Lactose is a milk sugar. In dairy products, the fermentation of this sugar plays a vital role. Lactose is a disaccharide of D-galactose and D-glucose bonded to each other by b-1,4 glycosidic linkage. Lactose cannot be taken up freely by the microbial cells. A specific transport system is required for the translocation of this sugar to the site of metabolism. Lactose transported through PTS gets phosphorylated as lactose-6-P, while the other system translocates it unphos — phorylated. Once lactose is translocated, it is fermented first undergoing hydrolysis into monosaccharides with the help of b-galactosidase, also called lactase. The former enzyme is present in the lactic acid bacteria. Approxi­mately, 80% of the galactose originated from lactose is metabolized via taga- tose pathway. Figure 9.5 shows the structure of lactose.

Fig. 9.2 Pathway of glucose degradation. a hexokinase, b phosphoglucose isomerise, c phosphofructokinase, d aldolase, e triosephosphate, f glyceraldehydes-3-P- defydrogenase, g phosphoglycerate kinase, h phosphoglycerate mutase, i enolase, j pyruvate kinase

— Fructose-6-P

9.4.1.4 Starch

Starch is a homopolysaccharide of D-glucose units that are joined to each other through a 1,4-glycosidic bond. Starch has two components, amylose and amylo- pectin (Fig. 9.6). Amylose is an unbranched molecule with molecular weight ranging from a few thousands to 5,000,00. One end of each chain with free hemiacetal group is reducing while the other is nonreducing in nature. The typical blue color with starch is due to its ability to form a helical structure. It is soluble in water. Amylopectin is a branched polysaccharide with b 1-6 linkage at every

25-30 glucose units. The molecular weight and branching per chain differ for different sources of starch.

Starch is widely distributed from lower microalgae such as Chlamydomonas to higher plants. In plants, it is the major storage material. A great diversity of microorganisms is able to utilize this polysaccharide. The hydrolysis of starch into glucose in biological systems is carried out with multiple enzymes. For the commercial application of amylolytic enzymes, the reader is referred to an earlier review [62].