Studies On Factors Affecting Xylose Fermentation By Pichia Stipitis

Agricultural and forestry wastesare rich in polysaccharides, which can bepre-treated and hydrolyzed to monosaccharides and then fermented to ethanol. Howto improve the utilization of xylose in the hydrolyzate of acid steam-exploded cornstover has been an important focusin the bioethanol industry. In this thesis, theconditions that affect xylose fermentation by Pichia stipitis, such as aerationrate,initial yeast concentration, pH and acetic acid tolerance, were investigated. Thefermentation efficiency of varied hydrolyzates and processes were compared. Thefermentation process of both glucose and xylose presented in the hydrolyzate wasoptimized. The main results are as follows:Liquid holdup and shaking speed affected the aeration rate of fermentation inthe flask. With the increase of liquid holdup and the reduction of shaking speed, theaeration rate reduced and the fermentation time extended accordingly. In afermentation of50g·L^-1 xylose in a250mL flask with a shaking speed of150r·min^-1 and a liquid holdup of75mL, the sugar utilization was nearly100%; thehighest ethanol concentration and ethanol yield reached18.56g·L^-1 and84.1%,respectively. When the xylose was fermented without air supply, the sugar utilizationcould reach99%, but the ethanol yield was only79.6%. Each liquid holdup had anoptimal shaking speed, for example, an optimal fermentation system was75mLliquid holdup with a shaking speed of150r·min^-1.In a2L fermentor, when theaeration rate was0.075L·min^-1(0.05L·L^-1·min^-1), the fermentation time was32h,the highest ethanol concentration and ethanol yield were18.29g·L^-1 and83.2%,respectively. When the aeration rate was0, the highest ethanol concentration andethanol yield reached19.09g·L^-1 and86.5%, respectively, but the fermentation timewas as long as96h. Therefore, the reasonable aeration rate in the2L fermentorwas0⁰.075L·min^-1.The initial yeast concentration affected the fermentation as well. When theinitial yeast concentration was7g·L^-1, the ethanol yield was79.9%, and the carbonrecovery reached80.1%which was the highest in the experiments.Pichia stipitis hada relatively wide pH range for fermentation. When initial pH wasfrom4.1to5.8, thesugar utilization was high and the ethanol yield varied slightly. Optimal fermentationpH range was4.8⁵.5in which the ethanol yield remained nearly constantly. Theyeast could tolerate acetic acid up to5g·L^-1; the replacement of the yeast in themedia with fresh inoculumsin a certain period could shorten the fermentation time effectively. The yeast could adapt to a certain concentration of acetic acid when theadaptation was performed by gradually increasing the amount of hydrolysate whichcontained acetic acid and other inhibitors. When the acetic acid concentration was2g·L^-1, the adapted yeast could get a sugar utilization of99.27%with a highestethanol concentration of18.03g·L^-1, which represented an ethanol yield of83.97%.At an aeration rate of200r·min^-1, the adapted yeast could increase the ethanol yieldby83.7%to95.2%. The increase of pH and aeration rate improved the tolerance toacetic acid of the yeast.Acid steam-exploded corn stover hydrolysate was concentrated at differentrates and fermented after enzymatic hydrolysis. The glucose and xylose in thehydrolyzate concentrated by two times wereutilized in4h and24h, respectively,with the ethanol yields being95.0%and89.9%,respectively. When the hydrolyzatewas concentrated to three times, fermentation of glucose and xylose required6h and36h, respectively, and the ethanol yields were94.8%and88.5%, respectively. Whenglucose and xylose presented in the hydrolysate, which was concentrated by twotimes, were co-fermented the fermentation time was20h and the ethanol yield was78.8%. Mixed strains could improve the co-fermentation of glucose and xylose; thefermentation time was reduced to8h and the ethanol yield was increased to90.73%.