Sulfur Amino Acid Stability. Hydrogen Peroxide Treatment of Casein, Egg White, and Soy Isolate

Abstract

ABSTRACT Egg white solids (EWS), soy protein isolate (SPI), and casein were treated with differing levels of H 2 O 2 at both 40 and 90°C. Total and reactive cysteine/cystine (Cys), total methionine, methionine sulfoxide and sulfone contents were determined for all samples. At 40°C methionine in all samples was readily converted to its sulfoxide, with little sulfone or cysteic acid being formed. At 90°C Cys was rapidly converted to the stable cysteic acid and methionine to its stable sulfone form. The production of the less stable methionine sulfoxide at 90°C was minimal. Of the three proteins studied, the methionine and Cys residues in EWS were least stable under oxidizing conditions, whereas those present in SPI and casein were more stable.