An Overview on Starch Structure and Chemical Nature

Abstract

Starch is considered the second most common biomass on earth, as it is produced by green plants as an energy reserve. It is found as granules of different morphologies (depending of the botanical source) in plant tissues, mainly seed, roots, tubers, leaves and fruits. (Odeku 2013; Pérez and Bertoft 2010; Zia ud et al. 2017). On a cellular level, starch is synthesized in two types of plastids, chloroplasts and amyloplasts, through three main pathways: The Calvin cycle, sucrose synthesis and storage starch biosynthesis. (Hsieh et al. 2019; Tappiban et al. 2019). Chemically, starch can be defined as a polysaccharide composed of α-D-glucopyranosyl units that can be linked in either α-D-(1–4) and/or α-D-(1–6) linkages. These molecular linkages form to types of molecules: the linear amylose formed by approximately 1000 glucose units linked in α-D-(1–4) manner and the branched amylopectin, formed by approximately 4000 glucose units, branched through α-D-(1–6) linkages, as shown in Fig. 2.1. The union of both amylose and amylopectin forms a semi-crystalline structure arranged as small granules with diameters between 1–100 μm. Most of the native starches have amylose percentages that range between 70 and 80% and amylopectin ranging from 20 to 30%. Furthermore, some types of starch can have a very high amylose content, such as starch extracted from amylomaize with a 70% of amylose and some can have very low amylose content such as waxy maize starch with a 1% amylose content.