Effect of gelatinization on starch-water interactions

The extent of gelatinization of a starch is a function of both initial water content, and thermal treatment. The influence of extent of gelatinization on starch-water interactions during thermally-induced transitions has been investigated. The interactions between water and waxy corn, normal corn, potato and pea starches during gelatinization (G + M1 endotherms) were monitored by DSC and 1H NMR relaxation. Additional unfrozen water (AUW), reflecting enhanced starch-water interactions, largely depends on the initial water content. For the samples subjected to higher heating temperature, a larger AUW is found presumably due to the greater disruption in granule structure. The results from DSC deconvolution suggest that, as water content increases, the later (M1) endotherm is much reduced and gradually incorporates into the dominant earlier (G) endotherm, reflecting solvent-assisted melting of amylopectin crystallites. NMR T2 distribution reveals two distinct water populations corresponding to intra- and extra-granular water, which rapidly exchange during gelatinization.; The influence of gelatinization on the glass transitions of waxy corn, normal corn, potato and pea starch gels were investigated by DSC. While the glass transition temperatures of the partially gelatinized gels are independent of the initial water content, the glass transition temperatures of the fully gelatinized gels increase as the initial water content increases. These observations might reflect the difference in the level of structural disruption associated with different extent of gelatinization, resulting in different gel structures as well as different concentrations of the glassy unfrozen matrix. Annealing at a temperature near the onset temperature of the glass transition increases the glass transition temperatures of the gels, possibly due to an increase in the extent of freeze-concentration as evidenced by a decrease in AUW.; Starch-water interactions during gelatinization and amylose-lipid complex dissociation were studied in corn starches with different amylose content. Deconvoluted DSC reveals different gelatinization mechanisms for low and high amylose corn starches. While the solvent-assisted melting of homogeneous amylopectin crystallites is suggested for waxy and normal corn starches, the melting of a mixture of amylopectin crystallites with different thermal stability may be an appropriate gelatinization mechanism for high amylose corn starches. Formation and dissociation of amylose-lipid complexes are shown to be promoted by available water.