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Native starches

Molecular structure

Starch is stored in plants in the form of granules. The granules first appear in the plant cell as minute points, which subsequently grow rapidly to fill the cell. The granule is made up of stratified layers of starch formed around a nucleus called a hilum. As these layers develop, the starch granule assumes a size and shape characteristic of the specific plant.

Tapioca starch granules vary in diameter from 5 to 35 microns, potato starch from 15 to 100 microns, maize from 5 to 25 microns, while rice starch granules are only about 3 to 8 microns in diameter. The shapes vary from near perfect spheres to flattened ovoids, elongated disks, polygons and many others. By observing the size and shape of the granule, the plant source of a starch can be identified even in mixtures of dry starch. Variations in starch granules from different plants may be observed microscopically.

The granule structure is not just a loose agglomeration of glucose polymers. It is systematically structured with the starch molecules oriented in specific spherocrystalline patterns. If the starch granules in aqueous suspension are observed through a microscope in polarized light, a typical Maltese cross pattern is observed.

Starch consists of two glucose polymers: amylose and amylopectin. These do not exist free in nature, but as components of the starch granules. The size, shape and structure of the granules vary substantially between botanical sources. The relative proportion and structural differences between amylose and amylopectin contribute to the significant differences in starch properties and application functionality. Starch granules are essentially insoluble in cold water. Their unique functional characteristics are the result of three major changes they undergo when heated with water: gelatinisation (viscosity increase), pasting and retrogradation. Retrogradation is especially evident when amylose containing starches are cooled, resulting in water release (syneresis).

In plants, starch occurs in the presence of other constituents such as protein, fibre, lipids, water and other organic and mineral compounds.