17 - Microbial production of xylitol and other polyols

Abstract

D-glucose is the prevalent carbohydrate in human and microbial metabolism. Other common monomeric sugars are D-fructose, D-mannose, D-galactose, D-xylose and L-arabinose. In large quantities these sugars are found in bound polymeric forms in plants. When sugars are reduced they are converted to sugar alcohols (polyols) which are rare in nature. The history of sugar chemistry goes back to 18th century when the first sugar alcohols were detected. All sugars and their reduced forms can be consumed by at least some living organisms and the metabolism of the most common ones is discussed in detail. Sugar alcohols such as sorbitol or xylitol have found use in human consumption owing to their low calorie content and their positive physiological effects such as prevention of tooth decay. The common sugar alcohols can be produced in reductive reactions by microorganisms and in this chapter we discuss the present status of these microbial production technologies, including the use of genetically modified organisms. Sugar alcohols can also be produced by chemical catalytic reduction which is in most cases the preferred method for their large scale production. Sugar alcohols are key intermediates owing to their non-chiral nature in biosynthesis of several rare sugars and their derivatives.