Structural and rheological properties of the clots formed by ruminant milks during dynamic in vitro gastric digestion: Effects of processing and species

Abstract

Ruminant milks coagulate during gastric digestion, which plays an important role in controlling digestion kinetics. Previous studies on cow milk showed that the structural and rheological properties of the clots formed in the gastric environment affect their breakdown and consequently their emptying rate into the intestine. This study investigated the impacts of homogenisation and heat treatment on the structural and rheological properties of the clots formed by sheep, goat and cow milks during simulated dynamic gastric digestion, with empasis on the non-bovine milks. Overall, with increasing intensity of processing, the gastric clots formed by ruminant milk became more fragmented, contained more moisture and had lower consistency. The consistency of the clots appeared to be associated with the presence of colloidal calcium phosphate in the casein micelles and the moisture content of the clot. A structural rearrangement of the clots may occur during the early stages of digestion because the forces maintaining the clot network change as the pH decreased. Between different species, we observed different impacts of individual processing treatments on the macrostructure and the consistency of the clots. Homogenisation had greater impacts on goat and sheep milk clots than cow milk clots whereas heat treatment (95 °C for 5 min) had the most effect on homogenised cow milk clots. Differences in the physicochemical properties of the ruminant milks, e.g. casein micelle size and ionic calcium concentration, may have been responsible for the variations observed between species. This study furthered our understanding of the gastric coagulation of ruminant milk and highlighted the species-specific processing impacts (processing × species interactions) on the structural and rheological properties of gastric milk clots.