Thermodynamic considerations of starch functionality in foods

Abstract

Functionality of starch macromolecules is considered from the viewpoint of their thermodynamic properties. The thermodynamic incompatibility, self-association and inclusion complexing of starch macromolecules are important for food formulation and digestion. Helical conformations of starch macromolecules increase their co-solubility with other biopolymers (molecular mimicry) and influence the thermodynamic activity of other macromolecules (molecular symbiosis). Molecular mimicry and molecular symbiosis are the basis of many biological functions, phase behaviour and rheology of biopolymer mixtures. Rotation of starch granules in shear flow (producing a ball-bearing effect) contributes to the fat-like texture of foods. Branched exopolysaccharides could be an evolutional predecessor of starch granules. Amylopectin-like exopolysaccharides form a biopolymer solution layer around the cell, which is not accessible to foreign macromolecules due to biopolymer incompatibility underlying a non-specific defence of the cell. However, this biopolymer solution around cells remains perfectly accessible to nutrients of low molecular weight allowing for necessary cell nutrition. This barrier solution layer of exopolysaccharides could then be regarded as a protective and alimentary capsule surrounding the cell. The principle of incompatibility of exopolysaccharide fractions could be extended to provide a mechanism by which exopolysaccharides leaving the cell are responsible for binding and evacuating metabolites. This binding of undesirable metabolites is analogous to the binding of self-antigens and in a molecular sense precedes the evolutionary development of a similar result: specific immunity. A colony of cells is formed as the assembly of individual cells with their surrounding exopolysaccharide solutions separated from the medium by the interfacial layer. The overlap of the interfacial layers produces a three-dimensional network of canals that would assist the diffusional transportation of nutrients from a larger surrounding area. At the level of individual cells, the thermodynamic properties of amylopectin-like exopolysaccharides predict the binding of nutrients within their helical structure by inclusion complexing. Carrying them to the surface of the cell would then produce an octopus-like effect of facilitated transport. The capacity of starch macromolecules to bind lipids and other hydrophobic ligands leads to a decrease in the rate of enzymatic hydrolyses of inclusion complexes, which would extend the contribution of the exopolysaccharide layer to the nutrition of the cell. An increase in concentration of cells and their competition for the exopolysaccharides as a source of energy could result in the accumulation of dietary energy reserve inside the cells in the form of starch-like granules. Like globular proteins, the primary structure of amylopectin (branch point distribution and side-chain lengths) appears to be responsible for the secondary and tertiary structure of macromolecules and the structure–function relationship in starch granules.