Dephytinization of two soy proteins increases phosphorus and protein utilization by rainbow trout, Oncorhynchus mykiss

The efficacy of pre-treating two soy proteins with phytase (dephytinization) was evaluated in 2 x 3 factorial trial with rainbow trout [two soy proteins, three treatments (no treatment, dephytinization, P supplement)], Two commercial soy proteins, Soycomil P and Hamlet Protein 300, were fed with no supplemental phosphorus (P), or with 3 g supplemental P kg(-1) diet. Experimental dephytinized batches of the soy proteins were prepared by wet incubation with microbial phytase (BASF Natuphos) and included in diets without supplemental P. The soy product contents were 600 g kg(-1) diets. P contents were 6-9 g kg(-1) in diets with and without supplemental P, respectively. Initially, 57-g fish were fed in triplicate tanks for 9 weeks. Dephytinization had a positive effect on weight gain, feed efficiency and on protein, P, calcium, magnesium and zinc utilization. Weight gain was better in fish fed dephytinized soy proteins than in fish fed untreated regular soy fortified with P. P retention averaged 33% in fish fed regular soy proteins and 72% in fish fed dephytinized soy protein. P load into the water from fish fed diets with 60% dephytinized soy proteins averaged 1.5 g P kg(-1) weight gain, compared to 4.3 g P kg(-1) weight gain in diets with untreated soy proteins. Although similarly low P loads cannot yet be expected from practical diets, dephytinization of soy with phytase could offer a powerful tool for nutrient discharge management in aquaculture. (C) 2002 Elsevier Science B.V. All rights reserved.