FOOD. Science. Technology. Quality

Food. SCIENCE. Technology. Quality

Food. Science. TECHNOLOGY. Quality

Food. Science. Technology. QUALITY




Structure of starch granules


Starch is produced in a semicrystalline granule form by higher plants for energy storage. The granule size and granule shape of starch differ with the botanical source. The diameter of starch granule varies from submicron to more than a hundred microns. The shape of the granules include spherical, oval, disk, polygonal, elongated, and kidney. Starch consists of amylopectin, a highly branched molecule, and amylose, primarily a linear molecule’ with few branches. Biosynthesis of starch is originated at the hilum, and the starch granule development is by apposition. The amylose content of starch granules increases with the maturity and the size of the starch granule, and amylose is found more concentrated at the periphery of the granule. The branch chain length of amylopectin, however, decreases as the granule size increases. Amylopectin molecules at the hilum consist of exceedingly long branches which are loosely packed with little crystallinity. These long branches are susceptible to iodine to develop a blue core in the granule. The outer chains of amylopectin are in a double helical crystalline structure. Starches which consist of amylopectin with longer branch chains (such as potato and high-amylose maize starches) display the B-type X-ray diffraction pattern, whereas those with shorter branch chains (such as wheat, rice, and maize) display the A-pattern. Starches with branch chain length in between (such as tapioca and banana) display the C-pattern. Amylose in the granule is dispersed among amylopectin. This is evident as amylose molecules are cross-linked to amylopectin, whereas amylose molecules are not found cross-linked among themselves. The molecular size of amylose increases with the increase of the granular size. Most amylose in the starch granule is present in a free form not complexed with lipids; however, about 21% amylose in non-waxy barley starch is present as lysophospholipid complex. 31P-NMR studies have shown that phospholipids are present in all the normal cereal starches investigated but not in tuber, root, and legume starches. With few exceptions (e.g., du waxy maize starch), most waxy starches donot contain phospholipids. Phosphate derivatives are primarily on amylopectin. Studies conducted by using DSC, X-ray, chemical analysis, and 31P-NMR of Naegeli dextrins showed that a substantial proportion of phosphate derivatives were located within the crystalline region of amylopectin and were protected from exhaustive acid hydrolysis.