Xylitol Production From Byproducts Generated During Sequential Acid-/Alkali-Pretreatment of Empty Palm Fruit Bunch Fiber by an Adapted Candida tropicalis

Abstract

Xylose is a pentose sugar with the potential to convert a variety of valuable chemical products. In this study, acidic pretreatment wastewater generated during a sequential acid-/alkali-pretreatment process was recycled to increase the hydrolyzed hemicellulose fraction from empty palm fruit bunch fiber (EPFBF), a lignocellulosic biomass. The xylose in the reused wastewater was subjected to overliming and an activated charcoal column was used to remove inhibitory compounds, for xylitol fermentation using the adapted C. tropicalis strain. The cell growth and xylose uptake rates in the adapted strain were 1.7-and 5-fold higher, respectively, compared to the wild-type strain. During batch fermentation using the adapted yeast strain and the post-pretreated xylose solution, 35.2 ± 0.8 g/L xylitol was obtained within 61 h for a production yield of 0.44 g xylitol/g xylose. These results indicate that xylose in the byproducts produced in the bioethanol process could be recovered for production of xylitol.

Isolation and Characterization of the Stress-Tolerant Candida tropicalis YHJ1 and Evaluation of Its Xylose Reductase for Xylitol Production From Acid Pre-treatment Wastewater

“…The EPFBF hydrolysate was prepared from the acid extract fraction for pretreatment through the sequential acid-/alkali-pretreatment process (Kim et al, 2012a; Kim, 2019).…”

“…For xylitol production, two xylose sources were used. One source was pure xylose chemical reagent, and the other was xylose solution prepared from the first dilute acid pretreatment of EPFBFs in the sequential acid-/alkali-pretreatment process (Kim et al, 2012a; Kim, 2019). The isolated C. tropicalis was cultivated in semi-defined medium containing 2% (w/v) xylose for 24 h, as described above.…”

“…Treatment of this wastewater accounted for approximately 20% of overall production costs (Humbird et al, 2011). Wastewater reuse is necessary for this process to be considered a sustainable or green technology, and the application of alternative processes to reduce non-utilized materials can keep overall production costs low (Capolupo and Faraco, 2016; Kim, 2019).…”

“…Among the various different types of pretreatment technologies, sequential acid-/alkali-pretreatment is effective for extracting hemicellulose, as well as for enhancing the cellulose content per unit biomass of empty palm fruit bunch fibers (EPFBFs) (Kim et al, 2012a; Kim and Kim, 2013). Repeated use of the acidic wastewater generated during the pretreatment process allows accumulation of the hydrolyzed hemicellulose fraction from EPFBF, which can be further used to produce xylitol, with hydrolyzed xylose produced in the post-pretreatment solution by an adapted yeast strain (Kim, 2019).…”

“…Nevertheless, the hemicellulose hydrolysate in the acidic pretreatment wastewater contains thermally degraded compounds, such as furfural and acetic acetate, as well as other aliphatic and aromatic compounds that are known to be toxic and inhibitory to microbial fermentation (Palmqvist and Hahn-Hägerdal, 2000; Jönsson and Martín, 2016). To overcome these inhibitory compounds and allow reutilization of the highly contented xylose present in the acidic pretreatment solution, a xylitol-fermenting yeast was adapted to the pretreatment solution through repeated batch cultivation (Liu, 2011; Mpabanga et al, 2012; Wang et al, 2015; Kim, 2019). The adapted yeast strain may have evolved tolerance toward the stresses caused by these inhibitory compounds, and thus improved cell growth and xylitol production.…”