Changes in food proteins reacted with nitrite at gastric pH

Casein and egg albumin were reacted with nitrite at pH 3. The reaction mixtures containing nitrite yellowed when the reaction began; after 24 hours of reaction with 7.5 X 10(-3) M nitrite, about 70% of added casein was sedimented as yellow precipitate. There was a considerable loss of lysine, arginine, tyrosine, and tryptophan residues. This was determined from the amino acid compositions of the proteins obtained by the reaction with 2.5 X 10(-2) M nitrite at pH 3 for 48 hours. An amino acid elution pattern of the treated casein showed five peaks due to new amino acids; three of these were provisionally identified to be cysteic acid, methionine sulfone, and 3-nitrotyrosine.