A new insight into the phase transition processes of food starches
Starch displays a unique thermal behavior, when heated in water, known as gelatinization. None of the theories documented in literature adequately explain the exact mechanisms of structural changes that occur in starch granules during gelatinization process. In this study, seven native starches, three modified starches, three resistant starches, and starch isolated from alkaline-cooked corn masa were examined to determine their phase transition behavior using light microscopy, scanning electron microscopy, differential scanning calorimetry, high-performance size exclusion chromatography, and X-ray diffraction. Starch polymers rearrange or reorganize at temperatures below the onset of the differential scanning calorimetric endotherm. This polymer rearrangement and subsequent granular restructuring that occur before complete granular disintegration is different from annealing. Analysis of modified starches revealed that cross-linking restricted or delayed these internal structural changes, and hydroxypropylation prevented the granular crystalline structure to a certain extent during phase transition. The resistant starches were more resistant to structural changes than other starches within the temperature range studied. The results concluded that starch gelatinization process is different from previously hypothesized direct order-to-disorder phase transition. It was found that starch granules heated in excess water undergo structural reorganization in the amorphous region before complete phase transition occurs. The nature and extent of these structural changes depend on starch type and the intensity of the hydrothermal treatment.
The phase transition associated granular structural changes also occur in starch containing foods. Alkaline cooked corn masa was used as a model system to study starch phase transition in food. The proteolytic enzyme thermolysin (EC 220.127.116.11) was used successfully to isolate starch from alkaline cooked corn masa. The isolated starch was analyzed to determine the nature and degree of starch phase transition that occurred during the masa production process. It was found that starch granules could morphologically remain intact while losing their structural order when they underwent phase transition during alkaline cooking while "trapped" in the protein matrices of masa.