Hydrothermal modification is one of the physical methods commonly used to modify the functional properties of starch granules while maintaining granular structure. Three parameters are varied in hydrothermal treatment of starch granules: temperature, moisture, and time. Hydrothermal treatments can be divided into two general areas: annealing and heat-moisture treatment. Annealing is usually performed in conditions of excess (>66%) or intermediate water content (40-55% w/w), while heat-moisture treat ment is defined for low-moisture conditions (<35% w/w). The temperature range used is generally between the glass transition temperature (Tg), the transition point between the glassy and rubbery state of starch, and the gelatinization temperature at which irreversible loss in crystallinity occurs. The time can be varied from hours to a week. Detailed conditions for a variety of starches can be found in the review by Jacobs and Delcour (1998), or determined based on Tg and gelatinization temperature of a specific starch sample.
Hydrothermal treatment results in changes in the crystalline and amorphous regions of starch granules and the interaction between these two regions. It results in an increase in perfection of crystallites, alteration of crystalline packing from B- or C- to A-type, and increased interaction of molecules within amorphous and crystalline regions. As the raw starch granule structure is related to starch digestibility, hydrothermal treatment can be used as a way of changing the slow digestion property of native starch granules resulting in lower GI. Shin et al. (2005) reported that hydrothermal treatment of sweet potato converted its C-type structure to A-type, and the SDS of the treated granule increased 200% compared with native starch granules. Owing to the broad range of physicochemical changes caused by hydrothermal treatment, more studies are needed to systematically investigate the effect on the slow digestion property and the related low glycemic response of hydrothermally treated starches.
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