Xylitol production from xylose

FIGURE 18.1 Processes for xylitol production from xylose. 

TABLE 18.2 Effects of Aeration on Xylitol Production from Xylose by Yeasts... 

Saccharomyces cerevisiae Xylitol production 95% xylitol conversion from xylose was obtained by transforming the XYLl gene of Pichia stipitis ncoding e a xylose reductase into S. cerevisiae, making this organism an efficient host for the production of xylitol, which serves as an attractive sweetener in the food industry 39... 

PC. (2008) Role of xylose transporters in xylitol production from engineered Escherichia coli. J. Biotechnol, 134, 246-252. 

Cirino PC, Chin JW, Ingram LO. (2006). Engineering Escherichia coli for xylitol production from glucose-xylose mixtures. Biotechnol Bioeng, 95, 1167-1176. 

Meyrial V, Delgenes JP, Moletta R, Navarro JM. (1991). Xylitol production from D-xylose by Candida guillermondii fermentation behaviour. Biotechnol Lett, 13, 281-286. 

Yahashi Y, Hatsu M, Horitsu H, Kawai K, Suzuki T, Takamizawa K. (1996). D-glucose feeding for improvement of xylitol productivity from D-xylose using Candida tropicalis immobilized on a non-woven fabric. Biotechnol Lett, 18, 1395-1400. 

Hydrolysate B from corn stover contained 4 g/L of glucose, 17.9 g/L of xylose, 5 g/L of arabinose, and 2.5 g/L of acetic acid. Glucose was readily fermented eighty-three percent of xylose was fermented in 23 h. The production of ethanol by fermentation of the com stover hydrolysate was 9 g/L (Fig. 3). The yield of ethanol from consumed sugars reached 93% of theoretical yield. We did not observe xylitol production and acetic acid consumption. 

In yeast and mycelial fungi, xylose is metabolized via coupled oxidation-reduction reactions . Xylose reductase is the enzyme involved in the reduction of xylose to xylitol. Sequential enzymatic events, through the oxidation of xylitol to xylulose, lead to the utilization of xylose. Many yeast species utilize xylose readily, but the ethanol production capability is very limited. Only a few yeast species effectively produce ethanol from xylose these include Pachysolen tan-nophilus, Candida shihatae and Pichia stipitis [80]. The production of ethanol from xylose by these three yeast strains has been studied extensively in recent years. Recently, genetically engineered yeast strains have been constructed for more effective conversion of xylose to ethanol. 

Although many facultatively fermentative yeasts utilize xylose as the carbon source for growth, the ability of these yeasts to produce ethanol from xylose is limited. Yeast strains that utilize xylose often produce xylitol from xylose extra-cellularly as a normal metabolic activity. However, only a few can produce significant quantities of ethanol. The prominent strains that produce ethanol from xylose include Pachysolen tannophilus, Candida shihatae and Pichia stipitis. However, the efficient production of ethanol from xylose is limited by the regulation of dissolved oxygen as well as by the imbalance of cofactors in the metabolic pathway during xylose utilization. In recent years, much effort has been put into improving yeast strains in order to produce ethanol from xylose more efficiently. 

Oxygen limitation had little effect on the activities of TAL and TKL. When OTR varied from 0 to 12.6 mmol/L-h, TAL and TKL aetivities always maintained at low levels (Fig. 4c). TAL and TKL activities always maintained at low levels, to show big control effects on xylose metabolism. Walfi idsson et al. [34] overexpressed TAL in XR- and XDH-expressing S. cerevisiae, whieh increased the cell growth but not ethanol production. Karhumaa et al. [30] found that increased XR and XDH activities redirected the production from xylitol to ethanol, whereas the rate of xylose consumption was governed by the overexpressed nonoxidative pentose phosphate pathway. 

The production of xylitol from xylose requires only a single step, xylose reductase (XR). The XR gene xyrA from Candida tropicalis was expressed in E. coli and 13.3 gl of xyhtol was produced from xylose [116]. A strain that co-consumes xylose and glucose was used to generate a 56 g 1" xylitol [117]. Another study used a transhydrogenase (PntAB) to achieve theoretical yield of xylitol from glucose in the presence of excess xylose [118]. 

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