STARCH STRUCTURE/FUNCTION RELATIONSHIPS: ACHIEVEMENTS AND CHALLENGES

M.J. Gidley

Unilever Research Colworth, Colworth House, Sharnbrook, Bedford, MK44 1LQ, UK

1 INTRODUCTION

The importance of starch, both biologically and technologically, is well known, as is its central role in the human diet. Many aspects of starch structure can be measured or detected by one or more chemical, physical, spectroscopic or microscopic methods. Functional performance of starches in technological applications can usually be assessed with appropriate end-use mimic tests, and underlying material properties can be measured using the arsenal of techniques available to the physical scientist. However, despite major progress in many aspects of starch structure / function relationships, there is still a significant gap in our ability to predict the functional properties of starch from a knowledge of structure. This is particularly relevant in the post-genomic era where it is, or soon will be, possible to select or modify plant sources of starch at the genetic level. There will therefore be a growing need to link structural features more precisely and predictably to defined functional performance. This review seeks to identify some of the key successes in describing and quantifying starch structure and function, as well as highlighting some of those areas where information or mechanistic understanding is lacking. Given the broad nature of the area, the choice of topics is necessarily subjective and will not include much in the way of introductory material.

2 LEVELS OF STRUCTURE IN STARCHES

Despite the textbook description as a 'simple' polymer of glucose, starch is one of Nature's most complex materials. There are two fundamental factors behind this complexity. One is the existence of characteristic structures over a wide range of distance scales, and the second is the heterogeneity of structure at all of these different distance scales both within a single granule as well as the natural variation inherent in populations of granules.

At each level of structure, there have been significant recent advances in methodology and information. Progress in tackling the heterogeneity problem has been slower reflecting the greater challenges involved.

2.1 Molecular Structure

Methods for probing the molecular structure of starch polymers are well advanced. A combination of chemical, enzymic and NMR methods are available for describing the branching pattern and branch length profile of polymers. When coupled with prior chromatographic fractionation, a high information content is obtained. It is now accepted that there is a continuum of starch polymer structures from purely linear to extensively branched, and that for some starches there is a difficulty in describing constituent polymers as purely amylose (lightly branched, molecular weight 104 - 106 Da) or amylopectin (heavily branched, molecular weight 106 - 108 Da). Although branching levels and branch lengths can be readily determined, it is more difficult to define the pattern of branching points. It is well established that branch points have a tendency to cluster, but it is more difficult to determine the local molecular architecture around branch points. NMR methods may help in this area in the future.

Molecular size is relatively easily determined for low or intermediate molecular weights, but sample preparation and handling requirements may make it difficult to identify true molecular weights for some large amylopectins. This is because above 108 Da, molecules may be sufficiently large that either they are mechanically unstable in solution or they are more colloidal than molecular in dimensions and consequently cannot be chromatographed or measured accurately by light scattering.

It is now clear that it cannot be assumed that molecular structure features are distributed evenly throughout granules. This can be shown by differential iodine staining or by sequential solubilisation from the outside of granules under defined chemical regimes. It is not known whether this distributional heterogeneity has any impact on functional properties.

2.2 Helices and Crystalline Order

There are significant stretches of contiguous linear glucan in all starches. This leads to the potential to form the repetitive glycosidic conformations characteristic of helices. A range of single- and double-stranded helices have been identified and characterised from native and treated starches. Starch chains usually show a high proportion of local glycosidic conformations...