Oxidative stability of selected vegetable oils after deep frying in different types of foods in Kenya

Abstract

Vegetable oils are triglycerides extracted from plants. Deep frying is one method which involves submerging the food in hot oil. When this oil is overused it undergoes a series of chemical reactions which may affect human health. This has made it important to study the oxidative stability of selected vegetable oils as relates to the different types of food fried. The main objective was to determine the relative thermal oxidative stability of the palm, corn, peanut, soybean, and sunflower oils after deep frying in different types of foods. In this study five types of foods were deep fried in five types of oils for 6 hrs. The oils were then divided into two equal portions. One portion was refrigerated at 40C and the other at room temperature for 5 days then used for frying for another 6 hrs. It was found that the storage conditions had significant effects on the oils. Fatty acid composition was done using gas chromatography where it was found that myristic, palmitic, linoleic and linolenic acid varied significantly (p≤0.05) per the type of oils. Soybean oil was found to have relatively higher linoleic (1025.59 ug/ml) and linolenic (43.90 ug/ml). Linoleic and linolenic acids were least concentrated in Palm 157.5 ug/ml and 0.7986 ug/ml. Peroxide values increased in all the oils after frying food. The iodine values of oils before and after frying food were compared and out of this, it was found that there was decrease in iodine value in all the oils after frying food. The highest decrease in Iodine value was observed in soybean after frying the five types of food and the values in g of I2/100g of oil were as follows chicken 27.0, chips 22.0, fish 35.5, mandazi 38.1 and smokies 17.3. The least decrease was indicated by palm: chicken 5.1, chips 4.6, fish 7.6, mandazi 2.0 and smokies 6.6. The study concluded that in regard to oil suitability, the parameters that were of major interest were peroxide, para-anisidine, iodine, refractive index and density values. Soybean oil proved to be relatively unstable; while palm and peanut were more stable compared to the other oils. In terms of food, the oil in which chicken and fish were fried contained more degradation product