Comparison of the frying performance of market samples of palm olein, corn oil and soya oil in Malaysia

Abstract

The frying performance of market samples of palm olein, corn and soya oils sold in Malaysia were compared. The oils were used for intermittent frying of potato slices at half hour intervals for a period of5hr per day for four consecutive days. Analyses of oils showed that during drying (i) the oxidation of unsaturated fatty acids, as shown by changes in iodine value and the fatty acid composition, was less in palm olein than in corn and soya oils, (ii) polymer formation was least in palm olein compared to corn and soya oils, (Hi) the rate of change in acid value was highest in palm olein and (iv) that after 4 days of frying, corn oil had the lightest colour. INTRODUCTION A major proportion of edible oils is used in deepfrying operations which is one of the most common methods of food preparation. The frying performance and quality of the oil under frying conditions are of interest to oil processors, commercial fry operators and individual consumers. During frying operations, oil is continuously or repeatedly exposed to high temperatures in the presence of air and moisture. Under these conditions, hydrolysis and both thermal and oxidative decomposition take place. Degradation of the fat results in the formation of volatile and non-volatile decomposition products. Among some of the physical and chemical changes that occur during frying are increased smoking, foaming, colour and viscosity and the formation of free fatty acids, hydroperoxides, carbonyls and other products of secondary oxidation and high molecular weight polymerized compounds. (Chang et at., 1978; Fritsch, 1981; Roth and Rock, 1972). The overuse of frying oils affects MA. AUGUSTIN, K.H. LEE AND K.T. YAU the flavour, stability, colour and texture of fried food. The nutritional value and safety of the oil can also be affected (Alexander, 1978). A number of frying oils, both locally produced and imported, are available in the Malaysian market. Palm olein, the liquid fraction of palm oil, is the major cooking oil produced locally. Corn and soya oils are among two of the more popular types of imported frying oils in the market. The prices of these oils in the local market vary, with the imported oils being more expensive than the locally produced palm olein. There has been no study on the performance of local market samples. It was thus of interest to compare the quality of palm olein, soya and corn oils during frying. It should be noted that this study was limited to the evaluation of market samples of frying oils available at local outlets only. MATERIALS AND METHODS Materials Random samples of refined, bleached and deodorized palm olein, corn oil and soya oil were bought at the supermarket. The oils for each batch were bought about a week before the start of the frying experiment. The same brand for each oil was used each time. Potatoes used in frying experiments were also from a local distributor and similar varieties could not be assured for each frying trial. Different varieties were used for all three trials. The potatoes were bought a few days prior to each frying trial. The potatoes were peeled and sliced to a thickness of 2mm using a mechanical slicer and slightly blotted dry prior to frying. Frying Experiments Fryings were done in Valentine Fryers. Four kg oil was heated up to 180°C in 10 min and fryings were started half hour after the temperature of the oil had reached l80°C. One hundred gm potato slices were fried for 3 minutes at 0.5hr intervals for 5hr per day for four consecutive days. This is equivalent to 10 fryings a day. The fryer was left uncovered during the frying period. At the end of each day, lOOg oil was removed from the fryer and kept in a cold room at 6°C until the day of analysis. The lid of the fryer was then put on and the oil was allowed to cool overnight. The fryings were continued the next day. Analysis of Oils The peroxide and p-anisidine values of the oils were determined before the start of the experiment. Other analyses of oils were completed within three weeks after frying. The peroxide and iodine values were determined according to AOCS methods (AOCS, 1974). IUPAC methods were used for the assessment of acid and />-anisidine values (IUPAC, 1979). The fatty arid profile of the oil was determined by gas chromatography after preparation of methyl esters (Christopherson et al., 1969; Timms, 1978). The gas chromatograph was fitted with a 10% SP 2330 column and a Flame Ionization Detector. The injector and detector temperatures were 250°C while the column temperature was 190°C. The flow rate of the carrier gas was 40ml/min. The % polar components was determined by column chromatography (Billek et al., 1978) while polymer content was determined according to the method of Peled et al. (1975). The dielectric constant was measured on a Food Oil Sensor (NI-20, Northern Instruments). The instrument was calibrated with standards provided by the supplier. Colour was measured in a cell in a Lovibond Tintometer. The initial oil was also screened for the presence of the antioxidants BHA, BHT and TBHQ using high pressure liquid chromatography. The method used for extraction and determination of antioxidants was based on that by Kirleis and Stine (1978) and Page (1979). RESULTS AND DISCUSSION Characteristics of the Fresh Oils The quality characteristics of the fresh oils are given in Table 1 while the fatty acid composition of the oils are given in Table 2. The fatty acid composition of all samples of palm olein are within the range for Malaysian palm olein (Tan and Oh; 1981). The fatty acid composition of all samples of corn and soya oil are within the respective ranges that were tentatively adopted by the Food and -Agricultural Organization/ World Health Organization Codex Alimentarius 296 PERTANIKA VOL. 10 NO. S, 1987 COMPARISON OF THE FRYING PERFORMANCE OF SAMPLES OF PALM OLEIN, CORN OIL & SOYA OIL TABLE 1 Quality characteristics of the fresh oib Quality characteristic Batch Palm olein Corn oil Soya oil Peroxide value (meq/kg)