Badal C. Saha and Rodney J. Bothast

Fermentation Biochemistry Research Unit, National Center
for Agricultural Utilization Research, Agricultural Research Service,
U. S. Department of Agriculture, 1815 North University Street,
Peoria, IL 61604

Xylitol, a five-carbon polyalcohol, has attracted much attention because of its potential use as a natural food sweetener, as a dental caries reducer and as a sugar substitute in diets for diabetics. Currently, it is produced chemically by catalytic reduction of xylose. Various microorganisms can convert xylose to xylitol. The present review describes microbial production of xylitol from xylose and xylose rich hemicellulose fractions present in various lignocellulosic biomass.

Xylitol, a pentitol of xylose, has attracted much attention because of its potential use as a natural food sweetener, as a dental caries reducer and as a sugar substitute for treatment of diabetics (7). It is a normal intermediary product of carbohydrate metabolism in humans and animals. The human body produces 5-15 g of xylitol a day during a normal metabolism (2). Xylitol is widely distributed in the plant kingdom, especially, in certain fruits and vegetables (7, 5, 4). However, extracting it from these sources is impractical because it is generally present in small quantities. Xylitol is currently produced chemically by catalytic reduction of xylose present in hemicellulose (xylan) hydrolyzate in alkaline conditions (5, 6). The recovery of xylitol from the xylan fraction reaches about 50-60% (4). Drawbacks of the chemical process are the requirements of high pressure and temperature, use of an expensive catalyst and use of extensive separation and purification steps to remove the by-products mainly derived from hemicellulose hydrolyzate (7). The bulk of xylitol produced is consumed in various food products such as chewing gum, candy, soft drinks and ice cream (2).