In vitro Digestibility of Whey Protein/K-Casein Complexes Isolated from Heated Concentrated Milk

The disulfide-linked complex of _K-casein and whey proteins that forms when concentrated milk is heated was isolated by centrifugation and column chromatography on Sephacryl S-1000. The rate of hydrolysis of β-lactoglobulin and _K-casein in the complex and the reduced and carboxymethylated components of the complex were measured by polyacrylamide gel electrophoresis. The rates of hydrolysis at pH 2.0 (pepsin) and pH 8.0 (trypsin and chymotrypsin) were similar for _k-casein in the complex and its reduced form. β-Lactoglobulin hydrolysis was faster for the reduced complex than for the complex which was much faster than for the native protein for all three enzymes. The results suggest that heating milk increases the digestibility of whey proteins, despite the formation of large protein complexes between the whey proteins and _K-casein.