3-38: Effects of reducing agents on fermentability and fermentation inhibitors in lignocellulosic hydrolysates

Fermentation inhibitors in lignocellulosic hydrolysates are an obstacle for development of efficient processes for production of cellulosic ethanol. The introduction of an additional process step and related costs for detoxification of lignocellulosic hydrolysates has commonly been raised as objections against detoxification. Recently published results show that detoxification of lignocellulosic hydrolysates, both softwood and sugarcane bagasse, can be performed with a one-step treatment at low temperature and slightly acidic pH (such as 25^oC and pH 5.5). This novel detoxification method is based on the addition of low concentrations of inexpensive reducing agents, such as sodium dithionite and sodium sulfite, directly to the fermentation vessel. The resulting fermentation improvements over untreated hydrolysates are in par with powerful detoxification methods such as alkali detoxification. Additional benefits with the use of reducing agents over other established detoxification methods are that reducing agents do not result in sugar degradation and have no negative effects on the fermenting microorganism at the concentrations that are used for detoxification. Investigations of the mechanism behind the effects of reducing agents using mass spectrometry (MS), i.e. UHPLC-ESI-TOF-MS and UHPLC-LTQ/Orbitrap-MS/MS, reveal that fermentation inhibitors including furan aldehydes, such as HMF and furfural, as well as lignin degradation products, such as coniferyl aldehyde, ferulic acid, and 4-hydroxybenzaldehyde, are primarily sulfonated and thereby rendered less toxic to the fermenting microorganism.