Evaluation of Heat-deteriorated Oils. I. TLC-FID Method for Determining Polar Compounds Content

Abstract

A simple and accurate method for determining polar compound (PC) content in heat-deteriorated oils was developed, since universal standards are necessary for assessing thermal rancidity in frying oil quality control. At present, the means used for standards differ according to countries. For instance, in Japan the upper limit of acid value (AV) is 2.5 and carbonyl value (CV) is 50, in the United States free fatty acid (FFA) content is less than 2.0%, and in EU countries PC content is below about 25% and smoke point is above 170°C. JOCS official method to determine PC content by column chromatography is complicated and time consuming, and thus in this study JOCS method was modified by TLC-FID, using soybean oil samples prepared under various heating conditions. TLC-FID method with hexane / diethyl ether (87/13 v/v) as developing solvent provided excellent linearity of y=1.02x + 0.53 with correlation coefficient (R2) of 0.9960, where y and x express PC content determined by JOCS and TLC-FID methods, respectively. Thermal rancidity of soybean oil samples heated under different air bubbling conditions was measured with PC content by TLC-FID, AV, CV, and anisidine value. PC content was clearly shown to be reflected in rancidity of sample oils with heating time. Correlation between PC content and AV displayed excellent linearity with correlation coefficient (R2) of 0.9310. The present TLC-FID method determining PC content may thus be concluded an effective means for evaluating the quality of heat-deteriorated oils.