Abstract
In Germany oxidized fatty acid content that are insoluble in petroleum ether (below 0.7%), smoke point (above 170°C) and acid value (below 2.5) have been adopted as the criteria for the quality of frying oils and fats. Among these criteria oxidized fatty acid content should be preferably considered as an important index which reflects thermal oxidation of frying oils and fats. However the measurement of oxidized fatty acid content is a time consuming work.
The authors therefore developed a new method for evaluation of the quality of frying oils and fats which is based on the change of relative interfacial tension about cyclohexane solution of frying oils and fats. The relative interfacial tension was measured with reference to the Japanese Industrial Standard (JIS). In measurement, instead of the standardized drop number tube, the authors used the original drop number tube which is composed of a injection needle and a measured glass tube.
Indication of high correlation between relative interfacial tension and oxidized fatty acid content (=0.919) or carbonyl value (=0.929) was given through the measurement of such indices about a large number of frying oils and fats sampled from fried food manufactures. On the other hand, the correlation between relative interfacial tension and acid value was not so much high (=0.731).
Relative interfacial tension should be considered as the index which reflects thermal oxidation of frying oils and fats more closely than hydrolysis of them.
