Comparative studies on physical properties of vegetable oils and their blends after frying

Physical properties of six commonly used oils and three blends consisting of three oils in each blend were studied after three successive frying of 'poories' (fried snack from flattened dough of refined wheat flour). The changes in viscosity, CIE trans-reflectance color and related parameters, UV-Visible spectra and UV-spectra of oil samples in solvent system (chloroform:methanol; 2:1, v/v) were studied. The results showed that viscosity and color of the oils changed to a much higher extent after first frying than subsequent fryings. The hue angle followed a similar trend. Changes in the UV-spectra in the solvent system indicated an increase in the formation of conjugated compounds after successive fryings. Peroxide values (PV) also increased after frying. Principal Component Analysis (PCA) plots of the data indicated that among oils examined groundnut oil and soy oil in combination with other oils were preferred for frying. Use of small amounts of unrefined oils (filtered) such as mustard oil or sesame oil which have a high content of natural antioxidants was beneficial as formation of conjugated compounds and increase in peroxide value was minimized after successive frying using blended oils.