Potential inhibitors from wet oxidation of wheat straw and their effect on ethanol production of Saccharomyces cerevisiae: Wet oxidation and fermentation by yeast

Alkaline wet oxidation (WO) (using water, 6.5 g/L sodium carbonate and 12 bar oxygen at 195°C) was used as pretreatment method for wheat straw (60 g/L), resulting in a hydrolysate and a cellulosic solid fraction. The hydrolysate consisted of soluble hemicellulose (8 g/L), low-molecular-weight carboxylic acids (3.9 g/L), phenols (0.27 g/L = 1.7 mM) and 2-furoic acid (0.007 g/L).

The wet oxidized wheat straw hydrolysate caused no inhibition of ethanol production by Saccharomyces cerevisiae ATCC 96581. Nine phenols and 2-furoic acid, identified to be present in the hydrolysate, were each tested in concentrations of 50–100 times the concentration found in the hydrolysate for their effect on fermentation by yeast.

At these high concentrations (10 mM), 4-hydroxybenzaldehyde, vanillin, 4-hydroxyacetophenone and acetovanillone caused a 53–67% decrease in the volumetric ethanol productivity in S. cerevisiae compared to controls with an ethanol productivity of 3.8 g/L. The phenol acids (4-hydroxy, vanillic and syringic acid), 2-furoic acid, syringaldehyde and acetosyringone were less inhibitory, causing a 5–16% decrease in ethanol productivity.

By adding the same aromatic compounds to hydrolysate (10 mM), it was shown that syringaldehyde and acetovanillone interacted negatively with hydrolysate components on the ethanol productivity. Fermentation in WO hydrolysate, that had been concentrated 6 times by freeze-drying, lasted 4 hours longer than in regular hydrolysate; however, the ethanol yield was the same.

The longer fermentation time could not be explained by an inhibitory action of phenols alone, but was more likely caused by inhibitory interactions of phenols with carboxylic acids, such as acetic and formic acid. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 81: 738–747, 2003.