Quality Evaluation of Oils and Fats by the Thermogravimetry. VI.

Abstract

The pyrolytical characteristics of various deteriorated oils were investigated by thermogravimetry(TG), derivative thermogravimetry (DTG)and differential scanning calorimetry (DSC). These oils were prepared as follows:soybean oil and lard were heated at 60°C to obtain autoxidized oil and 180°C to obtain thermally oxidized oil, and deteriorated oil was obtained by frying chicken in shortening, The relationship between the pyrolytical characteristics and chemical deterioration was also studied.1) In autoxidized oil, an oxidative reaction occured at 150°C200°C. This reaction could be detected more clearly during heating on TG and DSC curves with the progress of deterioration. In thermally oxidized oil, this reaction was not detected, but pyrolysis after weight started to decrease occured more quickly than that of the unoxidized oil, In the deteriorated oil, the temperature at which weight first started to decrease shifted to a lower region as deterioration proceeded.2) The characteristics found on TG and DSC curves were confirmed to be due to peroxidative products in the autoxidized oil lipid components having carbonyl groups thermally oxidized oil and polar lipids containing free fatty acids, mono-and di-glycerides in deteriorated oil may be useful for determining the conditions for oil deterioration.3) For autoxidized oil, the difference between the temperatures for initial and 25% weight reduction was closely correlated to peroxide, carbonyl value(COV)and p-anisidine values (AnV). For thermally oxidized oil, the decomposition rate between 250°C and 300°C was correlated to COV in the region below 30 and AnV in the region below 120. For deteriorated oil, the initial temperature of pyrolysis was correlated to COV, AnV, and polar lipid content.