ABSTRACT
Before discussing the physical chemistry it is necessary to first consider macromolecular structure. There are many excellent recent reviews on starch structure and biosynthesis, so these aspects will only briefly be considered [1,2]. Starch consists of two main polysaccharides, amylose and amylopectin. Both polysaccharides are based on chains of 1! 4 linked a-D-glucose (Fig. 1a) but whereas amylose is essentially linear, amylopectin is highly branched containing on average one branch point which is 1! 4! 6 linked for every 20-25 straight chain residues. Linear regions of the amylose chain form a dark blue complex with polyiodide ions in aqueous solution at room temperature [3]. In this complex, the amylose chain adopts a helical conformation with the polyiodide ion occupying the central cavity. This interaction is a basis for defining amylose as that starch polysaccharide which, under standardized conditions, binds 20% of its weight of iodine while under the same conditions amylopectin generally binds <1% w/w. This iodine binding allows a distinction to be made between amylose and amylopectin, and permits the determination of the amylose content of native starch [3]. Most starches contain between 20% and 25% w/w amylose although some waxy starches contain very little, if any, amylose (<1%) and other starches, such as amylomaize, contain in the region of 65% amylose. Although a fraction of the amylose population is linear and is quantitatively hydrolyzed by the exo-acting enzyme h-amylase, a fraction is lightly branched [4-8]. For example, purified wheat amyloses were hydrolyzed from79% to 85%byh-amylase. Isoamylolysis allowed quantitation of a short chain fraction representing between 3.2% and 7.6% w/w of the amylose depending on the variety of wheat examined [9]. Thus the fraction of amylose that is branched contains short chain branches on a main chain. Typical molecular
weights of extracted amylose are in the region of 105 to 106 g mol1.
