To compare the effects of technological treatments of milk on the gastric emptying of proteins and peptides, three diets were studied: raw milk, pasteurized milk, and yoghurt. In the preruminant calf, all effluents leaving the stomach during 12 h were collected and analyzed for N emptying, NPN level, amino acid composition, and characterization of proteins and peptides by SDS-polyacrylamide gel electrophoresis. With raw milk, casein coagulation is almost immediate in the stomach and casein is evacuated in the form of peptides. With pasteurized milk, casein coagulation is slower. Casein is also evacuated in the form of peptides. With yoghurt diet in which casein was gelified by acidification to pH 4 before ingestion, there is no coagulation in the stomach. Casein is evacuated during the whole digestion process in intact and degraded form. For the three diets, a-lactalbumin was degraded when the pH value was under 3.5. /3-Lactoglobulin did not seem to be proteolyzed. Amino acid compositions of the effluents are not so variable in the process of digestion with pasteurized milk as in digestion with raw milk. With yoghurt, amino acid compositions of the effluents are almost identical during the whole digestion process.
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