Effect of granule size on the structural and physicochemical characteristics of wheat starch

Granule size distribution directly influences the chemical composition of starch, affecting its functionality, characteristics of gelatinization, enzymatic susceptibility and crystallinity. Wheat starch granules exhibit a bimodal size distribution and are divided into type A (10-40 mu m) and type B (<= 10 mu m) granules. In this work, wheat starches obtained from four different cultivars were isolated and fractionated by size. This fractionation allowed the granules to be divided into large (> 15 mu m) and small granules (<= 10 mu m). Starches of different sizes were analyzed to determine their lysophospholipid and amylose content, X-ray patterns, enzymatic susceptibility, swelling power, and thermal and pasting properties. They were also examined under a scanning electron microscope. Independent of the cultivar studied, large granules showed an average diameter of 22 mu m and were lentil-shaped, while small granules showed an average diameter of 6 mu m and were spherical. The large granules exhibited lower lysophospholipids content, higher amylose content and a lower index of crystallinity (IC) than the small granules. Despite their higher crystallinity small granules were more susceptible to hydrolysis than large ones, suggesting that the enzymatic susceptibility of small granules is related to its larger surface area.