Loss of birefringence and swelling behavior in native starch granules: Microstructural and thermal properties

Starch granules imbibe water and swell when exposed to abundant water and temperatures above their gelatinization point. The degree of gelatinization of four native starches, wheat, potato, cassava and corn, was determined by the enthalpic transitions and simultaneous events between loss of birefringence and swelling to quantify the process in situ and in real time. In all cases, the following three stages were identified: low granular swelling, with little water absorption and 100% birefringence; gradual leading to complete loss of birefringence with the absorption of a large amount of water (approximately 50%); and complete granular swelling to equilibrium. A clear gap between the beginning and end of the loss of birefringence and swelling was observed. When the birefringence reached zero value, 50% swelling was reached at 55.7, 62.0, 68.7 and 70.6 °C for wheat, potato, cassava and corn, respectively. A good correlation between the degree of gelatinization measured by differential scanning calorimeter and microscopy was found for potato, corn and cassava with r² values of 0.98, 0.99 and 0.98, respectively, and an r² of 0.70 for wheat. Therefore, the loss of birefringence and swelling does not characterize gelatinization in equivalent ways.