Rapeseed/canola (Brassica napus L.) stands as an underdeveloped reservoir of beneficial nutrients for aquaculture. Aqueous extraction of soluble proteins from defatted rapeseed/canola meal represents an innovative processing method that is safe , environmentally friendly and with minimal impact on protein denaturation ( Aider and Barbana, 2011; Campbell et al., 2016) . This processing method can create a rapeseed/canola protein concentrate (~78 % crude protein) with low levels of phytic acid and insoluble fibers, and that has potential to contribute to circular economy by increasing the use of crop by-products . In this research, t he process was applied to produce an aqueous-extracted protein concentrate (APC) destined to serve as ingredient in aquaculture feeds . A series of studies were conducted to (1) evaluate the long-term effect of graded inclusion levels of APC on growth, feed and nutrient utilization, body composition, fillet color and gut histology of post-smolt Atlantic salmon , and (2) estimate the nutrient digestibility of APC in juvenile Atlantic salmon in freshwater and in post-smolt Atlantic salmon.
Materials and methods
Rapeseed/canola processing : D efatted meals obtained from rapeseed/canola crushing plants were processed at a pilot-scale facility through different steps involving protein extraction , precipitation and separation using water as solvent and no chemical such as hexane.
Long-term efficacy : Post-smolt salmon (228.0±4.9g) were fed graded inclusion levels of APC over a six -month study. Eight isoproteic and isolipidic experimental diets containing 0 (Diets A, E), 10% (Diets B, F), 15% (Diets C, G) and 20% (Diets D, H) APC were randomly allocated to 24 750-liter tanks at 33 fish per tank. Diets A through D were formulated to mimic commercial salmon feeds in geographies where processed animal proteins (PAPs) are used ( e.g. Americas), whereas diets E through H included no PAP (European-type diets) . The proximate (protein, lipid, dry matter, ash) and amino acid composition of the test ingredient, diets and whole-body of salmon were analysed according to AOAC methods. Effect of APC inclusion levels on growth performance, nutrient utilization, fillet color, gut histology and pellet physical characteristics were measured and analysed statistically.
Nutrient digestibility studies : Juvenile salmon (57.3±6.7g ) were fed with a reference diet and three test diets combining the reference diet with APC in 90:10, 80:20 and 70:30 ratios in freshwater (14.9±0.4°C). Feces were collected using the settling column passive method. Another digestibility study was conducted with post-smolt Atlantic salmon (227.7±4.1g ) also fed a reference diet and three test diets combining the reference diet with APC in 90:10, 80:20 and 70:30 ratios in saltwater (25ppt; 13.7±0.5°C). Feces were collected using the manual stripping method. For both studies, the proximate (protein, lipid, dry matter, ash) and amino acid composition of the test ingredient, diets and feces were analysed according to AOAC methods. The apparent digestibility coefficients were estimated using equations from NRC (2011) and results were analysed statistically.
Rapeseed/canola processing : The crude protein content of APC was consistent across the three lots processed during this project at 77-79% (as-fed). The amino acid profile was comparable to that of fishmeal.
Long-term efficacy : G rowth performances, measured using the thermal-unit growth coefficient (TGC), varied between 0.143 and 0.160, and were not significantly different among treatments (P>0.05). Feed conversion ratios (FCR) were ≤1.08 and, although there was a significant APC effect (P=0.003), differences were marginal and did not correlate with APC inclusion. Over the 168-day study, the best TGC and FCR were obtained with salmon fed 10% CPC, regardless of formula type. Proximate composition, nutrient deposition rates and retention efficiencies were similar across treatments. Although not significant , fillet redness improved with APC inclusion. APC inclusion level correlated positively and significantly with distal intestine villi length (P=0.031). Villi length of salmon fed 20% CPC (1053.3 ± 28.0 µm) was significantly superior to that of fish fed the control diet (931.7 ± 21.4 µm) (P=0.006). The proportion of distal intestine with no inflammation of the lamina propria and submucosa increased with APC inclusion.
Digestibility studies: The results of both digestibility studies are summarized at Table 1.
Discussion and concluding remarks
Rapeseed/canola processing : The environmentally friendly aqueous processing method is suitable for producing protein concentrates from oilseeds.
Long-term efficacy: A PC stood as a promising sustainable and safe protein alternative for salmon feeds in this long-term study. With a high protein content and an amino acid profile comparable to fishmeal , dietary inclusion of A PC at all levels tested in this study supported growth and optimal nutrient utilization while improving fillet redness and gut health of post-smolt Atlantic salmon.
Digestibility studies: Digestibility of protein, lipid and essential amino acids of APC was high at all life stages and unaffected by APC inclusion level.
To conclude, the next steps will consist in describing further the positive effects of APC on fish welfare/health in challenged fish and assessing the environmental impact/performance of this innovative ingredient.
Aider, M., Barbana, C., 2011. Canola proteins: composition, extraction, functional properties, bioactivity, applications as a food ingredient and allergenicity – a practical and critical review. Trends Food Sci. Tech. 22, 21-39.
Campbell, L., Rempel, C.B., Wanasundara, J.P.D., 2016. Canola/rapeseed protein: future opportunities and directions – Workshop proceedings of IRC 2015. Plants 5, 17.
NRC, 2011. Nutrient Requirements of Fish and Shrimp. Animal Nutrition Series. National Research Council of the National Academies. The National Academies Press, Washington, D.C. USA. 376 pages.