A review of processing of feed ingredients to enhance diet digestibility in finfish

Replacement of fish meal with plant proteins in aquaculture diets presents several problems. Firstly, aquaculture diets, particularly diets for carnivorous fish species, are nutrient dense and may contain up to 450 g crude protein (CP)/kg. Such diets preclude the use of ingredients with only moderate CP content, such as pulses including peas and faba beans or oilseed meals including canola/rapeseed meal and flax. Secondly, virtually all crops contain heat-labile and heat-stable secondary compounds including protease inhibitors, tannins, lectins, phytate, dietary fibre and starch. Removal of heat-labile secondary compounds may be accomplished by extrusion or other heat treatment. However, elimination of heat-stable secondary compounds, and increasing the nutrient concentration of diets, requires fractionation of crops. Fractionation technologies range from low technology processes such as dehulling to medium technologies such as air classification to sophisticated technologies such as aqueous and solvent protein purification. Studies on the nutritional value of processed plant proteins in various fish species have consistently shown improved digestibility and growth compared to feeding unprocessed ingredients. This review examines effects of processing technologies on nutritional properties of soybean meal, canola meal, peas, lupins and flax in aquaculture diets.