UC Davis

Gluconic acid was evaluated to be a promising pretreatment agent on wheat straw. The pretreatment was tested at different gluconic acid concentrations (0.125-1M), temperatures (160-190°C), and reaction times (30-120 minutes). After each pretreatment, the slurries were separated into hemicellulose hydrolysate and pretreated solid residue. The sugar yields in stage 1 (hemicellulose hydrolysate) and stage 2 (enzymatic hydrolysis of pretreated solid residue) were reported. The highest overall xylose achieved 89.6±3.4% yield that was observed at the pretreatment condition of 0.125M gluconic acid concentration, 170°C pretreatment temperature, and 30-minutes reaction time. The highest overall glucose achieved 91.5±2.0% yield at the pretreatment condition of 0.5M gluconic acid concentration, 190°C pretreatment temperature, and 30-minutes reaction time. The overall sugar (glucose and xylose combined) yield of 83.5±2.4% was achieved at the pretreatment condition of 0.5M gluconic acid concentration, 170°C pretreatment temperature, and 30-minutes reaction time. Preference of utilizing lower gluconic acid concentration was also evaluated. The overall sugar yield of 81.1±0.2% was achieved at the pretreatment condition of 0.125M gluconic acid concentration, 170°C pretreatment temperature, and 60-minutes reaction time.The evaluation for ethanol production utilized the hemicellulose hydrolysate and pretreated solid residue generated from the pretreatment condition of 0.125M gluconic acid concentration, 170°C pretreatment temperature, and 60-minutes reaction time because of lower gluconic acid concentration consumption and high overall sugar yield. Roughly 4.2±0.14% of the gluconic acid was lost during the pretreatment process and a portion may be lost from being stuck to pretreated solid residue. The remaining 75.6±0.59% of gluconate in the detoxified hemicellulose hydrolysate was fermented to ethanol along with other hemicellulose sugars (glucose, xylose, and arabinose). The engineered microorganisms such as Escherichia coli AH003 and E. coli SL100 were evaluated for ethanol production in the detoxified hemicellulose hydrolysate. The theoretical maximum yield obtained from the detoxicated hemicellulose hydrolysate fermentation was 107.5±1.2% from E. coli AH003 using Luria-Bertani (LB) media and 90.4±1.8% from E. coli SL100 using low-salt (AM1) media. Additionally, E. coli AH003 and Saccharomyces cerevisiae D5A were used as the ethanologens to convert the pretreated solid residue to ethanol via simultaneous saccharification and fermentation (SSF) with cellulase loading of 20 FPU and ?-glucosidase loading of 20IU/g cellulose. The ethanol yield obtained from E. coli AH003 in LB media from SSF was unclear since glucose, xylose, and a small amount of gluconate could be hydrolyzed from the pretreated solid residue. Meanwhile, S. cerevisiae D5A exclusively consumed glucose in lean media and the ethanol yield was 92.8±2.0% with cellulose conversion of 70.8±0.8%. Overall, evaluation of fermentation suggests that casamino acids in LB media could promote high cell growth and ethanol yield.