Carbocation scavengers for soft- and hardwood pretreatment: Impact on enzymatic cellulose saccharification and microbial fermentation

Background: The suppression of lignin repolymerization/condensation by the addition of carbocation scavengers to lignocellulosic biomass pretreatment can improve both the bioconversion of cellulose and the quality of the obtained lignin fraction. In recent years, a growing number of effective scavenger molecules has been identified. However, the effects of the various compounds on enzymatic cellulose conversion have not yet been systematically investigated for different lignocellulosic substrates. In addition, studies about potential toxic effects of the scavenger additives on microbial fermentation are lacking. Results: 2-Naphthol, 2-naphthol-7-sulfonate, mannitol and syringic acid were tested in the autohydrolysis pretreatment of softwood (spruce) and hardwood (beech). 2-Naphthol-7-sulfonate proved as the most effective and enhanced the enzymatic saccharification of softwood and hardwood cellulose by up to 277% and 20%, respectively. 2-Naphthol allowed to improve softwood cellulose conversion by 30%, but was hardly effective for hardwood. Mannitol and syringic acid did not prove effective for softwood, but could increase the saccharification of hardwood cellulose by 12% and 18%, respectively. In cultivation experiments with S. Cerevisiae and E. coli , 2-Naphthol proved toxic for both microorganisms at concentrations ≥ 0.03% w/w. In contrast, 2-naphthol-7-sulfonate showed no inhibiting effects on S. Cerevisiae at the tested concentrations (up to 0.34% w/w), and inhibited E. coli growth only very slightly at concentrations ≥ 0.17% w/w. In addition, it was practically completely consumed in pretreatment, so that its remaining concentration in a biorefinery fermentation process would be much lower. Mannitol showed no inhibiting effects on S. Cerevisiae at the tested concentrations (up to 0.34% w/w), and inhibited E. coli growth only at concentrations ≥ 0.13% w/w. Syringic acid prolonged the lag phase of S. Cerevisiae at the tested concentrations (0.03–0.28% w/w), and inhibited E. coli growth at concentrations ≥ 0.14% w/w. Conclusion: The experiments and a review of the literature show, that the scavenger effect for enhancing bioconversion is very case specific. It depends on biomass species, scavenger compound, pretreatment method, and other factors. 2-Naphthol-7-sulfonate proved as the most efficient additive, leading to the highest digestibility increase with both soft- and hardwood and being the most suitable compound for a fermentation process.