Harumi Kobayashi

The effects of pCO2 on yeast growth and metabolism under continuous fermentation

SummaryThe effects of pCO2 were investigated by changing the aeration rate, the purging gas and the total pressure in a chemostat cultivatioa Under glucose supply limitation, an increase in pCO2 from 44 kPa to 195 kPa resulted in 25 % decrease in cell concentration, 8 % increase in ethanol concentration, and 50 % decrease in glycerol concentration. Under oxygen supply limitation, similar dependency of ethanol and glycerol on pCO2 was observed, however, no influence of pCO2 on the cell yield was observed. The change in ethanol yield by pCO2 appeared to be caused by the equilibrium shift of pyruvate dehydrogenase system.

Effects of oxygen supply on yeast growth and metabolism in continuous fermentation

Abstract The yield changes in cell mass and metabolites with changes in the oxygen supply rate were investigated in continuous ethanol fermentation. With increases in oxygen concentration in the purging gas from 5.3 to 39.3 %, the specific oxygen uptake rate (qO2) increased from 0.158 to 1.24 mmol/g/h. With this change, cell mass increased from 13.2 to 14.9 g/l and glycerol decreased from 4.8 to 0.99 g/l, although little change in ethanol yield was observed. At a higher oxygen concentration and/or at a lower respiratory quotient (RQ), glycerol disappeared, acetaldehyde, acetoin and 2,3-butanediol increased, and ethanol started to decrease. The yields of iso-butylalcohol and iso-amylalcohol also increased with increases in the oxygen supply rate when RQ was lower than approximately 10. Reduction in the redox balance (NADH/NAD) in the cells by qO2, appeared to reduce initially the rate of glycerol-3-phosphate formation and next the rate of ethanol formation, resulting in the accumulation of acetaldehyde and formation of 2,3-butanediol through acetoin. Fatty acid composition changed with changes in the oxygen supply rate. The value for unsaturation, Δ mol−1, increased from 0.745 to 0.836 with the increase in qO2 from 0.158 to 1.79 mmol/g/h. Increases in oleic acid (C18:1) and decreases in palmitic acid (C16:0) were the major changes with the increases in Δ mol−1.

Optimization of two-stage continuous ethanol fermentation using flocculating yeast

SummaryTwo-stage two-stream continuous ethanol fermentation using flocculating yeast was studied to produce high ethanol concentration from blackstrap cane-molasses. Optimizing pH, RQ, cell concentration, and medium molasses concentration of the first stage to give a good cell yield without a significant decrease in ethanol yield, the final ethanol concentration of as high as 92.0–93.5 g/l at an overall dilution rate of 0.10 l/h was obtained. This process could work stably as long as 3 weeks without any decreases in the efficiency.