Starch
This is the most widely distributed substance in the vegetable kingdom and is the chief reserve carbohydrate of plants. Starch consists of single repeat units of D-glucose linked together through 1 and 4 carbons by a-linkages (cis) [10]. There are two types of starch molecules, amylose and amylopectin. The first one is mainly a linear polymer. Its molecular weight can range from 30,000 to 1,000,000, though it is mostly 200,000–300,000. Amylose is often pictured in a spiral form due to the conformation of the a-glucoside bonds:
Amylopectin, on the other hand, is branched through carbon 6:
The ratios of amylopectin to amylose in many natural starches are about 3:1. The main commercial source of starch in this country is corn. Lesser amounts of industrial starch are obtained from potatoes, wheat, and tapioca (not necessarily in that order). The extraction of starch from plant material is done by grinding the plant tissues in water. The slurry is then filtered to obtain a suspension of starch granules. These granules are then collected with the aid of a centrifuge and dried. When a water suspension of starch granules is heated to 60–80C, the granules swell and rupture. This results in formation a viscous colloidal dispersion containing some dissolved starch molecules. Cooling this dispersion results in formation of a gel, due to aggregation of the amylose molecules. It is essentially a crystallization phenomenon, known as retrogration. By comparison, amylopectin molecules cannot associate so readily due to branching and will not gel under these conditions. Starches are modified chemically in various ways. Some acetate and phosphate esters are produced commercially, as well as hydroxyalkyl and tertiary aminoalkyl ethers. Both unmodified and modified starches are used principally in papermaking, paper coating, paper adhesives, textile sizes, and as food thickeners. There are many reports in the literature on graft copolymers of starch. The work is often done in search of biodegradable materials for packaging and agricultural mulches. Most chemical modifications of starch parallel those of cellulose.
