Among hexoses, glucose is the immediate metabolizing sugar that can be fer­mented through different pathways such as glycolysis. The orientation of the — H and -OH groups around the carbon atom adjacent to the terminal primary alcohol carbon (carbon 5 in glucose) determines whether the sugar belongs to the D or L series. When the — OH group on this carbon is on the right side, the sugar is the D — isomer; when it is on the left, it is the L-isomer. Most of the monosaccharides occurring in mammals are D sugars (Fig. 9.1), and the enzymes responsible for their metabolism are specific for this configuration. In solution, glucose is dex­trorotatory—hence the alternative name dextrose, often used in clinical practice. Other important hexoses like galactose and mannose are first either converted into

glucose before fermentation or their products after initial metabolism join the glycolytic sequence. Figure 9.2 shows the pathway of glucose degradation.

9.4.1.1 Sucrose

This disaccharide is most commonly used as the carbon and energy source by fermentative microorganisms. It is a non-reducing sugar consisting of one mole­cule each of D-glucose and D-fructose linked through a-1, b-2 glycosidic bond (Fig. 9.3). In the fermentation process, sucrose is first hydrolyzed by invertase (sucrase) to D-glucose and D-fructose. D-glucose directly enters the glycolysis while fructose joins the main stream after phosphorylation with ATP in a hexo — kinase-catalyzed reaction. Sucrose can also be fermented through its initial breakdown by sucrose phosphorylase (Fig. 9.4).