Wheat starch A- and B-type granule microstructure and reactivity.

摘要

Wheat starch consists of at least two distinct granule populations: the larger A-type and the smaller B-type granules, which are differentiated by both size and shape. Wheat starch A- and B-type granules possess differential compositions, molecular/granular structures, and physical (swelling, gelatinization, and pasting) properties. Despite efforts to characterize both granule types, A- and B-type granule microstructural characteristics (e.g., surface pores, channels) and reactivity are not yet fully understood. The primary goal of this study was to investigate the role of both starch characteristics and granule microstructure in A- and B-type granule reactions with commercial chemical modifying reagents. For ease of discussion, this study is subdivided into four separate parts: (1) characterization of the physicochemical properties and amylopectin fine structure of A- and B-type starch granules of waxy and normal wheat genotypes, (2) elucidation of the microstructural features within wheat starch A- and B-type granules, (3) investigation of A- and B-type granule relative reactivities and modified starch properties in commercial substitution and cross-linking reactions, and (4) investigation of granular reaction patterns of chemically modified wheat starch A- and B-type granules.;The nature of pores and channels within wheat starch A- and B-type granules was investigated using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). For A-type starch granules, relatively large channels were observed in the equatorial groove region, while finer channels originated from other regions of the granule. This work reports the first visualization of B-type granule channels, which most frequently occurred as less-defined voids (as opposed to the fine, discrete channels of A-type granules) extending to granule surfaces.;Relative A- and B-type granule reactivities were investigated as function of reagent level (low, intermediate, high) and reagent type (propylene oxide analog, POA; phosphorus oxychloride, POCl3; sodium trimetaphophate, STMP). For POA derivatization, no differences in the relative reactivity of A-type and B-type granules were observed. For POCl3 reactions, B-type (relative to A-type) granules of waxy wheat starch exhibited higher reactivity, while no reactivity preference in regard to granule type was observed within normal wheat starch.;Granular reaction patterns for POA, POCl3, and STMP A- and B-type starch granule derivatives were visualized using reflectance confocal laser scanning microscopy (R-CLSM), and anaglyphic 3D images were constructed representing all starch fraction derivatives. Heavy reaction at the external surfaces of granules was observed for all derivatives, and the degree of reaction at the granule surface could not be differentiated among the reagent types. For internal reaction patterns of A-type granules derivatized with POA, reagent effectively accessed the hilum region of granules through channels, and entered the granule matrix from inner cavity surfaces and laterally through channel surfaces, producing the most homogeneous reaction patterns of the reagents investigated. (Abstract shortened by UMI.)