Elucidation of starch granule microstructure and reactivity

Kerry C Huber, Purdue University

Abstract

It is hypothesized that granule architecture influences patterns of reaction during chemical modification. Knowledge of granule microstructure provided in this study has advanced understanding of starch granule reactivity. A method for observing granular reaction patterns of modified starch is described that could facilitate characterization of starch reaction systems. Most starch granules of common corn, waxy maize, sorghum, and millet were shown to possess radial, tube-like channels, which penetrated inward toward a cavity at the hilum. Channels varied in their frequency and distribution among granules, as well as in their depth of penetration. Central cavities, which were observed in almost all granules, exhibited a variety of shapes and sizes. Channels were not observed in starch granules isolated from corn kernels that had never been dried; rather they appeared to be formed during granule dehydration. The same is true of cavities, although the data is less conclusive. Since channels and cavities increase available surface area for reaction and reagent entry into granules, their behavior was further investigated. Under slight swelling conditions (water), cavities were observed to swell somewhat closed, while channels appeared to remain open. Results from a model system (involving infiltration of granules with aqueous dye solution) supported the hypothesis that channels and cavities impact starch granule reactions. To observe actual granular reaction patterns within modified starch granules, starch derivatives were converted to thallium(I) salts and viewed by SEM compositional backscattered electron imaging. Observation of a phosphorylated starch derivative and a hydroxypropyl starch analog revealed that granular patterns of reaction were influenced by both starch and reagent types. For waxy corn and sorghum starches, flow of reagent into the granule matrix seemed to occur through channels (laterally) and cavities (from the inside out). For potato starch granules, which do not possess channels, reagent diffused inward through exterior granule surfaces. With regard to reagent, POCl3 (highly reactive) was more inclined to react at granule surfaces, while the propylene oxide analog (less reactive) appeared to diffuse further into granules prior to reacting. Together, the data support the hypothesis that granule architecture has an influence on the locus of reaction within a starch granule.

Degree

Ph.D.

Advisors

BeMiller, Purdue University.

Subject Area

Food science|Agricultural chemicals

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