Effect of chitin level on the <i>in vitro</i> protein bioaccessibility in diets for juvenile rainbow trout ( <i>Oncorhynchus mykiss</i> )

The inclusion of chitin-containing ingredients in rainbow trout (Oncorhynchus mykiss) diets has been claimed to be beneficial for fish health, but controversial results on growth performance and nutrient availability have been reported in literature. The present study aimed to evaluate in vitro the effect of increasing chitin level (1.5%, 3% and 4.5%) to simulate increasing insect meal inclusion in aquafeed (15%, 30% and 45%) on the protein degradation using a species-specific digestive enzymatic extract. The first in vitro assay investigated if and to what extent increasing chitin levels in a high digestible blend of purified proteins could affect protein hydrolysis. A second in vitro assay investigated the interference on protein hydrolysis by the same chitin levels included in a complete aquafeed formulation. The coefficients of protein degradation, soluble protein clearance and amino acids released over time were evaluated. The hydrolysis was maintained for 90 min and samples were collected at 0, 15, 30, 60 and 90 min. The overall results showed no substantial differences in terms of protein degradation and amino acids released after 90 min, indicating that an inclusion level of up to 4.5% chitin in the diet does not interfere with the in vitro protein bioaccessibility. All the evaluated parameters resulted in higher values in the protein blends than in the complete aquafeed; not due to the chitin presence but probably to the presence of other nutrients, like carbohydrates and lipids which interfered with the protein degradation.