Solid state fermentation of rapeseed cake with Aspergillus niger for degrading glucosinolates and upgrading nutritional value

Abstract

Background: Rapeseed cake is a good source of protein for animal feed but its utilization is limited due to the presence of anti-nutritional substances, such as glucosinolates (Gls), phytic acid, tannins etc. In the present study, a solid state fermentation (SSF) using Aspergillus niger was carried out with the purpose of degrading glucosinolates and improving the nutritional quality of rapeseed cake (RSC). The effects of medium composition and incubation conditions on the Gls content in fermented rapeseed cake (FRSC) were investigated, and chemical composition and amino acid in vitro digestibility of RSC substrate fermented under optimal conditions were determined. Results: After 72 h of incubation at 34°C, a 76.89% decrease in Gls of RSC was obtained in solid medium containing 70% RSC, 30% wheat bran at optimal moisture content 60% (w/w). Compared to unfermented RSC, trichloroacetic acid soluble protein (TCA-SP), crude protein and ether extract contents of the FRSC were increased (P < 0.05) 103.71, 23.02 and 23.54%, respectively. As expected, the contents of NDF and phytic acid declined (P < 0.05) by 9.12 and 44.60%, respectively. Total amino acids (TAA) and essential amino acids (EAA) contents as well as AA in vitro digestibility of FRSC were improved significantly (P < 0.05). Moreover, the enzyme activity of endoglucanase, xylanase, acid protease and phytase were increased (P < 0.05) during SSF. Conclusions: Our results indicate that the solid state fermentation offers an effective approach to improving the quality of proteins sources such as rapeseed cake.