Digestibility of modified milk proteins: nutritional implications.

Chemical changes in the structure of several milk proteins were quantitated and the effects on digestibility evaluated using a number of in vitro techniques developed in our laboratory. An assay for digestibility was developed and standardized incorporating immobilized gastric, pancreatic, and intestinal mucosal proteinases and peptidases and thereby simulating in vivo digestion. The method displayed a direct relationship to digestibilities determined in human feeding studies. Various degrees of racemization were induced in a number of proteins and quantitated with the aid of a bioreactor containing coimmobilized D-amino acid oxidase and catalase. Digestibility values were directly related to the degree of racemization and to the amount of lysinoalanine. In a separate study of these proteins, various degrees of Maillard reaction were induced, and the available lysine was quantitated using the fluorometric o-phthalaldehyde method. Digestibility values determined for these modified proteins were also directly related to the amount of available lysine. A bioreactor containing various immobilized proteinases and peptidases catalyzed complete hydrolysis of a number of proteins including beta-lactoglobulin. Because this method does not degrade residues such as tryptophan and methionine nor hydrolyze aldosyl lysine to give unmodified lysine, analysis of these hydrolysates yields more nutritionally meaningful chemical scores. In summary, these techniques appear to be useful for quantitating and monitoring changes in the structures of proteins resulting from processing and storage, which affect their nutritional quality.