Comprehensive Analysis of Monomeric Phenolics in Dilute Acid Plant Hydrolysates

We carried out a comprehensive analysis of the phenolic compounds in hydrolysate produced by dilute acid pretreatment of 20 potential lignocellulosic biofuel feedstocks, including grasses, hardwoods and softwood, and agaves. We find that the phenolic fraction is dominated by Hibbert's ketones, most of which had not so far been characterized in hydrolysate. Using gas chromatography/mass spectrometry, a range of 43-68 monomeric phenolic compounds were identified in each of the feedstocks, including from 13 to 20 Hibbert's ketones and related structures, which represented 28-82 % of phenolics formed during pretreatment. The total concentration of phenolics ranged from 87 to 1,077 mu g/mL (equivalent to 78-969 mg phenolics released per 100 g of biomass used) across the feedstocks studied. While total amount of phenolics produced does not correlate with the Klason lignin in the feedstock, the distribution of compound types produced is reflective of the S and G monolignol ratios of the feedstock. Since phenolic compounds are particularly inhibitory to microbial processes and cellulolytic enzymes, our results indicate there is sufficient variation across feedstocks that design strategies are likely to benefit from both general and targeted approaches to detoxification.