Effect of acetic acid and pH on the cofermentation of glucose and xylose to ethanol by a genetically engineered strain of Saccharomyces cerevisiae

Abstract

A current challenge of the cellulosic ethanol industry is the effect of inhibitors present in biomass hydrolysates. Acetic acid is an example of one such inhibitor that is released during the pretreatment of hemicellulose. This study examined the effect of acetic acid on the cofermentation of glucose and xylose under controlled pH conditions by Saccharomyces cerevisiae 424A(LNH-ST), a genetically engineered industrial yeast strain. Acetic acid concentrations of 7.5 and 15 g L(-1), representing the range of concentrations expected in actual biomass hydrolysates, were tested under controlled pH conditions of 5, 5.5, and 6. The presence of acetic acid in the fermentation media led to a significant decrease in the observed maximum cell biomass concentration. Glucose- and xylose-specific consumption rates decreased as the acetic acid concentration increased, with the inhibitory effect being more severe for xylose consumption. The ethanol production rates also decreased when acetic acid was present, but ethanol metabolic yields increased under the same conditions. The results also revealed that the inhibitory effect of acetic acid could be reduced by increasing media pH, thus confirming that the undissociated form of acetic acid is the inhibitory form of the molecule.

The relationship of glycerol and glycolysis metabolism patway under hyperosmotic stress in Dunaliella salina

“…The adaption to high salt environments occurs by means of glycerol synthesis to balance external osmotic pressure. Intracellular glycerol content has been found to increase with increasing content of NaCl in media [1][2][3]. It has been confirmed that yeast and other microorganisms also balance external osmotic pressure by synthesizing glycerol under hyperosmotic stress [4][5][6][7].…”

“…This experiment was performed to examine the expression pattern of DsPFK on mRNA levels under hyperosmotic stress, a condition known to involve high glycerol production in D. salina [1][2][3]. In Figure 1A, the relative level of DsPFK mRNA gradually increased from 90 min to 6 h under hyperosmotic stress with a highest level at 6 h, suggesting that expression of DsPFK was salinity-dependent.…”

The amino‐terminal tail of Hxt11 confers membrane stability to the Hxt2 sugar transporter and improves xylose fermentation in the presence of acetic acid

“…Glucose‐ and xylose‐specific consumption rates decrease with the concentration of acetic acid, but the inhibitory effect is more severe for xylose consumption as compared to glucose consumption (Casey et al, 2010). Thus, the presence of acetic acid in fermentation media leads to a significant decrease in the observed maximum cell biomass concentration during xylose consumption.…”

“…For instance, the acetic acid concentration in corn stove and poplar lignocellulosic biomass can be up to 5.6 and 3.6 (w/w)%, respectively (Lu et al, 2009). Acetic acid negatively affects cell growth, the xylose fermentation rate, and ethanol production (Casey et al, 2010; Hasunuma et al, 2011; Sakihama et al, 2015). When the undissociated form of acetic acid enters the cell, the intracellular pH needs to be recovered by pumping out protons at the expense of ATP, which may negatively influence cell growth (Wei et al, 2013).…”

Biorefinery Processes for Biomass Conversion to Liquid Fuel

“…In 2010, Purde University scientists improved a strain of yeast that can produce more biofuel from cellulosic plant material by fermenting all five types of the plant's sugars: galactose, manose, glucose, xylose and arabinose. Arabinose makes up about 10 percent of the sugars contained in cellulosic biomass (Casey et al, 2010).…”