Aerobic Ethanol Production with a Flocculent Yeast in a Biomass Recycling System

摘要

The aerobic ethanol production by Saccharomyces uvarum H2032 was studied in continuous culture with biomass recycling. A sedimenter was used as the separation device. On the basis of elementary balances, it was demonstrated that the overall ethanol yield in aerobic conditions can be as high as 95% of the theoretical yield if the biomass yield is kept below 0.05 and the respiratory coefficient (RQ) above 20. The effect of the most important operating parameters was investigated in order to assess their effect on the performance of the system. Cultivation temperatures of 34-35 sup 0 C were optimal for ethanol production. In this range the production of biomass and glycerol (the most important by-products of ethanol production) was significantly reduced. The cell mass concentration in the bioreactor was a direct function of the sedimenting temperature. Cell mass concentrations above 75 gl sup -1 could be achieved with a sedimenting temperature of 35 sup 0 C. Low sedimenting temperatures (15 sup 0 C) limited the biomass concentration to less than 55 gl sup -1 . The aerobic ethanol production (respiro-fermentative metabolism) resembled very closely the anaerobic metabolism of the cells. The ethanol had an inhibitory effect on the specific substrate and oxygen uptake rates. The ethanol inhibition on growth was ascribed to the inhibition of the energy yielding pathways, glycolysis and respiratory chain. A mathematical model of the system was formulated on the basis of the metabolic rates observed in steady state conditions. The mathematical simulations of the system indicated that an ethanol productivity of 60 gl sup -1 h sup -1 can be achieved with the residual glucose concentration remaining low (2 gl sup -1 ). The high dilution rates necessary to operate the system at this productivity suggest that the use of filters or centrifuges may be necessary for biomass separation. (ERA citation 11:050274)