Properties of Damaged Starch Granules. II. Crystallinity, Molecular Order and Gelatinisation of Ball-milled Starches

Wheat and maize starches were ball milled to obtain various levels of damaged starch, with corresponding losses of short-range crystalline order (measured by wide-angle X-ray diffraction) and double-helix content (measured by 13C-CP/MAS-NMR) in the dry starches. Regression analyses indicated that all damaged starch in the dry wheat and maize starches was amorphous. Order in damaged maize starches could be restored by wetting and drying the damaged starches. On wetting, the damaged portions of the granules comprised soluble material and swollen gel that were quantified together as damaged starch using an enzymic assay. The undamaged portion comprised surviving native starch granules and birefringent remnants of larger granules that had been partially damaged. Both the enthalpy and temperature of gelatinisation of the starches, and of the recovered native granules plus remnant material, decreased with increasing levels of starch damage. Starches isolated from millstreams showed the same pattern of behaviour. A model is presented to explain the observed patterns of damage. It is concluded that amorphous starch within the native granules (which does not include crystalline amylopectin) should be distinguished from amorphous damaged starch since they differ in composition and in their physical properties