Structure dependent toxicity of lignin phenolics and PEG detoxification in VHG ethanol fermentation
The inhibition of lignin phenolics on fermenting microbes has become one of the major barriers in developing an economically viable process for cellulosic ethanol production. In this study, the toxicity to yeast cells and the inhibition to the very high gravity (VHG) fermentation of the phenolic compounds were investigated in the absence or presence of polyethylene glycol (PEG). It was found that the inhibitory effects of phenolic compounds on VHG ethanol fermentation depend on the activity of their hydroxyl (–OH) hydrogen. 250 g L−1 PEG-1000 detoxified 2.0 g L−1 guaiacol in the fermentation broth, and boosted the ethanol concentration from 131 g L−1 to 173 g L−1. The inhibitory effect of 5.0 g L−1 guaiacol on ethanol fermentation was also alleviated, and the ethanol concentration was increased from 51 g L−1 to 151 g L−1 after detoxification with 250 g L−1 PEG-1000. The 1H-NMR of hydroxyl group (–OH) of phenolic compounds in PEG revealed the role of hydrogen bonding formation on the in situ detoxification mechanism of PEG, and the order in the strength of the intermolecular hydrogen bond between phenolic compounds and PEG. Furthermore, the kinetics of VHG ethanol fermentation in the presence of phenolic compounds were determined. The obtained kinetic model (phenolic compounds inhibitory effect) fits well the kinetics of ethanol production from lignocellulosic hydrolyzates using batch VHG ethanol fermentation technology.