Abstract
Milk protein coagulation is not only important during the processing of milk into various dairy products, but also during digestion of milk. This review focusses on the gastric coagulation of milk proteins. During this process, coagulation of casein micelles and milk fat globules can occur due to pepsin-induced hydrolysis of the proteins that provide steric stabilisation. The gastric coagulation leads to delayed gastric emptying of casein and fat. Native whey proteins are not susceptible to gastric coagulation or delayed gastric emptying. Both heat treatment and homogenisation of milk lead to weaker gastric curds being formed, which are broken down more easily due to proteolysis and deformation. Incorporation of denatured whey proteins in gastric curds of heated milk delays their emptying. Understanding gastric coagulation and digestion behaviour allows tailoring of gastric transit via compositional differences or processing.
Highlights
Protein quality is often expressed in as the so-called Digestible Indispensable Amino Acid Score (DIAAS), which essentially reflects the ability of the protein to provide sufficient levels of digestible IAAs in proportion to requirements (Wolfe et al, 2018; Wolfe, Rutherfurd, Kim, & Moughan, 2016)
We aim to present these insights and discuss them both in the context of physiological importance, milk protein structure, interactions and digestibility and changes therein as a result of processing
While the N-terminal part of k-casein is attached to primary casein particles (PCP) that in turn are associated with calcium phosphate nanoclusters, the C-terminal part of k-casein protrudes from the surface as a so-called hairy layer, providing the micelle with steric stabilisation
Summary
Caseins and whey in milk behave very differently under gastric conditions, with the caseins coagulating to form a gastric curd and the whey proteins remaining in solution (Boirie et al, 1997). Gastric coagulation is of physiological relevance and ensures a controlled transit of protein through the stomach; this, in turn, ensures a more sustained release of amino acids into the blood following intestinal digestion and absorption (Lacroix et al, 2006; Mahe et al, 1996). Lacroix et al (2006) showed that in the absence of gastric coagulation, i.e., when using only soluble milk protein, digestion and absorption were too rapid to be sustained in terms of anabolic response and led to increased losses of nitrogen in the form of urea and amino acid through oxidation. The appearance of casein curds in stool after consumption of hard curd milk by infants was observed, which were related to the formation of firm coagula in the stomach (Brennemann, 1911; Courtney, 1912; Hess, 1913). By combining in vitro and in vivo observations on this topic with mechanistic insights for milk protein coagulation under different circumstances (e.g., during the manufacture of cheese or yoghurt) notable advances have been gained, which are discussed
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