[On the occurence of isopeptide bonds in heated milk protein (author's transl)].

Milk, milk products and individual milk proteins were subjected to different heat treatments either as powders or in aquous systems. After complete hydrolysis of the peptide bonds (alpha-amide bonds) by a system of four proteinases or peptidases, respectively, the samples were analysed for isopeptides. For this purpose, two chromatographic ion exchange systems were developed, each of which separates Nepsilon-(beta-aspartyl-)lysine (Asp Lys) as well as Nepsilon-(gamma-glutamyl-)lysine (Glu Lys) from the common amino acids. In samples, heated 24 h for at least 120 degrees C, 2-5% of the lysine residues are incorporated in Glu Lys-bonds. Under the heating conditions used in dairy practice, no isopeptide bonds were formed.