Starch Homepage

................................ Home .......... Contacts ........... Iowa State University .......... Opportunities ........... Biotechnology.......... Links ................................

Starches ......... Amylopectin ......... Amylose .......... Structure ............ Pathway .......... Mutants ............ Enzymes .......... Clones ........... Modifications ............ Functionality .......... EnCapsulation ........... Processing


Starch Structure.

Text from Keeling (©1998)
Starch is a polymer of glucose made by plants. It is a mixture of 25% amylose and 75% amylopectin. Amylose is linear a1,4-glucan, whilst amylopectin is a highly branched chain of a1,4- and a1,6-glucan chains. Mutants are known with low amylose content and others with higher (e.g. from 50-90% amylose) than normal levels of amylose. In cereal plants the starch is laid down in the endosperm tissue of the grain. In potato the storage organ is the tuber. In tomato and many other fruits, starch is laid-down transiently before ripening, during ripening the starch is degraded back to sucrose. Starch derived from corn grain during wet-milling can be isolated in a remarkably pure state, contaminated only by a small percentages of proteins and lipids. Starch from all species has a very minute fraction of phosphorylated glucans with potato tubers carrying the highest known percentage (100 times higher than all cereals).

Biotechnological developments in starch synthesis hold great promise for modifying the basic structural composition of starch. Opportunities for structural modifications of starch can be broken-down into a few main classes:

These structural changes may be achieved by an empirical approach in which individual enzymes in the pathway are under and over expressed. However, there is evidence that this kind of approach has its limits and even with the conventional mutants we can see that there are reasons to doubt that we adequately understand the mechanism of starch assembly. In our research we are focussing on what we believe are the major technical challenges facing this new science today. In particular we are faced with a poor understanding of how the enzymes of starch biosynthesis control and contribute to starch structure. Furthermore we are also faced with trying to develop a better understanding of how starch structure and functionality are interrelated. Ultimately, it is only starch functionality that matters to the consumer and end-user/processor. Thus, we have two major technical challenges, first to improve our understanding of the links between functionality and starch fine structure, and second to establish a linkage between starch fine structure and enzymology and genetics.