Influence of lignocellulose-derived aromatic compounds on oxygen-limited growth and ethanolic fermentation by Saccharomyces cerevisiae

Phenolic compounds released and generated during hydrolysis inhibit ferment ation of lignocellulose hydrolysates to ethanol by Saccharomyces cerevisiae . A wide variety of aromatic compounds form from lignin, which is partially degraded during acid hydrolysis of the lignocellulosic raw material. Aroma tic compounds may also form as a result of sugar degradation and are presen t in wood as extractives. The influence of hydroxy-methoxy-benzaldehydes, d iphenols/quinones, and phenylpropane derivatives on S. cerevisiae cell grow th and ethanol formation was assayed using a defined medium and oxygen-limi ted conditions. The inhibition effected by the hydroxy-methoxybenzaldehydes was highly dependent on the positions of the substituents. A major differe nce in inhibition by the oxidized and reduced form of a diphenol/quinone wa s observed, the oxidized form being the more inhibitory. The phenylpropane derivatives were examined with respect to difference in toxicity depending on the oxidation-reduction state of the gamma-carbon, the presence and posi tion of unsaturated bonds in the aliphatic side chain, and the number and i dentity of hydroxyl and methoxyl substituents. Transformations of aromatic compounds occurring during the fermentation included aldehyde reduction, qu inone reduction, and double bond saturation. Aromatic alcohols were detecte d as products of reductions of the corresponding aldehydes, namely hydroxy- methoxy-benzaldehydcs and coniferyl aldehyde. High molecular mass compounds and the corresponding diphenol were detected as products of quinone reduct ion. Together with coniferyl alcohol, dihydroconiferyl alcohol was identifi ed as a major transformation product of coniferyl aldehyde.