Extrusion processing improves rainbow trout digestibility of microalgal Nannochloropsis oculata co-product biomass for more sustainable aquaculture diets

Abstract

Aquaculture feeds include fishmeal (FM) and fish oil (FO), derived from wild-caught marine fish such as anchovy and sardines, and pose ecological, food security, and economic drawbacks for sustainable aquaculture, the world's fastest-growing agricultural sector. Protein-rich microalgal co-product is a promising alternative to unsustainably sourced fishmeal in aquaculture diets. Microalgal co-product is a defatted biomass left over after extracting omega-3-rich oil for human nutraceuticals and crude oil for fuels. In this study, we report the first evaluations of nutritive values and digestibility of raw, extrusion- and enzymatic-processed co-product of the marine microalga Nannochloropsis oculata (N. oculata co-product). Results allowed us to evaluate the feasibility of using processed biomass in aquafeed for rainbow trout (Oncorhynchus mykiss), an important model for all salmonid aquaculture. Extrusion processing temperature (90 °C vs 127 °C) influenced the nutritional value of N. oculata. Protein and energy levels were significantly higher in co-product from both extrusion temperature treatments than in non-extruded raw co-product. Essential amino acid levels did not differ between extrusion temperatures, except that methionine was significantly lower in coproduct from 127 °C extrusion processing than from 90 °C extrusion processing. The protein level in extrusion processed co-product was not significantly influenced by pre-cooking. We detected the highest digestibility of crude protein, energy, most of the amino acids, and omega 3 polyunsaturated fatty acids (n3 PUFA) in extruded (90 °C) N. oculata co-product ingredient. Nutrient digestibility of enzymatic processed N. oculata co-product were mostly the same as extrusion processing or lower for certain nutrients. Overall, extrusion processing of N. oculata at 90 °C provided the best outcome for digestible protein, and yielded an excellent source of digestible protein, amino acids, and long chain omega-3 profile for rainbow trout and which could be an alternative to replace fishmeal in rainbow trout diets.