A Review Article : Ethanol Fermentation by Saccharomyces cerevisiae using Agricultural Waste

Bioethanol is one of the most promising and eco-friendly alternatives to fossil fuels, which is produced from renewable sources. Bioethanol can be produced from different kinds of raw materials. Conventional crops such as corn and sugarcane are unable to meet the global demand of bioethanol production due to their primary value of food and feed. Agricultural wastes are cost effective, renewable and abundant. To do this, very high gravity (VHG) fermentation which involves use of medium containing high sugar concentration(>250g/L) must be implemented to achieve high ethanol concentration. However, VHG fermentation leads to significant stress for Saccharomyces cerevisiae due to osmotic pressure at the beginning of the fermentation and high ethanol content at the end. At this review, rice straw is the most abundant waste compared to the other major wastes and potentially produce 205 billion liters bioethanol per year, which is the highest among these four mentioned agricultural wastes.

HIGH SOLID FERMENTATION AT PILOT SCALE FOR POTABALE ETHANOL PRODUCTION FROM RICE

In recent year, besides conventional process comprising separated liquefaction, saccharification and fermentation steps, few ethanol distilleries in Vietnam started using Simultaneous Saccharification and Fermentation (SSF) for ethanol production. In this work, we developed an SSF process of rice mash at high solids for potable ethanol production at pilot scale of 30 liters. After investigation on liquefaction and boiling time of five SSF processes, we selected the most suitable one (SSF3) to apply to pilot scale. At pilot scale, rice flour (RF) was dissolved in water to reach 315.4 g/l dry matter, then the mixture was liquefied at 85 oC for 60 min by using an alpha-amylase (Liquozyme SC DS at 45.36 KNU-S/kg RF) and a beta-glucanase (Optimash TBG at 2.812 U/kg CF). The starch slurry was then boiled for 30 min. SSF of liquefied mash was performed at 30 oC with the simultaneous addition of a gluco-amylase (Spirizyme SC DS at 540 AGU/kg RF) and a protease (FermgenTM at 600 SAPU/kg RF), active dry yeast (Ethanol Red at 1.5×107cells/l), urea (12 mM) and KH2PO4 (4 mM). Under these conditions, SSF process finished after 120 h and achieved 16.7 % v/v ethanol, which was equivalent to 87.1 % of the theoretical yield. Therefore, the VHG fermentation of rice flour could be a great potential to be applied in potable ethanol industry in Vietnam.