4-12: In-situ detoxification of dilute-acid lignocellulose hydrolysates with reducing agents

Lignocellulosic feedstocks can be converted to liquid biofuels and green chemicals in fermentation processes based on microbial biocatalysts, such as yeast and bacteria. Dilute acid hydrolysis or pretreatment of lignocellulosic feedstocks typically generate compounds that inhibit the fermenting microorganisms. Chemical detoxification, or conditioning, offers a way to dramatically improve the fermentability, even for strongly inhibitory hydrolysates. For example, optimization of alkali detoxification has resulted in more efficient detoxification procedures while the risks for degradation of sugars have been minimized [1]. However, alkali detoxification would still require an additional process step before the fermentation.

This investigation shows that addition of appropriate concentrations of reducing agents, such as dithionite, was an efficient way to improve the fermentability of a strongly inhibitory dilute-acid hydrolysate of spruce, which was fermented with Saccharomyces cerevisiae yeast. Addition of 15 mM sodium dithionite at pH 5.5 compared equally or favorably with the fermentability of an alkali-detoxified hydrolysate and of a synthetic sugar solution without inhibitors. After 9 h fermentation, the dithionite-treated hydrolysate reached an ethanol productivity of 1.7 g×L^-1×h^-1, whereas an untreated hydrolysate reached a productivity of only 0.2 g×L^-1×h^-1. Furthermore, the reducing agent could be added in situ in the fermenter, i.e. just before, simultaneously, or even after the inoculum with yeast. The approach circumvents the need to perform chemical detoxification as a separate treatment step and makes industrial implementation of chemical detoxification more feasible.

_________________

[1] Alriksson B, Sjöde A, Nilvebrant N-O, Jönsson LJ (2006) Appl. Biochem. Biotechnol. 129-132: 599-611.