Xylitol fermentation

Many factors impact the xylose to xylitol fermentation process, including oxygen delivery rate and concentration, pH, temperature, and the presence of other sugars (in addition to xylose), nutrients, and inhibitors [4, 6]. The least studied of these factors is the effect of specific inhibitors derived from biomass pretreatment on the growth of xylitol-... 

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. 

Xylitol (Fig. lg) is found in the primrose (38) and in minor quantity in mushrooms (39). It can be obtained from glucose in 11.6% overall yield by a sequential fermentation process through D-arabinitol and D-xylulose (28). 

The hemicellulose from the pulping of trees is an underused resource. A small amount is currently being hydrolyzed to xylose for hydrogenation to the sweetener xylitol. A good use could be as a substrate for fermentation. 

Fig. 1. Fermentation of standard sugar mixture (glucose and xylose) by S. cerevisiae 424A(LNH-ST). ( ) Glucose ( ) D-xylose (A) ethanol ( ) glycerol ( ) xylitol.

Our standard sugar mixture (Std. Mix) for fermentation contained YEP, and 70 g/L of pure glucose and 40 g/L of pure D-xylose. Glucose in Std. Mix was completely fermented in 10 h. Higher rates of xylose fermentation started when glucose concentration dropped below 2.5%. Of the xylose 88.5% was fermented in 30 h (Fig. 1). The production of xylitol dur-... 

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. 

Zadivar et al. (12) reported fermentation of a pure glucose/xylose mixture using their xylose-fermenting S. cerevisiae, but xylose mainly converted to xylitol with ethanol only as a minor product (12). Furthermore, the time needed for completion of the fermentation exceeded 100 h with an initial xylose concentration of 50 g/L. 

Fig. 2. Time course of substrate, cell dry weight, and fermentation products in fermentation of casamino acids supplemented with BSG OCL posthydrolysate (SH medium) (A) and casamino acids supplemented concentrated posthydrolysate (CSH medium) (B) by D. hansenii CCMI 941. ( ) Glucose, (O) xylose, (A) arabinose, ( ) cell dry weight, ( ) ethanol, ( ) xylitol, and (A) arabitol.

In fermentation samples, owing to the partial overlap of arabinose, xylitol, and arabitol, in the column Aminex HPX-87H, those components were also analyzed using a Sugar-Pak I column (Waters) operating at 90°C in a Merck-Hitachi HPLC system (Merck) equipped with an RI detector (L-7490 Merck). The mobile phase was 50 mg/L of calcium EDTA at a flow rate of 0.5 mL/min. 

Hemicelluloses can be hydrolysed into their component sugars and used as a fermentation feedstock for the production of ethanol and other alcohols (e.g. butanol, arabitol, glycol and xylitol), organic acids (e.g. acetic acid), acetone and gases (e.g. methane and hydrogen). The wider monosaccharide profile offers opportunities to develop different products to those derived from glucose alone. 

The xylose produced via enzymatic hydrolysis of the hemicellulose fraction could be converted via microbial fermentation into xylitol (which has been listed by the US Department of Energy among the top 12 value-added platform chemicals) for the production of a range of chemicals, including xylaric acid, propylene glycol, ethylene glycol and a mixture of hydroxyl-furans and polyesters (Werpy and Petersen, 2004). 

Xylitol, another polyhydroxy compound, is used as a sweetener in sugarless gum. It has approximately the same number of calories per gram as does sucrose and is not a low-calorie sweetener. However, because it does not have a carbonyl group, it is not fermented by bacteria in the mouth and does not promote tooth decay. 

The pentoses, such as xylose (Xyl), that result from the hydrolysis of lignocel-lulose (see above) resist fermentation by Saccharomyces, because it lacks an efficient mechanism to convert Xyl into xylulose (Xlu). The isomerization redox interconversion pathway of Xyl and Xlu, via xylitol, that is native to Saccharomyces, is inefficient due to a cofactor incompatibility (see Fig. 8.5) and results in a redox imbalance and the accumulation of xylitol [24]. Many bacteria, in contrast,... 

The benefits of polyols in food and confectionery products include reduced caloric content, reduced glycemic response, and reduced cariogenicity. The dental literature is replete with reports that demonstrate unique dental benefits for specific polyols, such as xylitol [52,53], while others [54,55] show that the genuine dental benefits of some polyols are similar. Polyols generally are not fermented by oral bacteria, so that acid production is minimized while... 

Although abundant in nature, and although several opportunities have been identified [64], heteropolysaccharides have largely escaped industrial exploitation. Exceptions are the conversion of pentoses into furfural (for temperature-resistant foundry resins) by acid hydrolysis the fermentation of pentoses into single cell protein Torula yeast) the catalytic reduction of xylose to xylitol, a dietary sweetener and the use of larch [Larix sp.)-specific arabinogalactan extracts as dispersants in printing inks. 

---