Xylitol production

Santos, J.C., Silva, S.S., Mussatto, S.I., Carvalho, W. and Cunha, M.A.A., Immobilized cells cultivated in semi-continuous mode in a fluidized bed reactor for xylitol production from sugarcane bagasse, World. Microbiol. Biotech., 21 (2005) 531-535. 

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. 

The major change in yeast metabolism induced by casamino acids supplementation was the marked increase in xylitol production, being the major metabolic product by D. hansenii grown in supplemented concentrated medium (Fig. 2B). Both arabitol and xylitol production in yeast are described to be augmented under stress conditions. Arabitol is usually found as a product of arabinose metabolism in oxygen-limited conditions (8,22), but arabitol production is not restricted to arabinose metabolism,... 

Xylitol production by P stipitis is much lower than any other xylose-fermenting yeast. 

The activities of XR and XDH decreased with the increase in oxygen limitation (Fig. 4a), which indicated that an increase in XR and XDH activities could lead to the increase in xylose metabolic rate. Karhumaa et al. [30] increased XR and XDH activities in Saccharomyces cerevisiae by genetic manipulation, which significantly increased ethanol but decreased xylitol productions. Not only XR and XDH activities but also the XR/XDH... 

C. guilliermondii produces the enzyme D-xylose reductase which catalyses a reaction where the proton carrier NADPH donates a hydrogen atom to D-xylose, and D-xylose is converted to xylitol as seen in Fig. 1. The xylitol can then be converted to D-xylulose, catalyzed by xylitol dehydrogenase, which is utilized in central metabolism [3]. Under semi-aerobic conditions, xylitol accumulation is favored compared to anaerobic or aerobic conditions. Under anaerobic conditions, the ratio of NAD(P)H to NAD(P) is low, and NAD(P)H is required for xylitol production. Under aerobic conditions, excess oxygen allows oxidation of NADH to NAD", and a resulting high NAD /NADH ratio results in a faster xylitol conversion rate to D-xylulose, eliminating the accumulation of xylitol [4, 5]. 

The xylitol production rate is highest at very high xylose concentrations (80 g/1) [8, 10]. However, concentrating the hemicellulose hydrolysates using vacuum evaporation to achieve high xylose concentrations also concentrates the non-volatile inhibitors [11]. At these inhibitor concentrations, volumetric productivity actually declines [12-14]. Many studies have demonstrated the inhibitory effect of these hydrolysis-derived compounds on growth and production of products, such as ethanol and xylitol [15-16]. Concentrated hydrolysate requires detoxification for optimum xylitol production. 

The xylitol producing yeast C. guilliermondii (ATTC No. 201935) was cultivated in a semisynthetic basal salt medium (BSM) for experiments investigating cell growth and xylitol production. This medium consisted of 40 g/1 xylose (EMD Chemicals, Darmstadt, Germany) as the carbon source, 5 g/1 ammonium sulfate as the nitrogen source, 0.5 g/1 magnesium sulfate, 0.1 g/1 calcium chloride, 1 g/1 potassium sulfate, and 1 g/1 yeast extract (Fisher Scientific, Rochester, NY). The medium was prepared separately for each experiment and autoclaved for 20 min at 121 °C to assure sterility. 

The batch xylitol fermentations conducted in the bench-scale fermentors were more typical of studies on inhibitor toxicities at high cell densities. It is possible that the xylitol production phase could also be considered at the microscale (100- to 200- xl volume), as this is a microaerophilic process in which low oxygen transfer rates are desirable. While this may not provide an accurate measure of the maximum volumetric productivity for a particular hydrolysate, it may be useful for investigating fundamental toxicological effects and inhibitor synergies on xylitol production. 

C. guilliermondii growth and xylitol production. For an efficient process, measures should be taken to adapt the organism to the inhibitors or remove the compounds to non-inhibitory levels. 

Fermentation Kinetics for Xylitol Production by a Pichia stipitis D-Xylulokinase Mutant Previously Grown in Spent Sulfite Liquor... 

Recently, the conversion of xylose into value-added chemicals, such as xylitol, ethanol, and lactic acid have made this process attractive to the fermentation industry [4]. In particular, bioconversion for xylitol production has been intensively studied during the last decade because xylitol can be used as a functional sweetener [5]. 

In the present work, we grew cells on SSL to determine the influence of ammonium sulfite spent liquor SSL on P. stipitis FPL-YS30 (xyl3-A) xylitol production. The present research used SSL derived from an ammonia sulfite pulping of southern pine. 

The highest xylitol production that we observed (31.6 g/1 Fig. 2a) was obtained using cells precultivated in YPD. This inoculum condition also showed the highest specific xylitol production rate 0.059 gxyiitoi/gceii h) and xylitol volumetric productivity Qp,... 

In general, cells precultivated in YPSSL and YPX showed similar and more stable values for specific xylose consumption and xylitol production rates than cells fi om YPD medium (data not shown). However, fermentation using cells precultivated in YPSSL showed lower values for specific xylose and xylitol production rates than cells precultivated in YPX. 

Table 1 Parameter kinetics for xylitol production by a / stipitis D-xylulokinase mutant previously grown in YP-SSL, YPX, and YPD media. ...

Die importance of bacteria and eukaiyotic microorganisms is illustrated in Table 7.1, which lists metabolite production by vaiious microorganisms, such as amino acid biosynthesis, ethanol production, lactic acid, and xylitol production, 7-AC A and 7-ADCA-pro-duction. 

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... 

Several lactate dehydrogenases (LDHs) were expressed in Sa. cerevisiae in order to produce lactic acid. Most successful was the expression of a fungal (Rhizo-pus oryzae) lactate dehydrogenase (LDH). A recombinant strain accumulated approximately 40% more lactic acid with a final concentration of 38 g lactic acid and a yield of 0.44 g of lactic acid per gram of glucose [89]. Xylitol is an attractive sweetener used in the food industry. Xylitol production in yeast was performed by the expression of xyll of P. stipitis, encoding a xylitose reductase [90]. 

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

Akinterinwa, O. and Cirino, PC. (2011) Anaerobic obligatory xylitol production in Escherichia coli strains devoid of native fermentation pathways. Appl. Environ. Microbiol, 77, 706—709. 

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