Abstract: To investigate the effects of processing technologies on dairy matrix structure and protein digestion and absorption, this study combined an in vitro simulated gastrointestinal digestion model with in vivo experiments in mice to systematically compare the differences in microstructural characteristics and protein digestion and absorption behaviors of typical liquid dairy products (milk) and a semi-solid dairy product (stirred yogurt). The results showed that the intensity of heat treatment affected the proteolytic kinetics of milk; the protein hydrolysis rate of milk treated at 135 ℃ for 2 s (M2) was faster than that of milk treated at 72 ℃ for 15 s (M1), but no significant difference in in vivo amino acid absorption was observed between the two milks. Compared with liquid milk products, the loose and porous gel network structure of yogurt significantly increased the accessibility of proteins to proteolytic enzymes during the gastric digestion phase, thereby accelerating protein hydrolysis. The degree of protein hydrolysis of yogurt at the gastric stage was significantly increased by 75.80% and 155.84% compared with M1 and M2, respectively. Moreover, the proportion of small molecule peptides (< 1 000 Da) in the digested products of yogurt increased by 34.75% and 36.42% compared with those of M1 and M2, respectively. In addition, yogurt digestion produced three unique bioactive peptides and 13 peptides with potential bioactivity. In vivo experiments further confirmed that postprandial plasma total amino acids and essential amino acids levels in mice were significantly elevated by 29.18% and 20.36% after ingestion of yogurt compared with M1, respectively, while the level of branched-chain amino acids was increased by 14.50% compared with M2. In conclusion, protein pre-hydrolysis induced by fermentation, together with the formation of a gel network structure, synergistically promoted the digestion and absorption of milk proteins, whereas increasing the intensity of heat treatment alone accelerated protein hydrolysis but had limited effects on improving amino acid absorption levels. This study provides a scientific basis for elucidating the regulatory effect of processing on dairy matrix structure and the mechanisms underlying milk protein digestion and absorption.

Key words: food matrix; milk; yogurt; protein digestion; peptide; amino acid