Corn starch structure, behavior, and reactivity
Abstract
Some but not all granules of corn (all cultivars and all developmental stages examined), sorghum, and millet starch had small openings (pores) randomly distributed over their surface, often in clusters and to different degrees. The pores were a real and normal feature of the native granule structure and not artifacts produced by isolation, specimen preparation, or observation techniques. Pores were also found along the equatorial groove of large granule of wheat, rye, and barley starch, but not on other (rice, oat, potato, tapioca, arrowroot, canna) starches. It was proposed that the pores affect the pattern of attack by amylases and at least some chemical reagents. Starch pastes and gels were examined by low-temperature scanning electron microscopy (LT-SEM) after rapid and slow cryopreparation and by traditional SEM after solvent exchange/critical point drying, with or without prior fixation. The rate of freezing and the state of the polymer network (dispersed or retrograded) had strong effects on the results. Rapid cryopreparation and LT-SEM gave results that agreed closely with the presently held theory of starch gel structures, i.e., they were composed of a continuous phase of bundles of molecules surrounding a discontinuous phase of swollen granule remnants commonly referred to as ghosts. Significant differences in the amount and size of the continuous phase and the strength and appearance of the ghosts of pastes and gels prepared from native and crosslinked common corn starch, native and crosslinked and stabilized waxy maize starch were observed using the preferred technique. It was concluded that cryopreparation combined with LT-SEM is a useful method for examination of gel structures with a minimum of induced artifacts. Isolated ghosts were also examined to determine any relationship between ghost structure and functional properties. Ghosts showed structural differences that correlated with the rheological behavior of each starch type. It is concluded that LT-SEM should be useful as a method for screening starch hybrids with unknown properties because the structures observed correlated with the known functional properties which are the basis of the applications of the starches.
Degree
Ph.D.
Advisors
BeMiller, Purdue University.
Subject Area
Food science
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