Microbial Production of Xylitol
Xylitol is a functional sweetener of five carbon atoms, and has the global market in food, bakery, confectionery, cosmetic, odontological, pharmaceutical, and medical sectors. There are different methods of xylitol production which includes chemical and biotechnological (microbial and enzymatic) processes. Presently, most of the xylitol is produced by chemical means at industrial level, which involves the reduction of D-xylose in the presence of nickel catalyst at elevated pressure and temperature. However, there are various limitations associated with chemical process such as high cost, high energy consumption, low product yield, and environmental hazards. Biotechnological processes are one of the alternatives to overcome these limitations with improved product yield, which involve enzymatic process using xylose reductase (XR) or fermentation using whole cells. However, the microbial production of xylitol involves additional advantages over enzymatic process i.e. low cost and less energy with high xylitol productivity, and environment friendly process. Most of the xylose utilizing microorganisms including bacteria, yeast, and fungi possess the ability to assimilate xylose into xylitol. Among microorganisms, yeasts are reported to be the best microorganisms for xylitol production followed by fungi, whereas, only a few bacteria have been reported for xylitol production. Microbial xylitol production has also been improved through immobilization strategies, and recombination DNA technology. Xylitol recovery and its purification are the most difficult and limiting steps of the fermentation process, which is further a matter of research. The present chapter describes the role of microbes in economical production of xylitol, strategies to improve the yield and productivity, recovery of xylitol, and commercial applications of xylitol.