Protein Cross-Linkage is as an Important Element of the Chemical Changes Associated with Heat Damage to Dietary Proteins

Thermal processing is central to the production of many feed ingredients. Proteins, and the amino acids that compose them, tend to be reactive and can undergo chemical changes during thermal processing. This “heat damage” can have deleterious effects on the nutritional quality of ingredients. The chemical changes associated with heat damage and their impacts on digestibility and bioavailability of amino acids are poorly characterized. Processing of proteins at high temperatures can result in protein cross-linking, the formation of covalent bonds between polypeptide chains within a protein or between proteins. There is emerging evidence that the presence of crosslinks in the structure of proteins, either due to disulphide bridges or cross-linked amino acids, may make parts of these proteins, more resistant to the action of proteases. This study aimed to examine protein cross-linking associated with thermal processing.

Freeze-dried chicken breast meat (CM) samples were subjected to thermal processing at different temperatures (70, 80, 95, 110, 120, and 130°C) for 45 min in an autoclave in absence (Control) or presence of chemical agents. L-serine and ferrous sulfate were used to stimulate amino acid crosslinking (Cross-Linking) and protein oxidation (ProtOx), respectively. Amino acids and crosslinked amino acids, namely lysinoalanine (LAL), lanthionine (LAN) and β-aminoalanine (BAL), were quantified using a standard amino acid analysis technique by UPLC. The content of sulfhydryl (S-H) and disulphide (S-S) bonds in CM was monitored using FT-Raman spectroscopy. Exposing CM samples to increasing temperatures in the absence or presence of L-serine and ferrous sulfate resulted in significant decreases in several essential amino acids (lysine, arginine, histidine, threonine, and methionine) and significant increases in the formation of cross-linked amino acids, notably LAL and LAN. Higher concentrations of cross-linked amino acids were observed in Cross-Linking and ProtOx treatments compared to Control groups. Processing temperatures exceeding 110°C were associated with increased concentrations of LAL and LAN. S-H and S-S bonds contents of CM decreased quadratically in the Control and ProtOx treatment groups with increasing temperature (p<0.05), indicating that the formation of disulphide bridge did not appear to be significant under the conditions examined. This may suggest that cystine in heat damaged proteins can suffer β-elimination of one of the sulfur atoms resulting in the formation of dehydroalanine residues that can react with certain amino acids to form cross-linked amino acids. This study highlights importance of protein cross-linking in heat damage and suggests that cross-linked amino acids can be practical indicators of heat damage for protein ingredients.

Key words: heat damage, proteins, digestibility.

Presented at the 2021 Animal Nutrition Conference of Canada. For information on the next edition, click here.