Laccase treatment of phenolic compounds for bioethanol production and the impact of these compounds on yeast physiology

摘要

Laccase treatment of phenolic compounds found in lignocellulosic hydrolysates is an alternative to reduce the growth inhibitory effect of these compounds on fermenting microorganisms. In order to determine the main factors that affect the efficiency of this biocatalytic approach, laccase oxidation of individual and mixtures of phenolic compounds was evaluated. Additionally, the effect of phenolic compounds on Saccharomyces cerevisiae cell physiology and their effect on yeast membrane liposome models were evaluated. Trametes versicolor laccase showed complete oxidation of six phenolic compounds, while Agaricus bisporus laccase showed oxidation percentages ranging between 30-100. High Performance Liquid Chromatography (HPLC) analyses suggested that compounds such as syringic acid, catechol and gallic acid were not completely polymerised after laccase treatment, which could explain the increase of negative effects observed when they were added to S. cerevisiae cultures. Ferulic acid and vanillin oxidation led to a 6.4 and 6.5-fold increase in ethanol production, respectively, compared with the untreated cultures. In phenolic mixtures, chemical interactions between phenolic compounds led to biotransformation of these compounds since different by-products were observed in HPLC chromatograms. Moreover, after laccase treatment, the negative effects of these mixtures were reduced, resulting in an increase in ethanol production yields. No correlation was observed between growth inhibition data, the effect on liposomes and the detoxification efficiencies. Thus, the efficiency of laccase detoxification was not related to the toxicity of the compounds but could be conditioned by the heterogeneity in the chemical structure and properties of these compounds (functional side groups, hydrophobicity, pKa).