis- olefinic doublebonds (=CH)Presence of aliphatic methylene (-CH2) group indicative of saturation levelPresence of saturated aldehydes as a marker of advanced oxidationAppearance of carbonyl compounds and other secondary oxidation productsPresence of trans- and cis- isomers due to cis-trans- isomerization uponthermal stressFormation of trans- isomers (conjugated trans and non-conjugated transrespectively)induced by conjugation and cis-trans- isomerization due to heatPossible presence of secondary oxidation products (aldehydes, ketones) withisolated trans- double bond indicative of advanced oxidationDisappearance of cis- double bonds indicative of reduced unsaturation110Tan S.L. et al. Foods and Raw Materials, 2022, vol. 10, no. 1, pp. 106–116near 1744 and 1728 cm–1. High absorbance of FPO at thisband could be explained by prolonged storage, whichintensified the oxidative reaction [39].The absorption band near 1680–1620 cm–1 couldbe assigned to the C=C stretch (Fig. 2d). The peak at1654 cm–1 showed a general declining trend inabsorbance with increasing adulteration levels, implyingthe disappearance of the cis- carbon-carbon double bondwithin the molecular structure. This could be due to thethermal and/or oxidative degradation of the oil samples.The accumulation of trans- fatty acids in all thesamples was further evident through an increasingtrend in absorbance from the cis- C=C stretch bandregion (1660–1630 cm–1) to the trans- C=C stretchband region (1680–1660 cm–1). This might be explainedby the occurrence of cis-trans isomerization inducedFigure 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–1AbsorbanceWavenumber, cm–1AbsorbanceWavenumber, cm–1AbsorbanceWavenumber, cm–1AbsorbanceWavenumber, cm–1AbsorbanceWavenumber, cm–1AbsorbanceWavenumber, cm–1a bc de f111Tan S.L. et al. Foods and Raw Materials, 2022, vol. 10, no. 1, pp. 106–116by thermal stress, changing the initial cis- geometricconfiguration into trans- and resulting in trans- fattyacids accumulation [19, 40, 41]. This finding wasfurther reinforced by the absorption bands at 968and 722 cm–1, corresponding to bending vibrationof -HC=CH- in trans- and cis- configuration,respectively (Fig. 2e and 2f).The UCO generally demonstrated a higherabsorbance intensity than the FPO at 968 cm–1.This observation related to the increment of transcompositioncaused by cis-trans- isomerization,resulting in deteriorated oil quality. Meanwhile, theband near 722 cm–1 was responsible for the cis- doublebonds of disubstituted olefins. The UCO showed lowerabsorbance values compared to the FPO, which wasprobably due to the reduction of cis- C=C doublebonds of unsaturated fatty acids. This observation wasconsistent with а previous study [19] that showed аprogressive decline in absorbance at 722 cm–1 indicativeof cis-trans- isomerization in unsaturated fatty acids,which subsequently resulted in double bonds vanishingfrom the cis- conformation.In addition, we found a very weak absorption bandat 987 cm–1, which indicated the presence of trans-,trans- and/or cis-, trans-conjugated diene groupsof hydroperoxides. Oil oxidation causes cis doublebonds to disappear, also leading to the isomerizationof cis- fatty acids to trans- isomers and hydroperoxide(primary oxidation products) generation [42]. Unstablehydroperoxides decompose into aldehydes, ketones, andother secondary oxidation products, which are morestable. These volatile compounds are responsible for theoff-odor of the oxidized oils.According to Guillen and Cabo, the absorptionband at 967 cm–1 indicates the possible presence ofsecondary oxidation products such as aldehydes andketones, which contain isolated trans- double bonds [43].As can be seen in Fig. 2e, the UCO showed a relativelyhigher absorbance at 987 and 967 cm–1 compared to theFPO. This was due to the generation of trans- isomersthat contributed to conjugated trans- isomers causedFigure 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 nmby the exposure of UCO to harsh frying that advancedoxidation.This observation was in agreement with the studiesby Lim et al. and Poiana et al. [10, 19]. Nevertheless,we also used the UV-Vis spectroscopy as an exceptionalalternative to the FTIR spectroscopy in detecting thepresence of primary (232 nm) and secondary (270 nm)oxidation products, which will be discussed later.Detection and quantification of FPO adulterationusing UV-Vis spectrophotometry. The UV-Visspectrophotometry is a simple analytical method todetect and quantify oil adulteration incidence. Thismethod evaluates the authenticity of oils by measuringabsorption bands between 200 and 400 nm [21]. TheUV-Vis spectra from 200 to 400 nm are considered to bedirectly related to oil quality [20, 21].Figure 3a illustrates a significant peak that weobserved within this range, from 200 to 400 nm. Thisfinding was consistent with the previous studies [20, 21],which detected oil adulteration incidence by observingthe molecular absorption of UV-Vis spectra within thedesignated range.Figure 3b shows an enlarged view of the UV-Visspectra. We found that the maximum absorption at 232and 270 nm was related to the presence of conjugateddienes and trienes, which served as the best indicator ofoil quality. This is because conjugated dienes and trienesare 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 maximumabsorption at 232 and 270 nm correlate with thepresence of oxidation products that are exceptionallypowerful in determining the adulteration incidence inoil. These absorptions are typically expressed as specificextinctions at 232 and 270 nm denoted by K232 and K270,respectively [20].Traditionally, the peroxide value and the anisidinevalue are often used together to measure the oxidativestatus of edible oils. They reflect the concentration ofprimary (hydroperoxides) and secondary (aldehydea bAbsorbanceWavelength, nmAbsorbanceWavelength, nm112Tan S.L. et al. Foods and Raw Materials, 2022, vol. 10, no. 1, pp. 106–116and ketones) oxidation products, respectively [44, 45].Repeated frying accelerates the accumulation ofoxidative products, thus contributing to higherperoxide and anisidine values indicative of deterioratedoil quality [46].Xu et al. reported that palm olein exhibited asignificant increment in the peroxide and anisidinevalues with increasing frying cycles [47]. Therefore,in counterfeit oil, their significant increase could beconsidered a result of quality degradation. Nevertheless,these chemical analyses are lengthy, expensive, andinvolve hazardous chemicals [48]. Thus, we preferred touse spectrophotometry to determine oxidative productsin the oil samples.Figure 4a shows maximum absorption of all theoil samples at 232 and 270 nm, while Fig. 4b shows anincreasing trend of K232 and K270 values with adulterantconcentrations. We found the absorbances at 270 nm tobe significantly higher than those at 232 nm.We found a linear relationship between adulterantconcentrations and absorbances at 232 and 270 nm. TheAO samples showed higher absorbances with increasingadulterant concentrations, while the FPO and the UCOsamples had the lowest and highest values, respectively.We also observed a tendency for all the samples toshow much higher absorption at 270 nm, comparedto 232 nm. This was due to the formation of relativelyunstable hydroperoxides (primary oxidation products),which directly correlated with absorbance at 232 nmand could decrease in number over time [10]. Theytended to decompose into more stable, complex formsof secondary oxidation products including aldehydes,ketones, and alcohols, which corresponded to theabsorption at 270 nm.This observation was in agreement with thatreported by Lim et al., Maskan and Bagci, andJolayemi et al. [10, 29, 49]. This experimental findingalso supported the application of specific absorbancesin the ultraviolet region at 232 and 270 nm to detectadulteration. They can serve as an oil quality indicatorFigure 5 Comparison of sensory attributes for chicken nuggetsfried in different sets of oil samplesthrough the measurement of primary and secondaryoxidation indicative of oxidative deterioration [50]. Thisfinding was consistent with that reported by Amereihet al., where high absorbance at these particularwavelengths indicated oil adulteration [21]. Thus, highquality oil shows low absorbances at 232 and 270 nmand vice versa.Effect of AO on the quality of fried chickennuggets. In this study, chicken nuggets were fried intwo sets of oil samples, FPO and 60% AO. Motivated byhealth concern, we only used 60% AO to simulate theadulterated palm cooking oil that was commonly foundin the night market. Figure 5 compares the averagescores of sensory attributes for the chicken nuggets friedin FPO (code 831) and 60% AO (code 524).We observed no significant difference (P > 0.05)between the chicken nuggets fried in FPO and thosefried in 60% AO in terms of sensory attributes includingflavor, color, juiciness, taste, and overall acceptability.However, there was a significant difference (P < 0.05) incrispness. These observations concluded that adulteratedoil with 60% UCO did not have a significant effect(P > 0.05) on the sensory perception of chicken nuggetsFigure 4 (a) Maximum absorption of fresh palm olein (FPO), used cooking oil (UCO), and adulterated oil (AO) with increasingadulterant concentrations at 232 and 270 nm, (b) Direct relationship between adulterant concentrations and UV absorbancesat 232 and 270 nmAbsorbanceWavelength, nmK extinctionAdulterated oil, %a b113Tan S.L. et al. Foods and Raw Materials, 2022, vol. 10, no. 1, pp. 106–116fried in it, compared to FPO. Our findings were similarto those by Enriquez-Fernandez et al., who reportedan insignificant difference (P > 0.05) between thefoods fried in used oil and fresh oil in terms of sensoryevaluation [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, weobserved that the 60% AO-fried nuggets were cookedfaster and therefore turned golden brown in a shortertime than those fried in FPO. 60% AO was much darkerand intense in color compared to FPO. Thus, our studyshowed a negligible effect of frying oil on the color ofchicken nuggets. This finding was in line with the resultsby Ahmad, but opposite to those reported by Li, whoemphasized that the color of frying oil influenced thecolor of fried foods [52, 53].Although taste differences were insignificant(P > 0.05) between the chicken nuggets fried in FPO andthose fried in 60% AO, we observed an appreciable gapin the scores. Some panelists mentioned an unpleasantrancid taste of the samples coded 524, which were friedin 60% AO. This rancid taste became more obviousand intense over time. This observation was furtherenhanced by Okparanta et al., who reported that rancidoil led to abnormal rancid taste in fried foods [54].However, there was a significant difference(P < 0.05) in crispiness, a desirable textural qualityof fried foods. The chicken nuggets fried in 60% AOtended to be perceived with increased crispiness,compared to those fried in FPO. This observation mightbe due to a considerable time gap between the fryingprocess of the samples and their sensory evaluation.The prolonged exposure to atmospheric air could have anoticeable influence on the sensory crispiness of both theFPO- and 60% AO-fried nuggets.This finding was consistent with those by Antonovaand Sung [55, 56]. In particular, Antonova reporteda correlation between increased holding time underambient conditions and decreased crispiness perceivedby the panelists [55]. Holding time, which is definedas the minimum and maximum time after frying thata product can be used for sensory evaluation, shouldbe determined for fried chicken nuggets to minimizevariation in the test results. The previous studiessuggested that breaded fried chicken nuggets should beserved for sensory evaluation within 10 min after frying,under ambient conditions, to avoid variation in the testresults [55]. Any longer than the suggested holding timecan have an impact on the panelists’ sensory perception.However, it is worth noting that the chicken nuggetsfried 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 andBordin et al., suggesting that the optimum quality offried food can be achieved with moderately alteredand reused frying oil, instead of fresh oil [57, 58].This is because of the role of surfactant compoundsin the frying process. These compounds accumulatein increasingly abused oils and facilitate the contactbetween foods and oil, thus contributing to bettercharacteristics of fried food products.CONCLUSIONIn conclusion, our study showed the effects ofadulteration with used cooking oil on both the oilproperties and the quality of fried chicken nuggets. Weobserved higher FFA contents in the oils as adulterantconcentrations increased. Pure UCO recorded thehighest FFA value and reached the discard point set bylegislation.The chemical characterization of oil properties byusing the FTIR spectral analyses determined somedifferences between FPO, UCO, and AO in terms ofthe exact position of band appearance and absorbanceintensities. Significant aberrations in the FTIR spectrawere observed at 3006, 2922, 2853, 2680, 1744, 1654,987, 968, and 722 cm–1.The UV-Vis spectral analysis used absorbancesat 232 and 270 nm (K232 and K270, respectively) asan indicator of oil adulteration. We found a linearincreasing relationship between the adulterantconcentrations and the K extinction values, whichenabled the detection and quantification of adulterationwith UCO.The sensory evaluation of the chicken nuggetsfried in FPO and AO showed no significant effects ofadulteration with UCO on their quality.CONTRIBUTIONThe authors were equally involved in writing themanuscript and are equally responsible for plagiarism.CONFLICT OF INTERESTThe authors declare that there is no conflict ofinterest.ACKNOWLEDGEMENTSThe authors gratefully acknowledge the technicalassistance from the laboratory staff with operatingthe instruments. The authors would also like to thankthe Faculty of Applied Sciences and Technology,Universiti Tun Hussein Onn Malaysia, for its supportthroughout the study, as well as CS Oil & Fats Sdn. Bhd.for supplying the pre-filtered UCO to be used as rawmaterial in this project.