Changes in the Digestibility of Dried Casein and Glucose Mixtures Occurring During Storage at Different Temperatures and Water Activities

The digestibility of a dried, stored casein and glucose mixture was determined as a function of storage temperature, water activity, and carbohydrate:protein ratio by using the immobilized digestive enzyme assay. The sample contained either a 3:1 or 10:1 molar ratio of glucose:casein e-amino groups and was stored for 30 d at either 30 or 60C and at a water activity of .2 or .5. The extent of digestion was measured using two biodigesters, one containing immobilized pepsin and the other, immobilized trypsin, a-chymotrypsin, and intestinal mucosal peptidases. The biodigesters were operated in a fluidized-bed mode that permitted analysis of samples containing insoluble material. Losses in digestibility compared with unstored dried casein:glucose mixtures ranged from 18 to 88%. The largest changes in digestibility were observed upon changes in the water activity or temperature of storage. As would be expected from the extent of Maillard reactions, the greatest loss of digestibility occurred during