Digestibility of canola protein products derived from the physical, enzymatic and chemical processing of commercial canola meal in rainbow trout Oncorhynchus mykiss (Walbaum) held in fresh water

Abstract

This study was undertaken to assess the relative importance of nutrient composition and levels of antinutritional factors present in various canola protein products derived from the processing of commercial canola meal (CCM) on digestibility in rainbow trout held in 9.9–11°C fresh water. Laboratory sieving of CCM (SCM) reduced levels of crude fibre, cellulose and hemicellulose by 31.6%, 16.6% and 56.4%, respectively. Aqueous methanol–ammonia washing of sieved canola meal (ACM) increased neutral detergent fibre (NDF, insoluble dietary fibre) and phytate, and dramatically decreased levels of total glucosinolates (> 88%) and phenolic compounds (≈ 59%) relative to CCM. Further treatment of ACM with phytase in acidified (pH 5.0) aqueous media (PCM), followed by filtration, increased levels of crude protein, crude fibre and NDF, and decreased levels of phytate (33%), phenolic compounds (81%) and glucosinolates (below detection limit) relative to CCM. Additional treatment of PCM with carbohydrase enzymes (SP-249 and/or Alpha Gal, an oligosaccharide-degrading enzyme) further elevated levels of NDF, cellulose, protein (SP-249 only) and indispensable amino acids (SP-249/Alpha Gal), and concurrently decreased levels of phytate relative to PCM. An industrial source of canola protein isolate (CPI), produced by first extracting CCM with aqueous salt solution and then recovering the protein fraction, had the best nutrient composition and lowest levels of the antinutritional factors mentioned above. The processing protocols had slight or marked negative effects (ACM and SP-249/Alpha Gal) on apparent dry matter digestibility. Protein digestibility coefficients were unaltered in the laboratory-processed products, except for the products treated with SP-249 and SP-249/Alpha-Gal. In these cases, the protein digestibility coefficients (77.4% and 79.5%) were significantly below that found for CCM (88.1%). With two of the treatments, ACM and SP-249/Alpha-Gal, processing depressed DE. In contrast, the DE level in CPI (20.6 MJ kg−1) and the digestibility coefficients for dry matter (77.1%) and protein (97.6%) were high. We conclude that high levels of fibre, either alone or together with phytate, have the greatest adverse effects on the digestibility of canola protein products for rainbow trout. The high digestibility of CPI and its low content of antinutritional factors may also enable this protein product to be an excellent substitute for premium quality fish meal in diets for salmonids.