Understanding Amylose Structure, What It Controls And What Controls It.

Abstract

Starch accounts for at least 92% (dry weight) of a milled rice grain. Starch is comprised of two fractions, amylose and amylopectin. Amylose content can range from 0% (in waxy rice) to about 30%. Amylose is essentially a linear molecule ranging from about 800 degrees of polymerization (DP) to about 10 000 DP. It carries a few widely spaced chains. Amylose plays a significant role in almost all of the cooking qualities of rice. The process of cooking of rice begins with the softening of the starch granules, which is primarily a function of amylopectin. The next process, swelling, is greatly affected by amylose. As the starch granules swell, amylose leaches from the granules into the solution phase. Behaviour observed in the field of synthetic polymer science suggests that the linear amylose molecules surround the swelling granules and inhibit the swelling. After amylose leaches from granules, it joins the continuous phase and van der Waal forces inside the helices of chains cause double helices to form. The double helices aggregate into a gel; the more double helices, the firmer the gel. The early stages of gel formation would occur in the interval between removing from heat and eating the rice. Long chains of amylose have a higher viscosity than short chains, and this limits the mobility of the long chains. Thus, with long chains, the formation of double helices and aggregations is slower, leading to a softer gel. Therefore, amylose structure could explain why two varieties with the same amylose content differ in cooked texture. In the later stages of gel formation, typically occurring well after cooking the rice, and when the temperature falls below 25 ºC, short chains of amylose will form double helices and crystallites much more readily than long chains of amylose. Therefore, rice that contains short chains of amylose are likely to be hard when cooled after cooking. The knowledge and information that could be provided by developing a method to measure amylose structure will provide a tool allowing greater insights into the effect of amylose structure on different cooking properties, with the ultimate aim of developing the knowledge into a selection tool for rice breeders. After developing a tool to measure amylose structure, it was applied to understanding a particular nutritional property of rice, namely resistant starch. Literature and early research indicated some link between resistant starch content and amylose content, however, detailed investigations of the structure of resistant starch, hypothesised to reveal more of the secrets of amylose, in actuality, revealed some of the secrets of amylopectin.