Several degradation products are formed or released during AFEX pretreatment process, which can be divided into five groups: 1) carbohydrates, 2) aliphatic acids, 3) nitrogenous compounds, 4) furans, 5) aromatic compounds. Some of these degradation products are expected to be inhibitory to Saccharomyces cerevisiae 424A(LNH-ST), thus affecting the xylose fermentation, as well as various saccharolytic enzymes. In this work, the effect of various AFEX degradation products on co-fermentation by Saccharomyces cerevisiae 424A(LNH-ST) were tested individually and in combinations. A synthetic media composed of similar nutrient composition as AFEX corn stover hydrolyzate was designed and successfully used as a control fermentation media in this study. Nitrogenous compounds showed the inhibitory effect on xylose fermentation, followed by carbohydrates, aliphatic acid and aromatic compounds. The inhibitory effect caused by nitrogenous compounds and carbohydrates were attributed to the presence of amides and oligomeric xylan, respectively. To our knowledge, this is the first report of inhibitory effect of oligomeric xylan and amides on xylose fermentation by Saccharomyces cerevisiae. When testing the amides (feruloyl amide, coumaroyl amide and acetamide) and their corresponding acids (ferulic acid, coumaric acid and acetic acid) individually, the amides were less inhibitory than their acids with respect to cell growth and xylose fermentation. From this point, the shift of degradation products from organic acids to amides may be responsible for the high fermentability of AFEX pretreated lignocellulosic biomass. We also evaluated the extent of enzymatic inhibition by various molecular fractions of AFEX treated corn stover degradation products. The pretreated biomass extracted products were fractionated using ultracentrifugation, gel filtration, and C18 resin solid phase extraction and further analyzed by mass spectrometry. The results indicate that both large and small molecule compounds play a role in enzyme/microbial inhibition.
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