Tea, coffee, and cocoa delay milk gastric coagulation in a biomimetic in vitro system (NERDT™) by hindering both acid- and pepsin-induced milk gelation.

This study investigated the effect of mixing whole milk with hot water extract of cocoa, coffee, or tea on milk intragastric coagulation during dynamic in vitro digestion. Milk diluted with water in the same proportion (70:30 v/v) was used as the control. The effect of adding cocoa, coffee, or tea extracts on acid- and pepsin-induced milk coagulation was first studied using a rheometer. The control exhibited a significantly shorter (P < 0.0001) gelation time in acid-induced coagulation than the milk mixtures containing extracts. The delaying effect of extracts on pepsin-induced coagulation was even more pronounced with coffee, causing a marked delay in gelation time, while tea and cocoa completely prevented gel formation under our experimental conditions. The milk mixtures, preheated to 60 °C, were then studied during simulated gastric digestion using the NEar Real Digestive Tract (NERDT™) dynamic in vitro digestion system. Samples collected at the stomach exit at different time points were investigated for pH, dry matter and microstructure. During gastric digestion of the control, protein coagulation occurred between 5 and 18 min (pH 6.3 ± 0.01 and 5.7 ± 0.28, respectively), whereas in the presence of cocoa, coffee, or tea, milk coagulation was observed between 18 and 27 min (pH 5.6 ± 0.07 and 4.84 ± 0.22 respectively). Confocal micrographs revealed larger protein aggregates in samples containing extracts, with entrapped milk fat clearly visible. The observed effects are likely due to the high polyphenol content of the extracts, which are known to interact with milk proteins. Our findings therefore suggest that protein-polyphenol interactions delay casein coagulation in the stomach and may alter gastric emptying and subsequent nutrient absorption along the small intestine.