Study of physicochemical properties of edible oil and evaluation of frying oil quality by Fourier Transform-Infrared (FT-IR) Spectroscopy

2.1 Material and chemicals

Hydrochloric acid (HCl), sodium hydroxide (NaOH), potassium hydroxide (KOH), idobromine (IBr), sodium thiosulphate (Na₂S₂O₃), potassium iodide (KI), and acetic acid (CH₃COOH) were of A.R Grade and purchased from Merck Darmstadt Germany.

Corn and Mustard oils were extracted from their dried, dehulled and powdered seeds purchased from the local market in Karachi Pakistan.

2.2 Density measurement

Densities of oil samples before and after frying were measured by an R.D bottle with a capacity of 10 mL.

2.3 Boiling point measurement

The boiling point of oil samples were measured by a thermometer ±1 °C. The boiling point depends upon the degree of unsaturation of fatty acids.

2.4 Viscosity measurement

The viscosity of oil samples before and after frying was measured by an Ostwald Viscometer techniconominal constant 0.05 Cs/c, ASTMAD 445 England. The flow time of oil samples were recorded with a stop watch (Japan, CBM, and Corp QSQ) least count ±0.01 s.

2.5 Saponification value measurement

The saponification value is determined by taking 1.0 g of oil sample in a conical flask to which is added 15 mL 1 N KOH and 10 mL of distilled water and heated under a reserved condenser for 30–40 min to ensure that the sample was fully dissolved. After this sample was cooled, phenolphthalein was added and titrated with 0.5 M of HCl until a pink endpoint was reached. A blank was determined with the same time conditions.

2.6 Iodine value (IV) measurement

A known weight of the oil sample is treated with an excess of iodobromine (IBr) in glacial acetic acid. Unreacted iodobromine is reacted with potassium iodide which converts it to iodine. The iodine concentration is then determined by titration with standard sodium thiosulphate.

2.7 Peroxide value (PV) Measurement

Peroxide value is a measure of peroxides contained in the oil. PV is determined by measuring iodine released from potassium iodide.

A known measured weight of oil samples is dissolved in acetic acid then chloroform and saturated KI mixture are added to the sample and the amount of iodine liberated from KI by the oxidative action of peroxides present in the oil is determined by titration with standard sodium thiosulphate using starch solution as an indicator. Titration was also performed for blanks.

2.8 Frying process

Potatoes were peeled and cut into pieces (approx 5 cm³) and were fried in the frying oil at constant temperature at three frying times. Frying experiments were conducted in duplicate in each frying medium.

2.9 Evaluation by FT-IR

FT-IR spectra of oil samples before and after frying were recorded with the help of a Fourier Transform Spectroscopy Model I-R Prestige 21 Shimadzu. It is used to study the saturation and unsaturation composition of heated and unheated oils at room temperature for monitoring the oxidation process in oils.

3.1 Viscosity and density

Oils are mixtures of triglycerides (TGs) and their viscosity depends on the nature of the TGs present in the oil. The viscosity changed due to the different arrangement of the fatty acids on the glycerol backbone of the triglyceride molecule. Therefore, viscosity is related to the chemical properties of the oils such as chain length and saturation/unsaturation. Table 1 shows that at room temperature of 35 °C the viscosity is high in Mustard oil as compared to Corn oil and Mustard oil has a high boiling point relative to Corn oil. It explains that the viscosity and density decreases with an increase in unsaturation and increases with high saturation and polymerization (Kim et al., 2010). Viscosity also depends on sheer stress and temperature. Sheer stress does not have much effect on the storage of oils which are used for edible purposes but the temperature does affect it. Results tabulated in Table 2 revealed that an increase in viscosities was observed for Corn oil while a decrease was observed for Mustard oil at the boiling point and at different frying times with a piece of potato. When the temperature increases the kinetic energy also increases which enhanced the movement of the molecules and reduces the intermolecular forces. The layers of the liquid easily pass over one another and thus contribute to the reduction of viscosity. This phenomenon is also verified by other researchers since oil viscosity depends on molecular structure and decreases with the unsaturation of fatty acids (Kim et al., 2010). The densities of both oils were decreased with the rise in temperature as well as when using the same oil for frying three times with a piece of potato. The densities of oils were related to the standard range of 0.898–0.907 g/ mL approved by the Standard Organization of Nigeria (SON, 2000). The results tabulated in Table 1 show that at room temperature of 35 °C the values of the densities are 0.9694 g/mL and 0.9223 g/mL for Mustard and Corn oils respectively. It may be due to the π bonds that make the bonding more rigid and rotation between C—C bonds becomes more strenuous.

During frying thermo-oxidative or lipid oxidation and hydrolytic reactions take place that results in deterioration in quality of the frying oil (Romero et al., 2000, 2003; Paul and Mittal, 1997). The primary oxidation products that develop in triacylglycerol are hydroperoxides, which may later break down to produce lower molecular weight compounds, such as free fatty acids, alcohols, aldehydes, and ketones, eventually leading to a rancid product (Barthel and Grosch, 1974). Lipid oxidation is a main deteriorative process which has an important implication in stipulations of the quality and value of fats and oils, particularly in relation to the off-flavors that develop as an outcome of autoxidation (Xiuzhu et al., 2007). Normally, frying oils undergo extensive degradation and complex chemical transformations when heated. The presence of air and water accelerated the deterioration of frying oil (Clark and Serbia, 1991) and resulted in an increase in the number of polar molecules as confirmed by viscosity and density data of oils (White, 1991).

3.2 Peroxide value

Peroxide value (PV) is used as a measure of the extent to which rancidity reactions have occurred during storage it could be used as an indication of the quality and stability of fats and oils (Ekwu and Nwagu, 2004). The peroxide value was also found to increase with the storage time, temperature and contact with air of the oil samples. The PV values tabulated in Tables 1 and 2 for both the oils range from 0.388–2.994 meq/kg. Results show that the peroxide values for Corn oil increased from 0.162 (35 °C) to 0.538 meq/kg (boiling point 140 °C) and 2.994 meq/kg (first frying) and then it decreased at second and third fryings gradually while for Mustard oil a decrease in peroxide value was observed from 0.830 (35 °C) to 0.388 meq/kg (boiling point 170 °C) and at three different fryings the pattern was the same as that in Corn oil. The initial PV was found to have occurred around 2.5–5 meq /kg oil which indicates a relatively good quality of these oils. There is a successive decrease in PV when the same oil was used for frying with a piece of potato. The peroxide value determines the extent to which the oil has undergone rancidity. Peroxide value ranges are closely related to the standard value of 10 meq/kg specified by Standard Organization of Nigeria (SON) (2000) and Nigerian Industrial Standard (NIS) (1992).

3.3 Saponification value

Saponification value (SV) is an index of average molecular mass of fatty acid in the oil sample. The SV value obtained for the oil samples in Table 1 showed 153.8 mg KOH/g for Corn oil and 125.6 mg KOH/g for Mustard oil. The values are below the expected range of 195–205 mg KOH/g of oil for edible palm oils as specified by SON (2000) and NIS (1992). The lower value of saponification values suggests that the mean molecular weight of fatty acids is lower or that the number of ester bonds is less. This might imply that the fat molecules did not interact with each other (Denniston et al., 2004).

3.4 Iodine value

Iodine value (IV) measures the degree of unsaturation in a fat or vegetable oil. It determines the stability of oils to oxidation, and allows the overall unsaturation of the fat to be determined qualitatively (AOCS, 1993; Asuquo et al., 2012). It was observed that measured iodine values for Corn and Mustard oils are 15.96 g and 8.10 g respectively. These low iodine values may have contributed to its greater oxidative storage stability. The oxidative and chemical changes in oils during storage are characterized by an increase in free fatty acid contents and a decrease in the total unsaturation of oils (Perkin, 1992).

All these physicochemical parameters such as viscosity, density, peroxide value, iodine value, and saponification values are qualitative properties of oils and do not indicate the position of the double bonds or the amount of olefinic carbon but rather it provides an overall status of unsaturation of the oils so it is not possible to point out the position of double bond(s) which are more susceptible to oxidation (Knothe and Dunn, 2003).

3.5 Spectral analysis

FT-IR spectroscopy is an excellent tool for analysis as the intensities of the bands in the spectrum are proportional to concentration. Mid IR spectra have been used to characterize edible oils and fats because they differ in the intensity and the exact frequency at which the max absorbance or transmittance of the band appears, according to the nature and composition of the sample (Guillen and Cabo, 2000). FT-IR spectra of Corn and Mustard oil samples showed that there exists a notable difference in the band at room temperature, at boiling point and between the same oil used for frying three times with a piece of potato. The oil composition affects the exact positions of the band and yields a shift when the proportion of fatty acids changed. At room temperature Figs. 1a and 2a the band around 3473 cm⁻¹ assigned to O—H stretching vibration of hydroperoxide, 3006.8–3007.0 cm⁻¹ as C—H stretching vibration of the cis-double bond ⚌C—H) and 2854.7–2925.8 cm⁻¹ shows C—H asymmetric and symmetric stretching vibrations of the aliphatic CH₂.

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Figure 1 FT-IR spectrum of corn oil. (a) At room temperature (35 °C), (b) boiling point, (c) first frying, and (d) 3rd frying.

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Figure 2 FT-IR spectrum of mustard oil. (a) At room temperature (35 °C), (b) boiling point, (c) first frying, and (d) 3rd frying.

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At different temperatures and during frying three times with a piece of potato, the percentage transmittance of almost all the peaks increased indicating a decrease in absorbance which may be due to the hydrolysis of oil during frying and the formation of free fatty acids and mono and diglycerides. These compounds accumulated in the frying oil with repeated use as the hydroperoxides also decrease which may be due to decomposition of hydroperoxides and secondary oxidation initiation.

At 35 °C Figs. 1(a) and 2(a) and at the boiling point of Corn oil (140 °C) and Mustard oil at 170 °C, the region of the double bond stretching shown at 1745.5 cm⁻¹ represents C⚌O ester carbonyl of triglycerides and at 1654.0–1658.7 cm⁻¹ indicates C⚌C stretching vibration of the cis olefins as shown in Figs. 1(b) and 2(b) respectively. The band trend is much more similar at the first frying and third frying with a piece of potato at 1745–1747.5 cm⁻¹ and 1660–1652.3 cm⁻¹ respectively as in Figs. 1(c and d) and 2 (c and d). This observation shows that there is presence of a saturated aldehyde functional group or other secondary oxidation products which causes an absorbance at 1728 cm⁻¹ to overlap with the stretching vibrations at 1746 cm⁻¹ of the ester carbonyl functional group of the triglycerides. The same patterns were observed at the boiling point. In Mustard oil Fig. 2(b) represents that at boiling point and for one and three times frying Fig. 2(c and d) there is a peak of double bond’s stretching which is —C⚌O free fatty acids at the region 1705.0 cm⁻¹ and the spectra showed overall signal pattern looked similar with each other. As the spectral regions undergoes several changes during oxidation processes at the room temperature and at different temperatures oils showed some regions of other deformations and bending at 1460–1462 cm⁻¹ of —C—H bending vibrations of the CH₂ and CH₃ aliphatic groups and at 1373.2–1377 cm⁻¹ of —C—H bending vibrations of CH₂ group were observed while at 908.2–970.2 cm⁻¹of CH⚌CH shows trans unsaturation.

For the determination of oxidation processes, at the boiling point and at three frying times of the Corn oil Fig. 1(b–d) the valuesof frequencies in the samples vary significantly from 2852.7–2926.0 cm⁻¹. The 2926.0 cm⁻¹ band is attributed to the symmetric stretching vibration of the aliphatic —CH₂ group. For Mustard oil at the boiling point Fig. 2(b), there is an additional peak at 3633.8 cm⁻¹ which shows that the secondary oxidized product has been formed.

FT-IR capability using finger print as tool to make a first differentiation and representative finger print (1163.0–723.3 cm⁻¹) in the infrared spectra of the two oils are shown in Figs. 1(a) and 2(a). The major peaks in these spectra that arise from the stretching vibration of C—O at 1163.0–1236.3 cm⁻¹ and at 723.3 cm⁻¹ represent (—CH₂), —CH⚌CH— overlapping of the CH₂ rocking and the out of plane vibration of cis disubstituted olefins. All these changes in the spectral regions showed several changes during the oxidation process. The frequency of the 3007.0 cm⁻¹ band depends on the oil composition as oil with a high proportion of linoleic or linoleic acyl groups show higher frequency data at this band than oils with a high proportion of oleic acyl group. The band at 2854 cm⁻¹ and the shoulder at 2962 cm⁻¹ increase their intensity but the band reduces its absorbance and increases percent transmittance.

The evaluated results of Mustard and Corn oils revealed that at the room temperature the degree of unsaturation is low and there is trans hydroperoxide forms of esters present with primary oxidized products and as the sample is treated at higher temperatures up to their respective boiling points when frying with a piece of potato the decrease in %T indicates an increase in the absorbance of peroxides. Secondary oxidation initiates the formation of secondary oxidation products and free fatty acids were also present. Thus each type of oil sample including fresh oil at room temperature and used oils (at different temperatures) showed very similar FT-IR spectra. However the heights of some specific bands were noticeable indicating the difference in quality of the two different oil samples used in this present study.