Vegetable seed oils are important part of daily human diet   as they not only give characteristics texture and flavor to food but they also contain essential fatty acids (EFAs)  which possesses high calorific value and are rich in energy for examples, sunflower, soybean, canola and others. These oils are polyunsaturated fatty acids which are susceptible to oxidation    and therefore, start to decompose the moment after they are collected from seeds and this process is accelerated by environmental factors including atmospheric oxidation, humidity and temperature. Apart from this, intense and prolonged frying of different foods including vegetables and meat at elevated temperature cause thermal oxidative degradation in polyunsaturated oils. As a result of thermal stress, the physical and chemical profile of edible oils is changed drastically. Physical parameters include unpleasant odour, change in color and viscosity while chemical reactions triggered by thermal oxidation yield a variety of different products. When oxygen molecules interact with polyunsaturated oils at high temperatures, it is mainly absorbed at double bond linkages and starts the degradation process. This results in the formation of various breakdown products including hydroperoxides and a range of carbonyl compounds e.g. aldehydes, ketones, free fatty acids (FFAs) and other compounds  . In addition to this, the extended frying also results cis to trans isomerization. The oxidation products largely depend on oil source, its composition, thermal stress and other environmental factors.
Many of these oxidative breakdown products are dangerous and therefore, make the edible oil unhealthy for human use. Commercial fryers used edible oils for prolonged times and in this perspective it is important to determine what the oxidation products are and when to change the frying oil. In order to develop oxidation resistance edible grade, shortenings    are developed which are partially hydrogenated oils and often contain edible emulsifiers. Shortenings are resistance towards oxidation at high frying temperature and their thermal stability is an important characteristic which determines the quality of food product.
The conventional chemical tests for determining edible oil quality include acid value, peroxide value, p-anisidine value, iodine value and others. These tests give empirical information about chemical profile of oxidized edible oil rather giving complete data. The conventional methods are laborious as they require extensive sample preparation protocols and also consume hazardous solvents and chemicals for testing edible oil quality. In order to ensure the fitness and quality of frying oil, it is imperative to accurately and rapidly analyze   the breakdown products in oxidized oil samples. For instance, modern spectroscopic tools such as attenuated total reflectance Fourier transformation infrared (ATR-FTIR)     is another method of choice for fast and sensitive analysis of oxidized edible oil without going through lengthy sample preparation steps. The unsaturation percentage, formation of carbonyl compounds, increase in trans isomers, presence of FFAs and information about other oxidation products can be obtained from FTIR data. On the other hand, gas chromatography mass spectroscopy (GC-MS)   is highly sensitive and sophisticated analytical method for separation and analysis of volatile compounds produces during thermal oxidation process.
In this work, we analyzed canola, sunflower and palm olein shortening by conventional volumetric method for monitoring acid values of these samples due to thermal stress. In the next phase, palm olein shortening was analyzed by ATR-FTIR to observe the change in chemical profile of oil including degree of unsaturation, formation of FFAs and aldehyde compounds. GC-MS investigation revealed the nature and composition of FFAs produced during thermal oxidation. The use of ATR-FTIR and GC-MS is suitable for analyzing breakdown products in thermally stressed palm olein shortening and thus, accessing its quality in quick time without extensive sample preparation steps.
2.1. Reagents and Chemicals
All the chemicals and reagents used for determination of acid value were obtained from Riedel-de-Haёn and Merck and used as received. These chemicals include potassium hydroxide (KOH), phenolphthalein (C20H14O4), isopropyl alcohol (C3H8O) and oxalic acid (C2H2O4).
2.2. Thermal Treatment of Edible Oil Samples
The oil samples were heated from 0 - 48 hours at 180˚C ± 5˚C on hotplate under continuous magnetic stirring to ensure uniform heating. The heated oil samples were collected after different time intervals and stored in air tight glass tubes to avoid contamination.
2.3. Acid Value Determination
The acid values of thermally stressed sunflower, canola and palm olein shortening were determined by volumetric method. For the determination of acid value, 5 gm of oil sample was dissolved in 50 mL of isopropyl alcohol in a conical flask and then 2 - 3 drops of phenolphthalein as indicator was added. The contents of flask were titrated against standardized 0.05N KOH till the pink color persists for 10 seconds which is considered as end point. The formula used for calculating acid value is described as follows.
In this expression, V represents the volume (mL) of KOH, N is the normality of KOH and W indicates the weight (gm) of oil sample.
2.4. ATR-FTIR Studies
Thermally stressed palm olein shortening samples were analyzed by Nicolet 6700 ATR-FTIR spectrometer. All the spectra were recorded in absorbance mode in the range from 650 to 4000 cm−1.
2.5. GC-MS Analysis
The chemical composition of untreated and thermally oxidized palm olein shortening samples was analyzed by JOEL GC-MS model JMS-A × 5050 H mass spectrometer (JOEL, Japan) coupled with Hewlett Packard 5890 gas chromatograph. Following conditions were maintained for GC-MS analysis of shortening samples i.e. helium as carrier gas, split ratio 1:100, electrical energy 70 eV, ionization current 200 μA, ionization source temperature 250˚C, column temperature with 6˚C/min rise to 230˚C. The comparative GC-MS data of untreated and thermally treated shortening samples for 48 hours was used to determine the formation of breakdown products due to oxidation. The chemical constituents of oxidized palm olein shortening were identified by their retention time and compared with NIST147.LIB.
3. Results and Discussion
The acid values determined for canola, sunflower and palm olein shortening are displayed in Figure 1. In this graph it can be seen that acid values for canola and sunflower oil increases considerably as the heating time increases indicating that thermal stress results significant oxidation of edible oils. While palm olein shortening remains stable for quite long time and the increase in acid value is much less as compare to canola and sunflower oil samples. This clearly demonstrates that palm olein shortening has good resistance against thermal oxidation.
In the next phase, two palm olein shortening samples heated for 24 hours and 48 hours at 180˚C ± 5˚C were analyzed by ATR-FTIR and their spectra were compared with untreated shortening sample, the recorded spectra are shown in Figure 2. The fingerprint region of FTIR i.e. 1650 - 650 cm−1 is more complex as it contains more peaks than functional groups region i.e. 4000 - 1650 cm−1. In this spectrum of untreated shortening, the absorbance at 721 cm−1 is related to rocking vibrations of −(CH2)n− functional groups of long chain fatty acids whereas a small weak absorbance peak at 868 cm−1 is related to =CH2 wagging vibrations.
Figure 1. Comparison of acid values of canola, sunflower and palm olein shortening as function of heating time, all the samples were heated at 180˚C ± 5˚C.
Figure 2. FTIR spectra of untreated, 24 and 48 hours heated shortening samples at 180˚C ± 5˚C.
Out of plane bending vibrations of −CH=CH− (cis) and −CH=CH− (trans) groups can be observed at 911 cm−1 and 962 cm−1, respectively. Ester groups of fatty acids showed strong absorbance band at 1158 cm−1 due to stretching vibrations of −C−O furthermore, different modes of bending vibrations of CH2 and CH3 groups can be observed around 1375 cm−1 and 1459 cm−1. A small peak at 1650 cm−1 indicates the C=C cis stretching vibrations while a strong and intense peak of carbonyl functional groups of triglycerides is shown at 1743 cm−1. Absorbance in the range of 2852 cm−1 and 2920 cm−1 shows CH2 symmetric and asymmetric stretching vibrations, respectively. And finally, absorbance at 3005 cm−1 was assigned to =C−H stretching vibration of cis double bond.
The FTIR data of untreated shortening is in accordance with the reported literature values     however, after heating for 24 and 48 hours at 180˚C ± 5˚C, there are some obvious changes in certain regions of spectra. For instance, from Figure 3 it can be seen that after heating the samples, the absorbance peak of carbonyl functional groups showed broadening comparing to untreated sample. The broadening in carbonyl peak is due to the formation of aldehyde groups or other secondary oxidation products resulted from thermal oxidation. In addition to peak broadening, the heated samples also showed shoulder peaks around 1700 cm−1 which corresponds to FFAs production due to oxidation. The formation of FFAs during frying is hazardous for human health and is easily identified by FTIR spectra. And furthermore, the disappearance of absorbance peak around 1650 cm−1 i.e. related to C=C stretching vibrations indicates the decrease in degree of unsaturation due to thermal oxidation.
The thermal oxidation of palm olein shortening is also indicated by decrease in stretching vibrations of C−H groups of CH2 as shown in Figure 4. It can be seen that absorbance peak corresponding −CH2 asymmetric stretching vibrations tends to decrease from fresh to oxidized samples. This decrease in absorbance of
Figure 3. FTIR spectra of untreated, 24 and 48 hours heated samples at 180˚C ± 5˚C to observe broadening in carbonyl groups peak, production of FFAs and disappearance of C=C stretching vibrations.
Figure 4. FTIR spectra of untreated, 24 and 48 hours heated samples at 180˚C ± 5˚C showing decrease in absorbance intensity of −C−H (CH2) asymmetric stretching vibrations.
stretching vibrations of −CH2 functional groups suggests the oxidation in palm olein shortening.
In next phase, the untreated and thermally oxidized palm olein shortening samples were analyzed by GC-MS to evaluate the nature and composition of fatty acids profile. It was observed that palm olein shortening heated at 48 hours contains mixture of saturated and unsaturated FFAs as major volatile components as the data is shown in Table 1. These include n-hexadecanoic acid (palmitic acid) and octadecanoic acid (stearic acid) as saturated fatty acids while 9-octadecenoic acid (oleic acid) as monounsaturated fatty acids. In case of
Table 1. GC-MS data of 48 hours heated palm olein shortening sample to observe FFAs produced due to thermal oxidation.
untreated sample, no FFAs were found. GC-MS provided important information about FFAs profile formed due to thermal oxidation.
Palm olein shortening samples are much more stable as they showed superior resistance towards oxidation comparing to canola and sunflower oils. The conventional volumetric methods for accessing the fitness and quality of edible oils are tedious, lengthy and they only offer empirical information about oxidation products. ATR-FTIR provided detailed chemical profile of degradation products as a result of thermal oxidation in relatively much faster and easier way without extensive laborious laboratory work and sample preparation steps. The formation of FFAs, degree of unsaturation and other related information of thermally stressed edible oil can be readily obtained. While GC-MS results provided important information about the profile of FFAs formed due to thermal oxidation. The combined data of ATR-FTIR and GC-MS techniques is useful for rapid and highly sensitive analysis of oxidized edible oils thus, monitoring its quality for healthy use.
Authors are thankful to Department of Chemistry, GC University Lahore for GC-MS analysis of palm olein shortening.