3.2.1 Starch

Starch is a polymeric carbohydrate made up of glucose units linked by glycosidic bonds. The starch is the most abundant carbohydrate in nature after the lignocel — lulosic complex and is present in high amounts in very important crops for human food, such as corn, wheat, potato, cassava, rye, oats, rice, sorghum, and barley. About 54 million tons per year of starch are produced for industrial purposes, from which 55% comes from the United States. From the total produced starch,

44.1 million tons come from corn, 2.6 million from cassava and rice, and 2.8 mil­lion from potato (Messias de Braganga and Fowler, 2004)

Starch is composed of two types of a-glucans, i. e., polymers based on glu­cose monomers linked by a-type glycosidic bonds, amylose and amylopectin, which represent 98 to 99% of the dry weight of starch kernels. The properties of these two polysaccharides along with the ratio between them in the starch kernels

FIGURE 3.3 Structure of amylose: n « 1,000.

determine the properties of the starch obtained from different plant sources. This ratio ranges from less than 15% amylose in waxy starch and 20 to 35% amylose in normal starch to greater than 40% amylose in amylo-starch. In addition, the water content of the starch kernels in equilibrium with the air goes from 10 to 12% in grains to 14 to 18% in tubers (Tester et al., 2004).

The amylose is a linear polysaccharide made up of D-glucopyranose units linked by a(1,4) glycosidic bonds, although it has been established that some molecules present several branching points due to the presence of a(1,6) (Buleon et al., 1998). These branching points are present in an amount of 9 to 20 per mole­cule (Sajilata and Singhal, 2005). The molecular weight of amylose is in the range 1×105-1×106 Da with a polymerization degree of 324 to 4,920 (1,000 on average). These variations depend on the starch origin. The basic structure of amylose is depicted in Figure 3.3. Two ends can be distinguished: one end with intact gly — cosidic hydroxyl (corresponding to the glucose residue of the right-hand side of Figure 3.3) called the reducing end, and the end with the glucose residue whose glycosidic hydroxyl is participating in the glycosidic bond with the following glu­cose residue (the left-hand side of Figure 3.3) called the nonreducing end. These ends are very important during the process of enzymatic hydrolysis of starch.

The amylopectin is a highly branched polymer made up of chains of D-glucopyranose units linked by a(1,4) bonds, but with 5 to 6% a(1,6) bonds leading to branching points (Buleon et al., 1998). These branching points occur every 15 to 30 glucose units on average. The amylopectin has a greater molecular weight than amylose (1×107-1×109 Da) with a higher polymerization degree of 9,600-15,900 (Tester et al., 2004). Because of the branches, the amylopectin pres­ents an elevated amount of chains that are differentiated by their inner or outer character. As in the case of amylose, the nonreducing ends of amylopectin can be distinguished (see the left-hand side of Figure 3.4a). The single reducing end of the amylopectin molecule can be observed on the right-hand side of Figure 3.4b.

The amylose and amylopectin are located in the starch kernel in a radial way in the form of concentric layers where there exist crystalline and amorphous zones of these two polysaccharides. This implies that kernel degradation, the first stage in the solubilization of starch in water, is very difficult at low temperatures. To achieve this goal, it is necessary to break the starch kernels through a hydrother­mal treatment involving the adsorption of hot water by the kernels and their swell­ing and destruction with the corresponding release of amylose and amylopectin in a soluble form. This process is known as starch gelatinization and has a crucial

importance during the industrial processing of this carbohydrate, especially dur­ing the production of starch hydrolyzates with sweetening properties (starch syr­ups), glucose, and ethanol. It is worth emphasizing that the solubilization of starch is required for the enzymatic attack of starch for producing glucose.