Starch from hull-less barley: IV. Morphological and structural changes in waxy, normal and high-amylose starch granules during heating

Abstract

Morphological and ultrastructural changes of waxy, normal and high-amylose barley starch granules heated in water to various temperatures were examined by scanning electron microscopy and transmission electron microscopy. Swelling and gelatinization/solubilization patterns of waxy hull-less barley (HB) starch were different from those of non-waxy HB starches. Waxy HB starch granules were split into two halves with a large cavity in the central region of the granule and started melting at the equatorial grooves during heating in water to 55 °C, whilst only a small amount of molecules leached out of granules. Heating resulted in rapid fragmentation of granules at temperatures exceeding 55 °C. However, with increasing heating temperature, the extent of release of the exudate became more pronounced while swollen starch granules remained intact in non-waxy HB starches. Knobs/protrusions (200–400 nm diameter) were present on the granule surfaces of non-waxy HB starches during gelatinization, but not present on the granule surface of gelatinised waxy HB starch. Ghost structure was formed and embedded in exudate/soluble matrix with a web network within ghosts at 90–100 °C. Bundles of brush-like strands in the exudate of high-amylose starch were formed after heating. Fine particles (∼10 nm diameter) arranged into chains were present on the granule remnants of all three types of barley starches. MALDI-MS analysis showed that distribution of branched chains in leached (at 65 °C) amylopectin was similar in both waxy and non-waxy HB starches. An increase in heating temperature from 65 to 100 °C increased the molecular size and branch chain length of the leached amylopectin fraction from high-amylose HB starch. The chain length distribution of amylopectin was identical in both the soluble and insoluble fractions obtained by heating high-amylose HB starch in water to 100 °C.