Oxidation of methionine and cystine in foods treated with hydrogen peroxide

The damage to sulphur‐containing amino acids in two experimental samples of casein and fish meal treated with hydrogen peroxide was studied. Methionine, cystine (cysteine), methionine sulphoxide (according to Neumann), methionine sulphone and cysteic acid contents were determined chemically. The nutritive value of proteins was determined biologically. In the treated samples, methionine appeared to be oxidised almost completely. The major part was oxidised to the methionine sulphoxide level. Both treated samples contained methionine sulphone and cysteic acid. It was noted that the percentage of total sulphur‐containing amino acids being oxidised to methionine sulphone and cysteic acid was about the same as the percentage decrease in n.p.u. The results suggest that peptide‐bound methionine sulphoxide is as available as peptide‐bound methionine.