Composition and properties of A- and B-type starch granules of wild-type, partial waxy, and waxy soft wheat

Starch A- and B-type granules were isolated from soft wheat (Triticum aestivum L.) genotypes representing the four granule bound starch synthase I (GBSSI) classes, and characterized according to composition and properties. While total (TAM) and apparent (AAM) amylose contents of both granule fractions decreased as starch waxy character increased, the A-type granules possessed higher TAM and AAM contents than B-type granules for a given genotype. From wild-type to waxy, a general transition was observed from B- to A-type starch granule fractions with higher levels of lipid-complexed amylose (LAM) and phospholipid. Within a genotype, A-type (relative to B-type) granules possessed higher gelatinization enthalpies, while B-type granules exhibited higher gelatinization peak and completion temperatures (broader gelatinization ranges) than A-type granules. Normal (wild-type) and waxy A- and B-type starch granule pasting rates were affected by starch granule lipids; the granule type within a genotype with the lowest LAM and phospholipid levels generally exhibited the shortest time to pasting. For normal and waxy starches, A-type granules exhibited higher pasting viscosities than B-type granules throughout the pasting profile. Thus, the A:B-type granule ratio is important to understanding overall wheat starch composition and behavior.