Products of the heat-promoted reaction between urea and the protein fraction of bovine milk

Summary

Enzymic digests of samples of heated skimmed milk were found to contain small amounts of S-carbamyl-L-cysteine. This material was concluded to have arisen from reaction of a cysteinyl residue in βlactoglobulin (β-lg) with cyanate derived from the native urea of bovine milk. After heating at 75 or 90 °C for 20 min skimmed milk contained 0·009 and 0·029 μmol/ml respectively of protein-bound S-carbamyl-cysteine. Evaporated milk contained 0·058 μmol/ml. The S-carbamylcysteine contents of samples of β-lg heated under similar conditions in equimolar 0·17 mM-KCNO at pH 6·6 were 0·016 and 0·062 μmol/ml respectively. At 60 °C no formation of S-carbamyl-cysteine was detected. Samples of evaporated milk and skimmed milk which had been heated for 20 min at 60, 75 or 90 °C were examined, after acid hydrolysis, for the presence of homocitrulline. None was found. Similarly α_s1- and β-caseins heated at 60 and 75 °C in 4 mM-urea at pH 6·6 yielded no homocitrulline, but after treatment at 90 °C for 20 min α_s1-casein contained 68 μg/g and β-casein contained 73 μg/g protein-bound homocitrulline. This was concluded to have been the product of reaction of cyanate and ε-amino groupings of lysyl residues of the caseins. The relative importance of carbamylation involving ε-amino groups or sulphydryl groups in milk is discussed.