Identification of crucial yeast inhibitors in bio-ethanol and improvement of fermentation at high pH and high total solids

Compounds inhibitory to enzymatic hydrolysis and fermentation are generated from neutral steam exploded corn stover in the process of producing bio-ethanol. In this study, weak acids were identified as main yeast inhibitors, while phenols and aldehyde contribute to the inhibition to a lower degree. Main weak acids in hydrolysates are acetic acid and formic acid, for which critical levels for yeast inhibition are 6 and 4 g/L, respectively. The inhibitory effect of these compounds can be greatly overcome by increasing pH of hydrolysates to 6.0–9.0, but there is a risk of bacterial contamination when fermenting at high pH. The relationship of pH, total solids of hydrolysates, fermentation and contamination was studied in detail. Results indicate that the contamination by bacteria when fermenting at high pH can be prevented effectively using hydrolysates with total solids of more than 20%. Meanwhile, ethanol yield is improved significantly.

► Acetic acid and formic acid in hydrolysates were identified as main yeast inhibitors.
► Critical levels of acetic acid and formic acid for yeast inhibition are 6 and 4 g/L, respectively.
► The inhibitory effect can be greatly overcome by increasing pH of lignocellulosic hydrolysates to 6.0–9.0.
► Contamination by bacteria at high pH can be prevented effectively using hydrolysates with total solids of more than 20%.