The amylose/amylopectin ratio is one of the main parameters measured in starch quality evaluation. In a diluted starch solution (<1% w/v) containing both amylose and amylopectin, amylose can be easily digested by a-amylase while amylopectin is slower to be digested due to its branching structure (Park and Rollings, 1994); however, in a concentrated starch system, especially one with a higher ratio of amylose/amylopectin, a firm gel will form and the digestibility will be decreased due to the high amounts of retrograded starch formed by amylose. Much of highly retrograded amylose is classified as RS. Studies have shown that the amylose/amylopectin ratio is an important determinant of starch quality and amylose content is highly correlated with RS content. Thus, foods produced with high-amylose starches generally have low GI value (Miller et al., 1992).
The amount of amylose and amylopectin within starch granules is genetically controlled by the series of enzymes involved in starch biosynthesis. Traditional breeding has been successfully used to produce starches (especially in maize) with a wide range of amylose and amylopectin ratios, such as waxy starches (essentially pure amylopectin) and high-amylose starches (50-70% amylose). Today, the ratio of amylose to amylopectin can be manipulated through modern genetic techniques, such as genetic engineering, to up- or down-regulate genes related to amylose and amylopectin synthesis. Modern techniques such as transposon insertion, site-directed mutagenesis, antisense inactivation, and TILLING (targeting induced local lesions in genomes) of mutagenized populations - as well as producing transgenic lines with the addition of foreign genes - are being used to produce starch mutants with desired properties. High-amylose starch has been an ideal starting material to make heat-stable RS (Brown, 2004) to decrease the postprandial glycemic response of processed foods. Conceivably, novel mutants could cause changes in the molecular weight, amount of amylose, and perhaps its branching structure to produce starches with higher amounts of SDS.
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