Chemical Changes of the Edible Oil by Heating (1)

On the soybean oil heated for 20 hours but intermittently several measurements were made on the following items, acid value, saponification value, peroxide value, iodine number, TBA value, carbonyl value, refractive index, and the amount of conjugated diene. Iodine number decreased consistently with the heating, while acid value, saponification value, refractive index and conjugated diene increased throughout the period. On the other hand TBA, carbonyl peroxide values did not show a definite change in one direction. These three tests often used for the measurement of degree of rancidity of the edible oils seemed to be sometimes inappropriate for the oils which were usually treated under a higher temperature such as 180-200°C in the process of cooking or frying.