How processing may affect milk protein digestion and overall physiological outcomes: A systematic review

Abstract

Dairy is one of the main sources for high quality protein in the human diet. Processing may, however, cause denaturation, aggregation, and chemical modifications of its amino acids, which may impact protein quality. This systematic review covers the effect of milk protein modifications as a result of heating, on protein digestion and its physiological impact. A total of 5363 records were retrieved through the Scopus database of which a total of 102 were included. Although the degree of modification highly depends on the exact processing conditions, heating of milk proteins can modify several amino acids. In vitro and animal studies demonstrate that glycation decreases protein digestibility, and hinders amino acid availability, especially for lysine. Other chemical modifications, including oxidation, racemization, dephosphorylation and cross-linking, are less well studied, but may also impact protein digestion, which may result in decreased amino acid bioavailability and functionality. On the other hand, protein denaturation does not affect overall digestibility, but can facilitate gastric hydrolysis, especially of b-lactoglobulin. Protein denaturation can also alter gastric emptying of the protein, consequently affecting digestive kinetics that can eventually result in different post-prandial plasma amino acid appearance. Apart from processing, the kinetics of protein digestion depend on the matrix in which the protein is heated. Altogether, protein modifications may be considered indicative for processing severity. Controlling dairy processing conditions can thus be a powerful way to preserve protein quality or to steer gastrointestinal digestion kinetics and subsequent release of amino acids. Related physiological consequences mainly point towards amino acid bioavailability and immunological consequences.

Dietary Advanced Glycation Endproducts and the Gastrointestinal Tract

“…Consequently, this leads to a direct hindrance of the proteolytic cleavage sites [28,29]. Additionally, indirect steric hindrance of the proteolytic cleavage sites can occur through binding to residues close to proteolytic cleavage sites, or it can cause cross-linking that inhibits access for proteases [26]. Alternatively, glycation may also promote proteolysis by opening up the tertiary structure of proteins (conjugation may interfere with refolding the protein after heating [29]), which was observed in the study by Moscovici [24].…”

“…In addition to the above mentioned experimental studies, an excellent systematic review by van Lieshout et al (2019) analysing the effects of glycation on protein digestibility concluded that glycation decreases protein digestibility [26]. One explanation for the reduction of protein digestibility by the MR lies in the nature of the reaction itself.…”

Lysine blockage of milk proteins in infant formula impairs overall protein digestibility and peptide release

“…However, most in vitro and animal studies on (isolated) milk proteins that focus more specifically on the effect of glycation suggest that lysine blockage impairs protein digestibility. 4 True ideal digestibility of lysine and other amino acids was reduced in rats when feeding a heated mixture of lactic casein or skimmed milk powder with lactose compared to when feeding a non-heated mixture. 5,6 Furthermore, a study in pigs demonstrates that 50% blockage of lysine in skimmed milk powder results in a significant decrease in post-prandial plasma appearance of lysine and total amino acids.…”

Reduced Occurrence of Gastrointestinal Symptoms in Chinese Infants Fed Minimally Processed Commercially Available Formula: A Cross-Sectional Observational Study

“…Maintaining Maillard reaction products as low as technically feasible is acknowledged as having a role in protein digestibility [15,16]. A recent systematic review by van Lieshout GAA et al concludes that glycation decreases protein digestibility and lowers amino acid availability, especially for lysine [36]. Undigested glycated protein can escape intestinal digestion and end up in the distal ileum and colon.…”