The nature of amylose-lipid interactions and their effects on the rheological properties of starch
The interaction of amylose with lipids and the formation of helical inclusion complexes were studied by viscometry, surface tension measurements, enzymic techniques, gas-liquid chromatography, X-ray crystallography and differential scanning calorimetry (DSC). The effect of fatty acids on the rheology of starch gels, and associated creep phenomena, were investigated by means of a viscoelastic analyser. The viscosity number of amylose solutions at pH 12 decreased with increasing concentration of added fatty acids (12:0 to 22:0) and reached a constant value presumably because of saturation of the amylose helix. At saturation, the molar ratio of fatty acid/amylose was dependent on the chain length of the fatty acid; the longer the chain length the lower the saturation molar ratio. The same stoichiometric relationship was valid for the insoluble complexes prepared at pH 4·6 with fatty acids and their monoglycerides. The precipitated complexes were completely degraded to glucose by amyloglucosidase. It was demonstrated that the mass of lipid required for saturation of a given mass of amylose could be predicted from the chain length of the lipid. It was found by DSC that there was a gradual increase in the dissociation temperature of the complexes which was related to the molecular weight of the fatty acids, but the dissociation enthalpy was essentially the same irrespective of the fatty acid chain length, suggesting a close structural similarity in the conformation. Complexes heated below their dissociation temperature showed structural transitions, probably from 7 to 6 glucosyl residues per helix, which were dependent on the chain length and melting point of the fatty acids. Viscosity measurements (at pH 12) with starch solutions containing added fatty acids, as well as creep and dynamic experiments with amylose and starch gels with added fatty acids, indicated that both the viscosity of solutions and the mechanical properties of the gels were dependent on the chain length and the concentration of the fatty acids, the concentration of the glucans and the ratio of amylopectin/amylose. Rheological tests provided evidence that amylopectin may interact with fatty acids, especially with those with a long chain. It was shown that structural failure of gels may be responsible for the inconsistent data reported in the literature.