Enhancing Prehydrolysates Fermentability by Adding Nucleophilic Amino Acids and Proteins in Biomass Pretreatment

摘要

Dilute acid pretreatment produced a considerable amount of carbonyl compounds in the biomass prehydrolysates, which significantly inhibited the sequential microbial fermentation. To reduce the release of carbonyl inhibitors, a novel approach of pretreatment with amino acids and proteins has been developed to improve the fermentability of prehydrolysates. Four percent (w/w) of cysteine (Cys), histidine (His), soy protein isolate (SPI), and bovine serum albumin (BSA) was added into dilute acid pretreatment of aspen (DAPA). The resulted prehydrolysates were fermented by Saccharomyces cerevisiae, and the glucose consumption rate in the prehydrolysates was increased from 0.32 to 1.35, 3.22, 1.02, and 1.61 g/L/h, respectively. The pretreated substrates were applied to enzymatic hydrolysis. Unexpectedly, it was observed that 72 h hydrolysis yields of DAPA-Cys and DAPA-His decreased from 71.35% (DAPA) to 63.93% and 28.11%, respectively, while the 72 h hydrolysis yield of DAPA-SPI increased to 75.04%, and the 72 hydrolysis yield of DAPA-BSA did not change. The results showed that BSA was the most effective additive to enhance the prehydrolysate fermentability. It increased the ethanol productivity of prehydrolysates from 0.15 (without addition) to 0.77 g/L/h. The final yield was promoted from 0.05 to 0.44 g/g glucose. In addition, a total of 22 potential inhibitors in prehydrolysates have been identified and quantified by gas chromatography/mass spectrometry (GC/MS). The results showed that addition of histidine in pretreatment reduced inhibitors much more than addition of cysteine, SPI, and BSA. The results suggests that addition of protein decreases inhibitors by reaction with aldehydes/ketones and also by buffering and reducing pretreatment severity, which results in less inhibitors produced. The reduced severity also decreases the cellulose digestibility, especially with the addition of cysteine and histidine. Hence, the overall effect on yield and productivity is a trade-off between the positive effect of reducing inhibitors and the undesirable effect of less sugar release from the substrates.