As noted in our previous publications (1-3), frying oil is replaced after use at 180 for about 20 h in the Japanese food manufacturing industry; 50% of the browning of frying oil is attributable to the thermal deterioration of the oil itself (Gardner color increases to 5), and the rest most probably to amino-carbonyl compounds formed from thermally oxidized oil/fatty acids and amino acids exuded from frying foodstuffs. Depending on the type of amino acids included, 200-2000 ppm of amino acids are enough to brown oil to G>10. Other substances, such as protein, starch, sugar, coloring substances, and blood, have little to do with the browning of oil in frying practice. Koga et al. (4) found that Gly, Ala, and Leu were browning substances but did not compare these amino acids with other substances related to frying or conduct a quantitative inves-441 JOS