Maillard reaction and protein crosslinking in relation to protein digestibility and gut microbiota in juvenile Nile tilapia (Oreochromis niloticus) raised at different water temperatures

High temperatures during the production of fish feed can induce Maillard reaction and protein crosslinking. These reactions are known to reduce protein digestibility and bioavailability of amino acids in mammals, but information in fish is scarce. Therefore, this research aimed to understand the effect of Maillard reaction and protein crosslinking on protein digestibility in Nile tilapia. An animal-based protein diet and plant-based protein diet were stored at 7 °C and 50 °C for seven days to induce a contrast in Maillard reaction and protein crosslinking. Fish were reared for 31 days at 24 °C or 32 °C to create a contrast in gut fermentation and study its role in protein digestion. Feed and faeces were collected to determine the apparent digestibility coefficient (ADC) of crude protein, lysine, Maillard reaction products (MRPs) as fructosyllysine, carboxymethyl-lysine and, carboxyethyl-lysine and crosslinked amino acids (CLAAs) as lanthionine and lysinoalanine. Surprisingly, the ADC of MRPs and CLAAs was similar to that of crude protein, indicating no effect of the Maillard reaction and protein crosslinking on protein digestibility. The gut microbiota composition was affected by water temperature and diet type but not by feed storage temperature, indicating the gut microbiota were not responsible for the high ADC of MRPs and CLAAs. However, positive correlations found between certain bacterial taxa and the ADC of MRPs might suggest involvement of gut microbiota in the digestion of heat-damaged proteins. This study shows that the effects of Maillard reaction and protein crosslinking on protein digestibility in fish deviates from the known effects in mammals.