Formation of structured clots, gastric emptying and hydrolysis kinetics of yak milk during in vitro dynamic gastrointestinal digestion: Impact of different heat treatments

Yak milk is highly nutritious in terms of abundant protein and fat, whereas little is known regarding the influence and mechanism of heat treatments on its gastric emptying and hydrolysis kinetics during digestion in the human gastrointestinal tract. In this study, a bionic human stomach-intestine system was employed to examine in vitro dynamic gastrointestinal digestion of fresh raw and heated yak milks. During gastric digestion, the formation of protein clots or aggregates was observed for all the milk samples. However, significantly more clots were found from autoclaved milk and raw milk, with a dry weight of 14.1 and 10.9 g from 200 g initial milk sample after 30 min digestion, respectively, compared to pasteurized (6.9 g) and microwaved (9.2 g) milks. For both raw and heated milks, the large-sized coalesced fat globules formed in the stomach were rapidly dissociated to small and uneven droplets upon digestion in the small intestinal conditions due to the highly efficient hydrolysis of milk fat globule membrane (MFGM) proteins and fat globules by pancreatic trypsin and lipase. Due to the formation of more dense-structured clots, a longer gastric emptying half-time was shown for the autoclaved (41.1 min) and raw (38.4 min) milks in comparison to that for the yak milks treated with microwave (34.2 min) and pasteurization (36.2 min). Consistently, the pasteurized milk (92.5 %) exhibited the highest protein digestibility at the end of digestion, followed by microwaved milk (87.8 %) ≈ autoclaved milk (86.1 %) > raw milk (80.8 %). The amount of free fatty acids (FFAs) released during digestion in general followed a similar order with that for proteolysis among the milk samples. This study highlights the crucial role of the formation of structured clots in the gastric emptying and hydrolysis of proteins and fat globules. This information will provide a mechanistic understanding of digestion kinetics of yak milk as impacted by heat treatments.