The impact of heating and drying on protease activities of ruminant milk before and after in vitro infant digestion

This study investigated the effect of heating (63 degrees C/30 min or 75 degrees C/15 s) and drying (spray-drying or freezedrying) on plasmin, cathepsin D, and elastase activities in bovine, ovine, and caprine milk, compared to nondried raw milk counterparts. Protease activities and protein hydrolysis were assessed before and after in vitro infant digestion with or without gastric and pancreatic enzymes. At 75 degrees C/15 s, plasmin activity in caprine and ovine milk decreased (69-75%, p<0.05), while cathepsin D activity in spray-dried bovine milk heated increased (2.8-fold, p<0.05). Plasmin and cathepsin D activities increased (<1.2-fold, p<0.05) after in vitro digestion with pancreatin, regardless of milk species. Endogenous milk enzymes hydrolyzed more proteins than gastric enzymes during gastric digestion and contributed to small intestinal digestion. In summary, milk proteases remained active after processing with effects dependent on the species of milk, and they contributed to in vitro protein hydrolysis in the stomach and small intestine.