A study on the physicochemical characteristics of popular cooking oils in Cambodia

Cooking oils tend to deteriorate in quality through oxidation reactions and microbial degradation, causing nutritional loss, consumer unacceptability, health risks such as cancer and inflammation. This study aims to assess the quality of five popular cooking oils sold in Cambodia, including two sunflower oils, two soybean oils, and a refined palm olein cooking oil. All the samples were collected from supermarkets in Phnom Penh, Cambodia. The peroxide value (PV), acid value (AV), and anisidine value (AnV) of the cooking oils ranged from 1.93 to 5.68 meq O2/kg oil, 0.11 to 0.26 mg KOH/g, and 28.71 to 62.49, respectively. The iodine value (IV) of soybean oil, sunflower oil, and refined palm olein oil is 121.53 to 125.91 g I2/100g, 127.39 to 128.89 g I2/100g, and 68.61 g I2/100g, respectively. The colour of the oils was presented as L, a, and b*, which ranged from 32.64 to 34.42, -1.48 to -0.34, and 1.50 to 5.25, respectively. The PV and AV of the cooking oils are within the acceptable range of the FAO’s Codex Alimentarius Standards which is 10 meq O2/kg and 0.6 mg KOH/g, respectively. According to the research findings, the popular cooking oils sold in Cambodia are in good quality for consumption.


Introduction
Fats and oils are essential nutrients that have a significant impact on various bodily functions.They are derived from plants, animals, or are modified in origin [1.2].Fats possess unique properties, including solubility in organic solvents, insolubility in water, and lower density than water at room temperature.The terms "fats" and "oils" describe the solid and liquid states of lipids at normal room temperature, respectively [3].
Improper production methods, handling, and storage can lead to cooking oils becoming rancid, losing their nutritional value, and flavor [4,5].There are many main factors affecting the physicochemical characteristics, rancidity or deterioration time including moisture, microorganism, oxygen, antioxidant, and exposure to the sunlight [6].
In qualities analysis of cooking oils, the important parameters including peroxide value (PO), acid value (AV) or free fatty acid, iodine value (IV), Anisidine value (AnV), Specific UV (233nm and 269nm), and color were observed [7].Consuming rancid cooking oils can lead to health hazards such as cancer and inflammation.Furthermore, opting for unsaturated cooking oils is preferable over IOP Publishing doi:10.1088/1755-1315/1297/1/012002 2 saturated fatty acid ones as a higher intake of saturated cooking oils raises the likelihood of developing cardiovascular disease [8,9].But using high unsaturated cooking oil in cooking method using high temperature can make the oil susceptible for rancidity [8][9][10].
The main objective of this study was to characterize the quality of five popular cooking oils sold in Cambodia's market.

Cooking oil samples
The cooking oil samples including pure sunflower oil, sunflower oil, two types of soybean oil, and refined palm olein cooking oil were purchased from supermarket in Phnom Penh, Cambodia.The cooking oil samples were stored in a dark place at room temperature prior to analysis.

Peroxide value
In this experiment, 5.0 g of the oil samples was mixed in 50 mL of 3:2 of acetic acid: chloroform and immediately reacted with 0.5 mL of saturated potassium iodide.A volume of 30 mL of distilled water was added into the mixture.The liberated iodine was then titrated with 0.1 M sodium-thiosulphate using 0.5 mL of 1% starch solution as an indicator until the dark blue color was removed.And the blank sample was performed without the sample [11].The peroxide value was determined using the equation (1).
Where B is amount in millilitre of sodium-thiosulphate used for blank titration, S refer to amount in millilitre of sodium-thiosulphate titrated by the oil sample, M is the molarity of sodium-thiosulphate, and W is sample weight.

Acid value
Five grams of oil sample were dissolved with 75 mL of freshly neutralized hot ethanol.Phenolphthalein was used as an endpoint indicator.The mixture was then titrated against 0.1 N of potassium hydroxide until the first pink color appeared which persisted at least 30s [12].The acid value was determined as equation (2).
where V represent the amount in milliliter of standard potassium hydroxide solution (mL), N is normality of standard potassium solution, W is sample weight in grams, and molecular weight of potassium hydroxide is 56.1 g/mol.

Iodine value
A specific amount of oil sample was weighed and put in 500 mL.Then, 15 mL of mixture of cyclohexane and acetic acid (1:1 v/v) and 25 mL of Wijs solution were added.And the blank test was determined without the oil sample.The solution was then shaken and put in a dark place for an hour and 2 hours for the sample obtaining IV<150 and IV>150, respectively.A volume of 20 mL of 10 % KI was added and followed by 100 mL of distilled water.The final solution was titrated with 0.1 mol/L sodiumthiosulphate solution using starch as an indicator [13].The iodine value was then calculated as follow: Where B is the amount in mL of sodium thiosulphate standard solution reacted with the blank, S represented in mL of sodium thiosulphate standard solution required for sample, M is molarity of the standard sodium thiosulphate, and W is weight in g of the sample.

Anisidine value
A gram of oil sample was dissolved and adjusted the volume to 25 mL volumetric flask using 2,2,4trimethylpentance.The unreacted test solution was prepared by transferring 5 mL of previous solution into the test tube and 1 mL of glacial acetic acid was added.The solution was incubated in a dark place at 23 °C for 10 min.The solution was transferred to a clear and dry spectrophotometer cell to observe the absorbance (A0) at 350nm noticed as the unreacted test solution.The absorbance of reacted test solution (A1) was analysed at 350nm, but glacial acetic acid was substituted by anisidine reagent.The absorbance of blank (A2) was observed according to the reacted test solution method by replacing the sample to 5 mL of iso-octane [12].The anisidine value of the vegetable oil sample was calculated as following formula: Where A0 means refer to absorbance of the unreacted test solution, A1 represent absorbance of reacted test solution, A2 is absorbance of the blank, Q is sample content (gram/mL, Q is 0.1 g/mL), V equal to 25 mL, the volume in which the test sample is dissolved, 1.2 refer to the correction factor for the dilution of the 5 mL of test solution with 1 mL of the reagent, and m is sample weight in grams.

Specific UV extinction
The oil sample was weighted 0.12 g to the nearest 0.0001 g and dissolved in 25 mL volumetric flask using iso-octan.The absorbances of the solution were analysed using spectrophotometer at 233, 269, and 446 nm to determine the UV extinction at 233 nm, UV extinction at 269 nm, and carotene content, respectively [12].The specific UV extinction at 233 nm and 269 nm were calculated as equation below: Where A is absorbance at wavelength (λ), C is concentration of the sample solution (g/100g), carotene content (mg/kg) was examined as follow: Where E is absorbance at 446 nm, C is concentration of the solution (g per 100 mL), and I is path length of the cell (cm)

Colour
The colour of cooking oil samples was determined following CIE lab using Chroma Meter (CR-400, KONICA MINOLTA, Japan).The colour parameters were presented as L* (lightness extends from 0 to 100 for black to white, respectively), a* (redness to greenness), and b* (yellowness to blueness).The instrument was standardized using white calibration plate.

Statistical analysis
For all parameters, the experiments were done in triplicate and the values of each parameter were reported in term of mean ± standard deviation (SD).The comparison of mean value was made using analysis of variance ANOVA followed by SPSS with (p<0.05)significant level.

Peroxide value
Peroxide value is the determination of active oxygen bound by cooking oil per kilogram oils.The mean value of peroxide value of five different cooking oils sold in Cambodia's supermarket was presented in Table 1.Peroxide value of the cooking oils showed the significantly different from each other which ranged from 1.93 to 5.68 meq O2/kg.The highest peroxide value was noticed on sunflower oil due to sunflower oil contain high polyunsaturated fatty acid that could cause the auto oxidation [14].In addition, the peroxide value in refined palm olein cooking oil was also high due to the shortest expiration date upcoming compared to other cooking oils.Furthermore, all cooking oils provided peroxide value in the Codex Alimentarius Commission standard (10 meq O2/kg).The other possible reasons for high peroxide value are transparent packaging, inappropriate storage conditions and distribution.Cooking oil that contains high amount of unsaturated fatty acids, particularly linoleic and linolenic acid, will increase an oxidation rate 25 times greater than that of oleic acid [15,16].

Acid value
Acid value is the free fatty acids (FFAs) produced by the hydrolysis of fats and oils.The amount of free fatty acids is influenced by many factors such as storage time, temperature, and moisture, because fats and oils are changed and degraded due to the various conditions including as storage, processing, heating, or frying.The presence of free fatty acids significantly affects the characteristics since they are less stable than neutral oil, making them susceptible to oxidation and rancidity.[17].Acid values of cooking oils are presented in Table 1.Acid values of cooking oils in different brands showed the significant (p<0.05)difference which ranged from 0.11 to 0.26 mg KOH/g.The highest acid value was found in refined palm olein cooking oil which is in the line with high value in peroxide value.Even the acid values are a little bit high, but it is still in the standard range which is lower than 0.6 mg KOH/g.This high acid value in cooking oils may be associated with inadequate neutralization, triglyceride hydrolysis, light exposure, lipase enzyme activity, and high storage temperatures [18,19].

Iodine value
Iodine value is the determination of iodine reaction with the double bonds of unsaturated fatty acid which refers to the saturation of cooking oils.Iodine value is also the parameters indicate the oxidation potential of the fats and oils.The higher iodine value indicates a low degree of saturation and greater susceptibility to oxidation [20].Iodine value of all cooking oils were presented in Table 1.There were significant (p<0.05)differences between refined palm olein cooking oil and other cooking oils.The iodine values of sunflower oil and pure sunflower oil were 127.39 and 128.89 g I2/100g, respectively.They were in same range of the previous study which ranged from 125 to 140 g I2/100g [21].In addition, iodine value of refined palm olein cooking oil was 68.61 g I2/100g which is a little bit higher than the previous study (56.77g I2/100g) [22].Furthermore, the soybean oil sold in Cambodian supermarket obtained lower iodine value than that found by a previous study [23].This would be due to the origin of the raw materials that were used.

Anisidine value
Oxidation refers to an unfavourable sequence of chemical reactions that deteriorate the characteristics of fats and oils, resulting in undesirable flavour and odour.Peroxide value is the prevalent parameter used to indicate the oxidation reaction in fats and oils, but to show the overall oxidation profile, anisidine value was used [24].The anisidine value is a measure of secondary oxidation compounds, specifically 2-alkenals and 2,4-alkadienals, which are produced when the decomposition of hydroperoxide.The anisidine value, along with the peroxide value, is used to assess the quality of oils by indicating the presence of second oxidation products.In terms of long-term storage stability, the anisidine value is considered more reliable than the peroxide value as an indicator of oxidative rancidity [25].The anisidine value of five cooking oils are presented in Table 2.The highest anisidine value was found in pure sunflower oil while the lowest anisidine was found in soybean oil 2. The of anisidine value in cooking oils may be due to the difference in expiration date and inappropriate conditions.Refined palm olein cooking oil 40.25 ± 1.93 b 3.34 ± 0.00 a 0.78 ± 0.01 a Note: the different superscript of the mean value within the same column indicates the significant (p<0.05)difference of mean value.

Specific UV extinction
The absorbance at ultra-violet wavelengths of the cooking oil products identified the deterioration and purity of the oil product.In the ultra-violet region, the autoxidation products of fats and oils present the spectra characteristics: hydroperoxide and conjugated dienes from decomposition were detected at 233 nm, while the secondary products of autoxidation and ethylenic diketone were detected at 268 nm [12].The specific UV extinction at 233 nm and 269 nm is shown in Table 2.The specific UV extinction at 233 nm ranged from 3.34 to 4.97 while at 269 nm, it ranged from 0.78 to 2.83.According to these results, it was in the first stage of reactions due to the higher hydroperoxide and conjugated dienes than secondary products of oxidation [26].

Colour
Colour is an important parameter of edible oils affected by both the refining process and the marketplace.Oil appearance might be an indicator of the problems having occurred during processing and storage.The colour of cooking oils is presented in Table 3.The significance of colour between oil brand was observed.The lightness of pure sunflower oils is higher than other four oils while the highest redness and yellowness were found in refined palm olein cooking oil.This could be due to the high pigment like carotenoids in palm fruit compared to other four oils.The high value in redness and yellowness may contribute to the present of carotenoids and other pigments which act as an antioxidants and increase the product shelflife.Carotenoids and other pigments contribute to the coloring of cooking oil, while the formation of polymers and the interaction of peroxide and aldehyde impact its redness and yellowness, respectively.[27].Variations in color of cooking oils can cause from the differences in oil types, degree of refining, and storage conditions [28,29].Note: the different superscript of the mean value within the same column indicates the significant (p<0.05)difference of mean value.

Conclusions
Among the five popular cooking oils sold in Cambodian supermarket, only sunflower oil samples exhibited rancidity levels surpassing half of the FAO standard.In addition, there were no samples that surpassed acid value standard set by FAO.Intriguingly, these deviations were not inherently tied to oil types, suggesting that production and expiration date disparities played a role.The observed high anisidine values were likely affected by the high peroxide values.Subtle variations in color were possibly attributed to oil types, pigments, and refining processes.The wide range of physicochemical qualities observed can be attributed to inappropriate transportation, storage, difference production and expiration date, on-shelf conditions.For consumers, seeking optimal cooking oils, prioritizing a substantial period before the expiration date emerges as a pivotal factor in making informed purchasing decision.

Table 1 :
Peroxide value, acid value, and iodine value of five popular cooking oils sold in Cambodian supermarket.

Table 2 :
Anisidine value and specific UV extinction at 233 nm and 269 nm of cooking oils.

Table 3 :
Color of cooking oils sold in Cambodian supermarket.