Color Deterioration of Oil during Frying

摘要

The food industries in Japan ban used edible oil in the amount of some hundred thousand tons every year. The color deterioration of the oil used for frying in the industries, and recovered on a large scale, was investigated. Virgin frying oil, whose mineral content was adjusted to that of recovered oil, was spiked with several components of fried foods (separately and in combination), then heated at 180°C for up to 70 h. From the change in oil viscosity, the apparent heating time of recovered oil was judged to be 20 h. It was found that in practice, starch, proteins, sugar, and pigments had little to do with the deterioration, whereas the amino acids, especially Cys, Met, Trp and the oil itself contributed to the deterioration. These amino acids seem to be exuded from foodstuffs during frying. The level of minerals in the oil affected the deterioration and viscosity increase of oil itself, in rate as well as in degree although the deterioration by amino acids was not affected much by mineral content. In conclusion, it is suggested that color deterioration of frying oil used in the Japanese food industry is attributable to (i) the amino-carbonyl reaction between thermally oxidized oil and amino acids exuded by fried stuffs, and (ii) coloring of oil itself influenced much by mineral content.