Trypsin inhibitor reduction improves the utility of soy protein concentrate from soymilk in the diet of the juvenile red sea bream, Pagrus major

Abstract

A rearing study was undertaken to investigate whether trypsin inhibitor (TI) activity reduction in soy protein concentrate (SPC) derived from soymilk can afford more replacement of fish meal (FM) protein in the diet of juvenile red sea bream, Pagrus major. A diet containing 67% FM as the sole protein source was used as the control (C). SPC products with TI activities of 36, 21 and 13 TIU/mg sample were prepared. Approximately 80% of FM protein in diet C was replaced by a combination of SPC products and corn gluten meal (4:1) and referred to as T36, T21 and T13, respectively. Aqueous krill extract (KE) was added at 1% in diets T36, T21 and T13 and termed T36K, T21K and T13K, respectively. Phytase was supplemented at 1000 FTU/kg diet in all SPC-based diets. Each of seven diets was fed to a group of 20 fish (mean weight 19 g) in triplicate twice per day to apparent satiation for 10 weeks. The final mean weight (P = 0.001), specific growth rate (SGR) (P = 0.001) and feed efficiency (FE) (P = 0.002) were significantly lower in fish fed the T36 and T36K diets than in those fed the other diets. However, there were no significant differences in final mean weight (P = 0.088), SGR (P = 0.314) and FE (P = 0.829) in fish fed diets T21, T13, T21K and T13K compared to the control diet. Although there were no significant differences in the digestibility of protein (P = 0.076) and energy (P = 0.066) among the treatments, the productive values of protein (P = 0.002) and energy (P = 0.001) were significantly reduced in fish fed the T36 and T36K diets. In contrast, a significantly lower phosphorus (P) retention efficiency (P = 0.026) in fish fed the control diet resulted in significantly higher P discharge (P = 0.001) to the environment compared with diets T21, T13, T21K and T13K. There were no major variations in hematocrit level or plasma constituents among the treatments, except glutamic oxaloacetic transaminase (GOT) and triglyceride. The results suggest that the reduction in TI activity at ≤21 TIU/mg sample (≤9.66 TIU/mg diet) in SPC from soymilk made it possible to replace 80% of FM protein without affecting growth performance and health status. Moreover, a significant reduction in P discharge suggests that ecological benefits can be achieved through improving diet utility by reducing TI activity in SPCs derived from soymilk.