Enzymatic Hydrolysis And Fermentation Performance Of Tamarisk After Steam Explosion Pretreatment

The conversion of tamarix, a deciduous shrub, to fermentable sugar for bio-ethanolproduction typically involves a thermal pretreatment step, followed by enzymatic hydrolysisand fermentation. In this study the effect of steam explosion on the digestibility of tamarix wasinvestigated. Steam explosion was conducted without acid impregnation. The main parametersconcernd included explosion temperature, residence time, and severity factor. After steampretreatment the resulted material was used as a whole, without solid-liquid separation andwashing, for enzymatic hydrolysis. Fermentation was conducted with a mixture of hydrolyzateand solid residue, or with the hydrolyzate only, to study the fermentability of high concentrationof sugars. The main results were as follows:Steam explosion without acid catalysis was a suitable pretreatment method for tamarixwhen following saccharification and fermentation were examined. The severity factor couldeffectively characterize the intensity of steam explosion pretreatment. Based on the celluloserecovery and the yield of enzymatic hydrolysis, the optimum condition of steam explosion wasat a severity factor value of4.239, in which the explosion temperature was210℃and theresidence time was10min. Under the optimum condition the steam-exploded tamarixcontained cellulose54%, the highest value among all the tests, together with a xylan content of1.4%. The enzymatic hydrolysis yield of this substrate was87%.The steam-exploded material was used as a whole, without solid-liquid separation andwashing, for enzymatic hydrolysis. In the hydrolysis the substrate concentration was7.5%(w/w); a cellulase loading of20FPIU/g cellulose was applied, together with a cellulase toβ-glucosidase ratio of1:1and a PEG6000addition of0.04g/g dry substrate. After48h, thehighest yield of enzymatic hydrolysis reached83%with a total sugar concentration of35g/Land a fermentable sugar ratio of98%.After enzymatic hydrolysis, the supernatant was concentrated by ultrafiltration andanalyzed for enzyme activity and protein content. SDS PAGE gel electrophoresis was alsoconducted to check the distribution of enzyme proteins. The results indicated that the apparentadsorption on solid residue of endoglucanase, exoglucanase and β-glucosidase was94.4%,70.3%and55.4%, respectively. In the process of vacuum concentration, the removal rates offurfural and levulinic acid were nearly100%, while that of acetic acid was about50%, and thatof HMF was only8%.In the fermentation with a mixture of hydrolyzate and solid residue and at the substrateconcentrations of5%and10%(w/w), glucose consumptions were more than98%in both cases;the conversion rates of glucose to ethanol were0.5and0.35, respectively. In7h, the ethanolconcentrations were12g/L and19g/L, respectively.The vacuum-concentrated enzymatic hydrolyzate with different glucose concentrationswere well fermented by Saccharomyces cerevisiae NL22at a yeast loading of20OD (opticaldensity). At the glucose concentration of131g/L, the glucose consumption rate was100%in8 h, and the ethanol produced was60g/L. The ethanol yield, that is the percentage of producedethanol to the theoretical value, was94%. When glucose concentrations were166g/L or189g/L, glucose consumed completely in12h; ethanol reached79g/L and87g/L, respectively. Inboth cases ethanol yields were93%. At the glucose concentration of220g/L, it took20h forcomplete glucose utilization, while the ethanol was100g/L and the ethanol yield was89%.