Antioxidant action and effectiveness of sulfur-containing amino acid during deep frying.

The aim of the study was to solve the mystery why sulfur-containing amino acids such as methionine can perform as an antioxidant during frying and hypothesized the antioxidative mechanisms. The results of this study revealed that sulfur-containing amino acids such as methionine failed to show DPPH· scavenging activity at room temperature but had valuable antioxidant activity based on OSI test at heated temperature. NMR analysis proved that methionine formed an intermediate molecule, 3-(methylthio)propylamine via decarboxylation during pyrolysis at heated temperature which was responsible for its antioxidant activity as shown by the OSI results. The mechanisms showed the proposed antioxidant behavior of methionine at heated temperature: (1) At heated temperature, 3-(methylthio)propylamine is generated by decarboxylation and (2) The antioxidant activity of 3-(methylthio)propylamine might be ascribed to the cooperation of amino group and the methylsulf-hydryl group in 3-methylthiopropylamine. From the frying study, methionine showed about 50% lower antioxidant capacity when compared TBHQ (tert-butylhydroquinone) based on OSI study, however, it has unexpected superior antioxidant activity under frying conditions that was on par with TBHQ. In summary, sulfur-containing amino acids with excellent antioxidant abilities might be useful for the food processing industry as antioxidant additives to extend shelf-life of food or food products.