In vitroproteolysis and functional properties of reductively alkylated β-casein derivatives

Summaryβ-Casein amino groups were modified with aldehydes and dialdehydes via reductive alkylation at pH 8·0. The degree of alkylation was controlled by the amount of the alkylating reagent applied. The initial rates of α-chymotrypsin-catalysed hydrolysis of alkylated β-casein were inversely related to the size of the modifying group. Proteolysis of modified β-casein with trypsin (18 h) or with α-chymotrypsin (48 h) depended on the nature and size of the substituent applied. The measurements of tryptophan fluorescence indicate that the modifications also induced conformation change. Solubilities of methyl-, ethyl- or benzyl-β-casein slightly increased; solubilities of β-casein dialdehyde derivatives were significantly lower than that of native β-casein. Emulsion stability of methyl- or ethyl- β-casein was higher than that of native β-casein in the acidic pH range. After modification with glyoxal, the emulsifying activity and the emulsion stability of β-casein decreased. The emulsifying activity of benzyl- β-casein was lower than that of native β-casein. Phthalylated β-casein displayed the poorest emulsion stability.