1 Starch Biosynthesis in Relation to Resistant Starch

Starch is present in amyloplasts as semi-crystalline intracellular waterinsoluble granules, with alternating crystalline and amorphous layers. Starch is a glucan homopolymer composed of one-quarter amylose (molecular mass 10–10Da) and three-quarters amylopectin (molecular mass 10–10Da), along with traces of lipids (0.1–1.0%) and proteins (0.05–0.5%). Amylose is essentially a linear glucan polymer, composed of a-1,4 linked glucose residues with a degree of polymerization (dp) ranging between 800 (in maize and wheat) to more than 4500 (in potato) with sparse branching (approximately one branch per 1000 residues) (Morrison & Karkalas, 1990; Alexander, 1995). Structural and functional aspects of these glucan polymers affect starch functionality and its end use. Amylose chains are capable of forming single or double helices. On the basis of orientation of its fibres in X-ray diffraction studies, amylose can be divided into Aand B-type allomorphs (Galliard et al., 1987). In B-type allomorph, six double helices are packed in an anti-parallel hexagonal mode surrounding the central water channel (36 H2O per unit cell). In A-type, the central water channel is replaced by another double helix, making the structure more compact. In this allomorph, only eight molecules of water per unit cell are inserted between the double helices (Galliard et al., 1987). Amylopectin is a highly branched glucan polymer, in which a-1,4 linked glucose residues are interspersed with a-1,6-glucosidic linkages (4–5%)