エネルギー多様化のための凝集性酵母による糖蜜からのエタノール生産

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To make the production of fuel alcohol from biomass both simpler and more efficient, we studied the processes of continuous and repeated-batch fermentation from cane molasses using flocculating yeasts. Continuous ethanol fermentation was tested at 30℃, pH4.5,using an uncooked molasses medium in a two-tower fermentation system. Although this system could be operated stably by the addition of antiseptic K_2S_2O_5 to the medium, certain drawbacks were encountered. To achieve a high level of ethanol production with molasses as a raw material, continuous fermentation was conducted using the fused and acclimatized flocculating yeast HA2 ; at a dilution rate of 0.4h^<-1> at 30℃, ethanol was produced at 25g/l.h, with an ethanol concentration of 63g/l. However, when the system was operated for 4 d at a dilution rate of 0.5h^<-1>, the fermentation became unstable as thefflocculating activity of the yeast decreased. Aeration stabilized the flocculating activity of yeast and allowed long-term operation of the fermentor at the higher dilution rate of 0.5^<-1>. As the concentration of ethanol produced(60g/l)was low, the effects of the concentration on the specific growth and specific production rates of ethanol were studied. Based on the findings of this study, laboratory-scale continuous fermentation was carried out using the two-tower system. In the second fermentor of the tower, 85g/l ethanol was produced at a dilution rate(D_t)of 0.2h^<-1> by active feeding of the fermented mash from the first fermentor into the second one by a pump(hereafter called"active feeding"). To maintain the number of viable cells above 10^9 cells/ml in the second fermentor, an active feeding ratio of more than 23% was required. Bench-scale continuous fermentation achieved an almost equally good performance. Heat evolution from continuous fermentation was found to be dependant upon productivity ; a productivity of 26.5 g/l.h gave 6.08 kcal/l.h. The construction of new plants with tower fermentors would be necessary for the introduction of continuous fermentation commercially. In contrast, repeated-batch fermentation would require only the partial remodeling of existing plants. A thermotolerant flocculating yeast, Saccharomyces cerevisiae KF-7,constructed by protoplast fusion, was selected for repeated-batch fermentation using a molasses medium(20w/v% total sugar)at 30℃ ; stable fermentations could be achieved with an ethanol productivity of 5 g/l.h. Even at 35℃, a productivity of 3.6g/l.h was obtained, but the concentration of ethanol produced was only 65g/l. This low concentration could be increased to 80-82g/l by aeration at 0.1vvm. At 33℃, the aeration time was decreased in steps to establish the minimum aeration required for efficient fermentation performance. The productivity and viable cell number were maintained at around 7.6g/l.h and 4.0×10^8 cells/ml even when aeration was applied for only the first 5h of fermentation. A linear correlation was found between the decrease in the trehalose content and the death of the yeast after the completion of each batch at all fermentation temperatures. In order to increase the ethanol concentration, an attempt was made to improve the salt tolerance of the thermotolerant strain KF-7 by maintaining a high concentration of KCl in the medium. The selected strain, K211,accumulated higher concentrations of glycogen and trehalose than KF-7 and also remained viable in the stationary phase during batch fermentation. As a result of repeated-batch fermentation tests using K211 at 0.1 vvm aeration, stable ethanol production was achieved with an ethanol concentration of 91g/l and a productivity of 2.7g/l.h at the higher temperature of 35℃ in a molasses medium containing 22w/v% total sugar.
1997-01-25

エネルギー多様化のための凝集性酵母による糖蜜からのエタノール生産

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