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

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.