Gluconic and xylonic acid production from lignocellulosic biomass by Gluconobacter oxydans

Abstract

To address the growing concerns over national energy security and climate change, significant amount of efforts have been made for developing sustainable biofuels and chemicals from renewable lignocellulosic biomass. Gluconic acid and xylonic acid are two important chemicals in food and pharmaceutical industries, which can be produced through fermentation by the microorganism Gluonobacter oxydans. The value-added chemicals production (such as gluconic acid and xylonic acid) from lignocellulosic biomass can generate extra revenue in the forest biorefinery process, which has the potential to significantly improve the viability of biofuels production from biomass. In this study, we first investigated the gluconic acid and xylonic acid fermentation on glucose and xylose respectively without pH control. A nearly 100% conversion of glucose was obtained resulting in a gluconic acid production yield of 100%. However, for xylonic acid fermentation, only 61% conversion of xylose was reached after 48 h with a xylonic acid production yield of 57%. The low conversion yield of xylose probably was caused by the quick pH dropping and pH-sensitive enzymes. Subsequently, the effects of pH control (addition of CaCO3, NaOAc buffering, NaOH neutralization) on gluconic acid and xylonic acid fermentation were further studied. Higher pH (>5.5) resulted in further oxidation of gluconic acid to 2-keto gluconic acid (2KGA) and 5-ketogluconic acid (5KGA) in the gluconic acid fermentation on glucose. With the addition of CaCO3 at 6 g/L, xylose was completely converted into xylonic acid in the 48 h fermentation by G. oxydans. iii Finally, we studied the gluconic acid and xylonic acid fermentation on pretreated woody biomass. Sweetgum (wood chips) was first pretreated with 50% ethanol and 1% H2SO4 at 170 ⁰C for 1 h. The pretreated substrate was washed and collected for gluconic acid production through SHF (separated hydrolysis and fermentation) and SSF (simultaneous saccharification and fermentation) with 2.5, 5.0 and 10.0 FPU/g enzyme loading. In SHF process, the hydrolyzed glucose was quickly converted to gluconic acid within 10 h, and considerable amount of KGA (up to 20 g/L) was also produced with the further extension of fermentation to 48 h. In the SSF process, the hydrolyzed glucose was accumulated during the fermentation, which indicated the fermentation conditions (pH 4.8 and 37 ºC) in the SSF process was not suitable for gluconic production by G. oxydans. In conclusion, we believe that pH plays a very important role in gluconic acid and xylonic acid fermentation by G. oxydans. For gluconic acid fermentation, relatively high pH (>5.5) results in the accumulation of ketogluconic acid. For xylonic acid production, pH should be governed under neutral condition.