Gastro-intestinal Digestibility of Processed Forms of Cow’s Milk Proteins under Simulated Infant and Adult Conditions Characterised by in vitro Methods

Human digestion involves chemical, biochemical and fermentative processes integrated to achieve optimal nutrient absorption along the gastro-intestinal (GI) tract. In particular, efficient gastro-intestinal enzymatic digestion of proteins is critical for maximum upper intestinal protein absorption and may influence the distribution of dietary protein between the ‘host’ and microbiota. However, the relative efficiency of infant versus adult conditions of GI digestion of cow’s milk products have not been systematically studied. Conditions for in vitro GI enzymatic digestion, optimised for either infant or adult conditions, were applied to test pasteurised, skimmed milk, skimmed milk powder (SMP) and whey and casein protein fractions of cow’s milk. Research methods included digestion monitoring by o-phthaldialdehyde (OPA) derivitisation of peptides and free amino acids, and digestate analysis to characterise low mass ‘absorbing’ products (size exclusion chromatography) and larger non-absorbing products (700-3500 Da) by MALDI-TOF-MS. The results demonstrated significantly lower digestion efficiency for infant versus adult digestion conditions, particularly in the gastric phase. Digestibility of pasteurised milk and SMP were generally lower than either whey or casein protein fractions. Lower overall protein digestibility was associated with lower and higher proportions of ‘absorbing’ and ‘non-absorbing’ peptides, respectively. The higher yield of ‘non-absorbing’ peptides for infant digestion may permit strategic release of peptides with receptor-antagonistic bioactivity along the gut, and/or may promote supply of nitrogen for the microbiota. The favouring of digestion-resistant peptides to the colon due to processing requires further research to also investigate potential negative relationships with health.