Quality evaluation of different repeatedly heated vegetable oils for deep-frying of yam fries

Abstract

Many studies have been conducted to evaluate the quality of repeatedly heated oils (RHO) used in deep-frying potato fries, but little is known about the quality of RHO used for deep-frying yam fries. Therefore, this study investigated the quality of RHO used for deep-frying yam fries, focusing on coconut oil (CNO), sunflower oil (SFO), and soybean oil (SBO). The oils’ quality was determined by changes in moisture content, acid value (AV), free fatty acid (FFA), iodine value (IV), peroxide value (PV), and total polar compounds (TPC). The oils’ degradation and the presence of primary and secondary oxidation products were evaluated with thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR), respectively. The TGA curves indicated that the thermal stability of RHO followed the trend: RSFO (217 °C) > RSBO (187 °C) > RCNO (158 °C). RHO recorded a sharp increase in AV, FFA, PV, and TPC. The presence of secondary oxidation compounds was registered at a peak intensity of around 1742 cm−1 using the FTIR technique. A significant decrease (p < 0.05) in IV, PUFAs, β-carotenes, and color content were registered, rendering RHO unsuitable for deep-frying of yam fries. Finally, the RHO quality decreased in the following order: RSFO > RSBO > RCNO, implying that oils with a high degree of saturation are less susceptible to thermal changes during deep-frying.