Nature of thermal transitions of native and acid-hydrolysed pea starch: Does gelatinization really happen?

Thermal transitions of native and acid-hydrolysed starch were investigated for the first time by employing differential scanning calorimetry (DSC) in combination with field emission scanning electron microscopy (FE-SEM) to examine morphological changes in the starch after hydrothermal treatment in the DSC pans. The characteristic, well-defined endothermic transition was observed in the DSC traces for native starch and starch after one day of acid hydrolysis, but after two days of hydrolysis the endothermic transition became broad and undefined. After heating in DSC pans, native starch was observed visually and with SEM to have formed a gel, whereas starch granules hydrolysed for one day appeared to have undergone only limited swelling and coalescence. Starch that had been hydrolysed for two or more days appeared powdery and SEM images revealed that the granules had undergone very little swelling. The thermal transitions of native and acid-hydrolysed starch are interpreted in terms of the theory of polymer swelling and dissolution. Native granules and granules with only minimal acid damage are able to swell within the constraints of the water-limited conditions of DSC, but as acid hydrolysis progresses the capacity of the granules to swell is decreased and endothermic transitions are increasingly due to dissolution of polymer chains. An exothermic transition that followed the main endothermic transition is proposed to be due to the condensation of water vapour on the lid of the DSC pan.