To elucidate the effects and mechanisms of casein/whey protein ratio and food matrixes on the gastrointestinal survival of probiotics in fermented milk, the probiotic fermented milk samples with different casein: whey protein mass ratio (4∶1, 2∶1, 1∶1 and 1∶1.5) were used as raw materials, with probiotics powder used as the control. An advanced dynamic in vitro gastrointestinal model was employed to mimic the stomach and small intestine digestive environments present in children. The changes of gastric/intestinal pH, gastric retention ratio, apparent aggregation structure, and survival rate of total lactic acid bacteria (including Lactobacillus bulgaricus, Streptococcus thermophilus and Lactobacillus acidophilus) and Bifidobacterium lactis during the simulated dynamic gastrointestinal digestion were measured. The probiotics powder showed remarkably lower gastric pH and higher intestinal pH during digestion compared to the fermented milk samples. The aggregates formed during the digestion of fermented milk in the stomach gradually disappeared in intestine. The gastrointestinal survival of both the total lactic acid bacteria and B. lactis in all fermented milk samples was significantly higher than that in the solid drink (P<0.05). With the increase of casein/whey protein ratio, the gastrointestinal survival rate of the total lactic acid bacteria was significantly increased (P<0.05), but the viability of the B. lactis was not remarkably affected. These results suggested that casein micelles in fermented milk might have higher buffering capacity than whey protein and tend to form denser coagula or aggregates in the stomach. These might generate certain protective effect on probiotics, thus improving the survival of probiotics during gastrointestinal digestion.
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