Chapter 6 - Structural Features of Starch Granules II

Abstract

Starch granules are commonly found in seeds, roots, tubers, stems, and leaves. Grain seeds, such as maize kernels, contain up to 75% starch on a dry starch basis. The stored starch granules can be converted by enzymes (amylases) to glucose, and the glucose is utilized to generate energy during germination or whenever energy is needed. Starch granules are densely packed with semicrystalline structures and have a density of about 1.5 g/cm3. Because of this stable semicrystalline structure, starch granules are not soluble in water at room temperature. In the granular form, starch can be isolated easily by gravity sedimentation, centrifugation, and filtration, and can be subjected to various chemical, physical, and enzymatic modifications with subsequent washing and processing. There are two major starch polymers: amylopectin and amylose, each having different properties. Biosynthesis of starch granules takes place primarily in the amyloplast. Minor components found in starch granules include polymers of sizes and with properties intermediate between those of amylose and amylopectin, starch lipids, monostarch phosphate ester groups, and proteins, particularly granule-bound starch synthase. Some minor components, especially phospholipids, free fatty acids, and phosphate ester groups, although at low concentrations, can drastically affect the properties of starch pastes and gels. An understanding of the internal organization of starch granules is crucial for scientists and engineers to optimize reaction conditions for chemical, physical, and enzymatic modifications.