In vitro gastrointestinal digestion of liquid and semi-liquid dairy matrixes

Protein digestion in two liquid dairy matrixes with different heat treatments (pasteurized and sterilized milks) and in one semi-liquid dairy matrix (stirred-yogurt) was investigated using an in vitro gastrointestinal digestion model. After buccal digestion, significantly lower amount of soluble proteins were measured in yogurt than in both milks. This difference between dairy matrixes decreased during gastric digestion and disappeared at the end of the duodenal digestion upon the proteolytic action of pepsin and pancreatin. Electrophoresis pattern of digested mixtures showed that casein digestion began at the gastric phase and was slower for pasteurized milk than sterilized milk and yogurt. At the end of duodenal digestion, no more intact caseins were present in all the dairy matrixes while faint bands of whey proteins were still visible for pasteurized milk and yogurt. The release of free amino acids during the duodenal phase varied according to their nature (acid, basic, neutral or hydrophobic) and seems to be governed by the specificity of the enzymes. These results suggest that the severity of milk's heat treatment influences the kinetics of protein digestion, mainly during the gastric phase, and that the impact of processing has to be considered to study protein digestion in dairy products.