Protease increases ethanol yield and decreases fermentation time in no-cook process during very-high-gravity ethanol production from rice

No-cook process or Simultaneous Liquefaction, Saccharification, and Fermentation process (SLSF) under very-high-gravity (VHG) condition possesses many advantages for potable ethanol production from rice. The important nutrient that affects the effectiveness of fermentation is nitrogen. Two sources of nitrogen are considered including added urea and indigenous rice protein which can be hydrolysed by an external protease. This study aims to assess the impact of urea and protease on rice conversion, yeast performance including the formation of ethanol and co-products in the SLSF-VHG. The results show that in comparison to the control process (without adding nitrogen), a protease (Fermgen™ 2.5X) supplementation of 600 SAPU/kg rice flour resulted in efficient breakdown of the rice structure along with 2.4% v/v higher ethanol yield and shorter fermentation time of 48 h. Meanwhile, the adding urea at concentration of 16 mM had a slight effect on the structural modification of the rice material without a significant increase (p < 0.05) of final ethanol yield. There was no significant difference (p < 0.05) in ethanol yield between urea and protease addition compared with protease alone. Our results suggested that the addition of protease provided enough nitrogen for yeast by hydrolysing the endogenous rice protein during SLSF-VHG process.