Abstract
This review deals with two kinds of physical modifications of starch which have been socalled “warm water treatment” and “heat-moisture treatment.”A temperature-composition diagram illustrating the gelatinization and the melt of starch granules has been constructed from the DSC data that show two kinds of endothermic transitions for the starch-water system. The diagram obtained corresponds to that of the crystalline polymerpoor solvent (diluent) system. Thermal phase transitions of the starch-water system have been discussed using the diagram.The effect of warm water treatment on starch granules has been studied: This treatment causes the sharpening and the rise of the gelatinization temperature range, and the reduction of the swelling power and the solubility. The structural changes in granules during the treatment have been interpreted in terms of the annealing of semicrystallites of starch molecules. In fact, photomicrographs presented in the paper well shows the annealing process of starch granules with increasing temperature. It was concluded that this treatment brings about increases in homogeneity of both individuals and populations of starch granules through the process of annealing of starch crystallites.Heat-moisture treatment brings about changes in the physical properties of starch through the process of the “melting” of starch granules. In the case of potato starch, the “B” pattern of X-ray diffraction is changed to “A” or “C” pattern, involving increases in heterogeneity of granular structure. The gelatinization properties of potato starches treated at various moisture levels and temperatures were examined and the relationships between the treatment conditions and the gelatinization properties have been clarified. From the results, it was concluded that heat-moisture treatment brings about irreversible disordering of granular structures with increasing of associative forces among starch molecules in granules.Degradation of the physically modified starches with amylases and mineral acids were also studied. Susceptibility of heat-moisture treated potato starches to amylases was much larger than that of untreated one, but not in the case of corn starch. Microscopic observations by SEM showed that the degradation of the treated potato starch with glucoamylase from R. niveus and α-amylase (liquefying type) from B. subtilis (BLA) proceeded in the different mode each other. Differences in the action mode of BLA on the digestion of corn and the treated potato starch were also shown. In contrast to the heat-moisture treated potato starch, the warm water treated potato starch showed less susceptibility to amylases than the untreated one did. Some scanning electron micrographs of acid-modified potato starch granules were also presented.In conclusion, the possibilities of the utilization of the physically modified starches were described.
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