The effect of microwave radiation on some thermal,rheological and structural properties of cassava starch

Three samples of native or untreated cassava starch were exposed to microwave radiation for periods of 5, 10 or 15 min. The temperature of each sample was measured immediately after each exposure time and the temperature of the samples was around 135 °C. The samples were cooled to room temperature and maintained in a desiccator with anhydrous calcium chloride. All the samples were analysed by thermogravimetry-derivative thermogravimetry, differential scanning calorimetry (DSC), rapid viscoamylographic analysis (RVA), X-ray diffraction powder patterns, non-contact atomic force microscopy and colour characteristics by reflectance spectrophotometry. The thermal behaviour, gelatinisation temperatures, enthalpy and pasting properties were determined. Relative to the time of microwave exposure, the peak viscosity and gelatinisation (RVA and DSC) increased slightly after 5 min, and, after 10 and 15 min, it decreased considerably. The degree of relative crystallinity (%) decreased, while the average roughness increased. The reflectance spectrophotometry showed that microwave action occured quickly and progressively, causing colour changes (mainly with trends to yellow) and very small differences to the starch samples that were heated at controlled temperature in a conventional oven.