(18c) Detoxification of Biomass Hydrolysates with Nucleophilic Amino Acids Enhances Microbial Fermentation
Producing biofuels from lignocellulosic biomass via biochemical conversion shows great promise for reducing the dependence on petroleum based fuels. Biomass typically needs to be pretreated prior to subsequent enzymatic hydrolysis to monomeric sugars and fermentation to biofuels by microorganisms. Pretreatment, however, generates a wide range of toxic compounds from the degradation of carbohydrates, lignin and extractives, which significantly inhibit microbial fermentation. Numerous studies have concentrated on identifying potential inhibitors and developing detoxification methods to remove their inhibition. However, due to the large number of degradation compounds and low concentrations, identification of the compounds those are the most potent to fermentation remains elusive. Consequently in the absence of correct targets, the detoxification or conditioning methods developed have not been cost effective or chemically selective.
Carbonyl compounds generated in biomass pretreatment hinder the biochemical conversion of biomass hydrolysates to biofuels. A novel approach of detoxifying hydrolysates with amino acids for ethanol production was developed. Among the 20 amino acids assessed for their detoxification efficiency and nucleophilicity, cysteine was the most effective one. It increased both ethanol productivity and final yield of biomass hydrolysates from 0.18 (untreated) to 1.77 g/L/h and from 0.02 to 0.42 g/g, respectively. Detoxification efficiency was followed by histidine and it increased the final yield to 0.42 g/g, then by lysine, tryptophan and asparagine. It was observed all five effective amino acids contained reactive side-chain functional groups, which played important roles in the amino acid detoxification reaction. The study further showed cysteine and glycine detoxifications were temperature and pH dependent. The mechanistic study using mass spectrometry revealed thiazolidine carboxylic acid, a Schiff base, was formed by condensation of aldehyde and cysteine.