Samples of corn and soybean oil (50 mL) were heated in Pyrex glass beakers from 303 °K to 343 °K using a hot plate 78HW-1 Jiangsu, China (Mainland). Samples were taken at intervals of 10 °K in separate glass vials and stored until analysis.

Samples of corn and soybean oil (50 mL) were taken in Pyrex glass beakers and exposed to microwave irradiation (model DW-112 C, Pakistan) operating at the high power setting (800W, 2450MHz) for 0.5 min, 1 min, 2 min, 4 min, 6 min and 10 min. Samples were collected at each time interval. The temperature of each oil sample was noted using a thermometer. After each heating treatment, the microwave oven was stopped for 30 minutes to cool before starting the next heating. The collected samples were stored in sealed vials until further analysis. The temperature profiles of the oils after microwave irradiation exposure at different heating times are shown in Figure 1.

The density of corn oil and soybean oil samples was measured using a relative density bottle and Sartorius electronic balance (model BL-1505, Germany) with ± 0.001g uncertainty. The viscosity of corn oil and soybean oil samples was determined using Ostwald viscometer (techniconominal constant 0.05Cs/c, ASTMAD 445 England). The oil sample was poured into a viscometer and the time of flow between the two marks of oil samples was measured using a stopwatch with a count of at least 0.2s. (Zahir et al., 2017).

The acid value of the oil samples (S) was measured by dissolving 4 g oil in 50 mL 2-propanol and titrated with a 0.1 N potassium hydroxide solution using a phenolphthalein indicator. A blank (B) determination was also made with the same procedure without the oil sample. The acid value was determined through the reported approach of AOCS Cd 3-63, 1993:

An accurately weighed 1 g oil sample (S) was dissolved in 15 mL of a 1 N ethanolic potassium hydroxide solution (KOH) and 10 mL double distilled water were added. Refluxed the contents for 40 minutes to convert the triglyceride molecules to glycerol. After cooling, titration was performed with 0.5 N hydrochloric acid (HCl) using phenolphthalein indicator. Blank (B) determination was also made using same procedure without oil samples. Saponification values were determined through the equation (AOCS Cd 3-25, 1997):

A 5 g oil sample (S) was dissolved in a 30 mL mixture of acetic acid and chloroform which was further reacted with 0.5 mL of a saturated potassium iodide (KI) solution. After one minute of shaking, 30 mL water were added and the contents were shaken vigorously to liberate iodine from the organic to the aqueous layer. Iodine was titrated with a 0.1 N sodium thiosulphate solution using a starch indicator. A blank (B) determination was made using the same procedure without the oil sample. Peroxide values were calculated using the equation (AOCS, 1997 method Cd 8-53):

A 5 g oil sample was dissolved in 25 mL iso-octane. The absorbance of the solution (A_b) at 350 nm was measured using iso-octane as the reagent blank. 5 mL of the above solution and 5 mL of iso-octane were pipetted out in separate test tubes which were further diluted with 1 mL of p-anisidine reagent (2.5% w/v in acetic acid). The absorbance of the sample solution (A_s) against the blank was recorded after 10 minutes at 350 nm using a UV-VIS spectrophotometer (UV-1800 Shimadzu, Koyoto Japan). The p-anisidine values for the oil samples were calculated using the equation (AOCS, 1997 method p2.4):

The contents of conjugated dienes (CDs) and conjugated trienes (CTs) in terms of specific extinctions at 232 nm and 268 nm, respectively were monitored using a UV-VIS spectrophotometer (UV-1800 Shimadzu, Koyoto Japan). The absorbances of the oil samples, properly diluted in iso-octane, were recorded and CDs and CTs values were calculated from the following equations (AOAC, 1997 p2.15):

Where A_s andA_b is the absorbance of the sample and blank, respectively, b is the path length and C represents the percent concentration of oil sample.

The FT-IR spectrometer (model IR-Prestige-21 Shimadzu Corporation, Kyoto, Japan) was used to record the spectra of the oil samples. A drop of each sample was poured between two disks of KBr to prepare a thin film. The spectra were scanned throughout the region from 4000 cm⁻¹ to 400 cm⁻¹ and recorded as percent transmittance values (Zahir et al., 2017).

The statistical analysis of the experimental data was done by taking a triplicate measurement of the sets for all parameters and the values were reported in terms of mean ± standard deviation (SD) in the respective tables. Comparison of the mean values was made using analysis of variance (ANOVA) followed by Tukey’s test on SPSS 15.0 at α = 0.05 significant level.

The density and viscosity data explain the physical nature of oils. The presence of double bonds and length of the hydrocarbon chain in the triglyceride molecule affect the density and viscosity of oils. The values for densities and viscosities of the oil samples were decreased after conventional heating (Adejumo et al., 2015), while they were increased after microwave heating (Li et al., 2018). It was observed that corn oil had lower values of density and viscosity as compared to soybean oil. The higher amount of polyunsaturated fatty acids (linoleic acid) of corn oil is responsible for lower density and viscosity values (Abbas et al., 2016). Soybean oil had higher viscosity values due to the presence of less unsaturated fatty acids as compared to corn oil. After conventional heating, the kinetic energy of the oils was increased and the degradation of oil molecules takes place, which lowers the oil viscosity (Diamante and Lan, 2014). After the application of microwave heating, these unsaturated fatty acids were converted into high-molecular-weight polymeric compounds, cyclic compounds, dimers, trimers, and epoxides, resulting in the formation of a large molecule and led to an increase in viscosity values (Srivastava and Semwal, 2015; Li et al., 2018).

The acid content of the oil is the measurement of fatty acids in oils. It was observed that with conventional heating, acid values were increased by 21.20% and 32.29% for corn oil and soybean oil, respectively. The same results were observed for the microwave heating of the oils for up to 1 min. Microwave heating at a higher temperature, with an increased exposure time of up to 10 min caused a 10.61% and a 44.56% decrease in the acid values of corn and soybean oil, respectively. Corn oil had a high amount of free fatty acids, as observed by the high acid values. During repeated conventional heating, the degradation of triglycerides and the formation of fatty acids were observed by increased acid values (Halim et al., 2016; Adejumo et al., 2015). With microwave heating, the amount of fatty acids increased (Taghvaei et al., 2014) and then these fatty acids degraded to hydro peroxides which were further converted to secondary oxidation products and lower acid values (Leong et al., 2015). The acid values for corn oil showed greater degradation with microwave radiation exposure compared to soybean oil, which confirmed the oxidative stability of soybean oil (Adejumo et al., 2015).

Oils with high saponification values are considered to be better edible sources which contain high proportions of low-molecular-weight and short-chain fatty acids (Adejumo et al., 2015; Hussain et al., 2015). The saponification value of soybean oil was higher compared to corn oil with conventional heating from 303 °K to 323 °K. Microwave heating (0.5 min to 10 min) increased the temperature from 343 °K to 543 °K compared to conventional heating. The lower saponification values for soybean oil with microwave heating compared to corn oil indicated less formation of short- chain fatty acids. The higher degradation rate of long-chain fatty acids to short-chain fatty acids is responsible for higher saponification values in corn oil (Rutckeviski et al., 2016). It was concluded that conventional heating of oil samples converts the triglyceride molecules to glycerol and fatty acids which showed a 35.90% and 5.46% decrease in saponification values for corn oil and soybean oil, respectively. However, significant increases in saponification values for corn oil (101.05%) and soybean oil (68.17%) were observed with microwave heating due to an increase in the amount of short-chain fatty acids (Rutckeviski et al., 2016).

The peroxide value is the chemical property used to measure the extent of rancidity in oils. Fresh oil has lower peroxide values compared to oxidized oil. The results tabulated in Tables 1 and 2 reveal that the peroxide values for corn oil are high compared to soybean oil. The conventional heating of oil showed a 46.40% and 54.14% increase in the amount of hydro peroxides due to the process of oxidation of fatty acids. The same observation was made for canola seed oil (Jalili et al., 2017). The peroxide values of microwave heated oil were initially increased upon irradiation but the continuous exposure of microwave irradiation showed 92.79% and 86.13% decreases in peroxide values with increased heating time, which resulted in the formation of secondary oxidation products by the breaking of hydro peroxides (Abbas et al., 2016; Ghosh et al., 2014).

The peroxide value cannot be used to determine the rancidity of oil at higher heating temperatures due to the instability of hydro peroxides. Various volatile and non-volatile compounds, including dimers, trimers, alcohols, polymers, and other secondary oxidation products are produced by the degradation of hydro peroxides, which strongly affects the stability of the oil and makes it more rancid. This fact led to the representation of rancidity of oil in terms of the p-anisidine value (p-AV) and total oxidation value (TOTOX value). The p-anisidine value of edible oil samples determines the amount of hydro peroxides as well as the aldehydic and ketonic forms of products (Leong et al., 2015). During conventional heating, p-anisidine values were increased by 31.53% and 24.83% for corn oil and soybean oil, respectively. Microwave heating showed a 536.13% increase in the p-anisidine values in soybean oil; whereas the p-anisidine values in the corn oil samples were initially increased by 580.0% (Ghosh et al., 2014) and then further heating caused a 256.82% decrease. P-anisidine values were high for microwave irradiated oil samples compared to conventional heated oil samples, which showed more chemical deterioration in oils due to exposure to microwave irradiation (Abbas et al., 2016).

The total oxidation value (TOTOX) expresses the oxidation state of the oil. The lower value of total oxidation signified better nutritional value of the oil. TOTOX values were calculated by the equation (Halim et al., 2016);

The results reported in Tables 1 and 2 reveal that the soybean oil samples had low TOTOX value compared to the corn oil. The lower TOTOX values for soybean oil indicated higher nutritional value than corn oil. It was also observed that corn oil showed 45.17% and 51.07% increases in TOTOX values, whereas soybean oil showed increases of 51.23% and 15.64% with conventional and microwave heating, respectively. It was also noticed that oil samples exposed to microwave irradiation had higher TOTOX values compared to the conventionally heated oil samples. (Abbas et. al., 2016). The effect of conventional heating (343 °K) and microwave heating (0.5 min) on the oxidative properties of oils are represented in Figure 2.

UV-spectroscopic indices in terms of conjugated dienes (CDs) and conjugated trienes (CTs) were monitored at 232 nm and 268 nm, respectively. The conjugated dienes (CDs) and conjugated trienes (CTs) in the oil were generated by the rearrangement of double bond positions in the monounsaturated fatty acids and polyunsaturated fatty acids, respectively (Li et al., 2018). Oils that have low mono and polyunsaturated fatty acids exhibit low formation of conjugated dienes and conjugated trienes, respectively. Increased values of CDs and CTs upon conventional and microwave heating, indicated the degradation of hydro peroxides and formation of conjugated dienes and conjugated trienes for both oils. Corn oil showed greater values of CDs and low values of CTs compared to the soybean oil. Corn oil contains a high amount of monounsaturated fatty acids, so, the formation of dienes was higher, which was confirmed by greater values of CDs (Ghosh et al., 2014). The conventional heating of corn and soybean oil showed 13.52% and 21.61% increases; whereas microwave heating showed 47.75% and 5.38% increases in CD values, respectively. On the other hand, the conventional heating of corn and soybean oil showed 17.82% and 31.08% increases in CT values; whereas microwave heating showed 150.99% and 224.26% increases in CT values, respectively. A previous report on Perah seed oil (Li et al., 2018) and pumpkin seed oil (Abbas et al., 2017) also confirmed the significant increase in CDs and CTs with increasing heating periods. The oxidative stability of oil decreased with the increase in UV-spectroscopic indices of oils. The low CD and CT values for soybean oil mean better oxidative stability upon conventional and microwave heating.

The Fourier Transform Infra-Red Spectroscopy analysis was used to study the oxidative rancidity of oils. Figures 3 and 4 show the FT-IR spectra of corn oil and soybean oil, respectively, with different heating treatments. During the continuous heating process, the hydrolysis of oil takes place, which resulted in an increase in the amount of hydro peroxides, free fatty acids, mono glycerides, and diglycerides. The formation of secondary oxidation products and characterization of the oils were made through the intensities and frequencies of the peaks and bands in the FT-IR spectra (Zahir et al., 2017; Sadoudi et al., 2014).

The assignment of the vibrations of functional groups in the infrared spectra of the oils are as follows: 3471.87–3441.01 cm⁻¹ (OH stretching vibration of hydro peroxides and FFAs of ≡C-H), 3100–3000 cm⁻¹ (strong peak of -CH (sp² and sp) stretching vibration of cis-double bonds in unsaturated fatty acids) (linoleic and linoleic acyl group), 3000–2850 cm⁻¹ (Two peaks of CH₃ symmetric stretching and sp² CH₂ asymmetric stretching of aldehyde, which confirms the presence of aldehydes), 2679.13 cm⁻¹ (-C=O fermi resonance) (Wu et al., 2015), 1745 cm⁻¹ (aliphatic ester -C=O stretching in the carbonyl group of triglycerides, which means the oil has high nutritional value) (Srivastava et al., 2015).

The FT-IR spectra of heated oil samples show a peak at around 3300 cm^−1, which indicates the formation of hydro peroxides, as observed by Srivastava et al., 2015. The intensity of the cis-double bond near 3008 cm⁻¹ remained almost unaltered or suffered a very slow shifting towards smaller values during oxidative stress. The bands at 2927 cm⁻¹ and 2854 cm⁻¹ increased their absorbance due to surrounding chemical changes as a consequence of the oxidation process (Abbas et al., 2017). Intensity at 1745cm⁻¹ was increased on heating as hydro peroxides decomposed to carbonyl compounds which are secondary oxidation products (carbonyl compounds i.e. aldehydes, ketones, dimers, trimers etc.) and shifted the peak towards a lower frequency ~1728 cm⁻¹ (Srivastava et al., 2015). The smallest variations in the intensities at 2854 cm⁻¹ and 1745 cm⁻¹ were observed for conventional heating, which was justified by the small variations in CD and CT values. Similar observations were reported by Srivastava et al., 2015 in heated virgin coconut oil. The intensity of the band near 1465 cm⁻¹ tended to increase with the oxidative treatment. These increments were found to be higher in the microwave heated oil, indicating that oxidation proceeded more slowly in the conventional heated oil samples compared to the microwave heated one (Abbas et al., 2017).

The peak heights of IR bands were used to calculate some ratios for the comparison of spectra. A symmetric deformation of CH₃ and CH₂+CH₃ appeared at 1375 cm⁻¹ and at 1460 cm⁻¹, respectively. The measurement of the degree of branching was made by the ratios of A₁₃₇₅/A₁₄₆₀. The degree of chain length was measured by the ratio of absorbance band of CH₂ rocking vibration at 720 cm⁻¹ and CH₂+CH₃ or CH₃ symmetric deformation at 1375 cm⁻¹ and 1460 cm^−1, respectively, or A₇₂₀/A₁₄₆₀ or A₇₂₀/A₁₃₇₅ (El-Bassoussi et al., 2010).

The degree of branching and chain length was measured for all treated oil samples and reported in Table 3 for conventional and microwave heating. The results showed that the degree of branching and the degree of chain length of the fatty acids of the oils decreased as the oil was conventionally heated. Microwave irradiation increased the branching pattern and chain length of fatty acids. The results also revealed that the ratios of A₁₃₇₅/A₁₄₆₀ had higher values for soybean oil compared to corn oil samples.