Xylitol has recently been recognized as one of the top 12 value-added chem­icals from biomass by the DOE [139]. This pentahydroxy sugar alcohol is commonly used to replace sucrose in food products and in toothpastes as a natural, non-nutritive sweetener that inhibits dental caries [140]. In add­ition, xylitol can serve as a valuable synthetic building block for deriva­tives intended for new polymer opportunities [139]. Production of xylitol, which typically involves hydrogenation of xylose derived from hemicellulose — xylan hydrolysates with an active catalyst such as nickel, ruthenium, or rhodium [139], is currently very limited. Numerous yeast strains have been developed that are capable of producing xylitol in complex medium [141 — 144]. Xylitol production (up to 237 gL-1) by Candidata tropicalis has been optimized by growth in complex media containing urea and numerous ex­pensive vitamin supplements [145]. More recently, strain PC09 was derived from E. coli W3110, which is capable of fermenting a broad range of sug­ars in mineral medium. PC09 can process glucose and xylose blends into xylitol by using an NAD(P)H-dependent xylose reductase from Candida boi — dinii (CbXR) to reduce xylose to xylitol, whereas glucose serves as the cell growth substrate and to regenerate the reducing equivalents [146]. Resting cells and controlled fermentations of PC09 produced 71 and 250 mM xylitol while consuming 15 and 150 mM glucose, respectively. In the controlled fer­mentations, approximately 25 mM xylulose was formed as co-product [146].

Because glucose was used to regenerate reducing equivalents and was not converted to xylitol, the xylitol yield was quantified in terms of a molar yield of reduced product formed per glucose consumed. In the case of zero growth, a maximum molar yield of 10-12 is expected; resting cells and controlled fermentations of PC09 had molar yields of 4.7 and 1.7, respectively. While the molar yield is relatively low compared to the theoretical maximum, this process could prove to be more economical after further optimization and metabolic engineering.

5.4