Optimization of ethanol production from food waste hydrolysate by co-culture of Zymomonas mobilis and Candida shehatae under non-sterile condition
A complete conversion of the hexose and pentose sugars in the food wastes hydrolysates (FWH) to ethanol is a prerequisite for maximizing the profitability of an industrial process for bioethanol production. Response surface methodology (RSM) was employed to optimize the effects of nitrogen source [(NH4)2SO4], phosphorus source (KH2PO4), yeast extract and inoculum size on ethanol production from FWH by co-culture of Zymomonas mobilis and Candida shehatae under non-sterile condition. The optimal conditions for ethanol production were 1.15 g/L of (NH4)2SO4, 0.95 g/L of KH2PO4, 1.38 g/L of yeast extract and 14.75%v/v of inoculum. The results indicate that the most significant parameters affecting ethanol production from FWH by co-culture under non-sterile condition was yeast extract. Ethanol production of 77.6 g/L obtained under optimized condition was 56% increased as compared with the use of raw FWH (34 g/L) and was in good agreement with the value predicted by quadratic model (79.98 g/L), thereby confirming its validity. Ethanol yield of FWH in batch fermentation by co-culture was 0.15 g-ethanol/g-food waste (77.6 g/L), which was 94.6% of the theoretical yield while Z. mobilis alone yielded 0.11 g-ethanol/g-food waste (54.2 g/L) and C. shehatae alone yielded 0.09 gethanol/ g-food waste (48 g/L). Ethanol production from FWH in 1-L fermentor by co-culture also gave ethanol yield of 0.16 g-ethanol/g-food waste (78.8 g/L) which was 96% of the theoretical yield. Despite of being a waste, an ethanol yield of 0.16 g-ethanol/g-food waste demonstrated the potential of food waste as a promising biomass resource for ethanol production.
Keywords: Co-culture, food waste hydrolysates, non-sterile fermentation, response surface methodology, optimization.