Ethanol Fermentation Of Xylose By High Density Limiting Oxygen Yeast And Mechanism Of Yeast Stress Ethanol Tolerance

Xylose is the major component of hemicellulose on the hydrolysis of lignocellulosic and full of converting xylose into ethanol is promising for the development of lignocellulosic ethanol. At present, due to the long fermentation period, low xylose utilizing rate and the yield of ethanol, it hinds widely use of the nature strains of fermenting xylose. In order to keep high cell density in the broth, the nature strain Pichia stipitis was preparation for high density culture and the culture conditions were optimized. Meanwhile, the performances of high density limiting oxygen feast employed in recycling, ethanol fermentation productivity and the tolerance of stress ethanol were investigated experimentally.In the process of dilute acid pretreatment, the hydrogen ion acted on exposed functional groups in the surface of straw, which the xylose relased from the chain of xylan. The surface structure loosen, the interal porosity and specific surface area increased were caused by acid pretreatment, which enzyme easily contacting straw to promote the rate of enzyme hydrolysis and saccharification. The effects of reaction time, concentration and solid-liquid rate in the pretreatment process were optimized with quadratic regression orthogonal design. The optimal pretreatment conditions were determined:reaction time 2h, concentration of acid 4.1%, liquid to solid ratio 22:1.The effects of fermentation showed that it is more important for oxidation-reduction equilibrium and cell growth, when the mass transfer of oxygen was limited with fabric layers. Meanwhile, the performances of sugar and oxygen uptake were guaranteed, the liquidity of broth was enhanced, cell viability was maintained and the inhibition of ethanol on cell growth and metabolism with fitting shaking speed. The results showed that under the condition of the initiate xylose concentration at 60 g/L, rotary rate 140 rpm, the high xylose utilizing rate of 99.57% was obtained.Compared with free cell, it showed that the xylose utilizing rate and ethanol yield with high density yeast was more than free cell. Though the result on fermentation of hemicellulose hydrolyzate was less than pure sugar, the efficiency was more than free cell. Obviously, it is advantage for high density cell can increase cell concentration, shorten fermentation time and improve xylose utilizing rate and ethanol productivity.Base on the above, the stress ethanol tolerance of high density yeast was investigated. The results showed that the cell concentration and ethanol yield was decreased, and the cell to xylose utilizing rate was slow in the xylose fermentation of additional high ethanol concentration. Meanwhile, compared with free cell, the xylose utilizing rate of high density yeast was higher, and the effects of high ethanol concentration were less. Based on the analysis, when high density cell stressed high ethanol concentration, the cellular and intercellular can keep the function of protection and resistance.