Phase transition of starch granules observed by microscope under shearless and shear conditions

The phase transitions of starch granules of cornstarches with different amylose/amylopectin contents (waxy: 0/100; maize: 23/77; Gelose 50: 50/50; Gelose 80: 80/20) were systematically studied under shearless and shear conditions. A microscope with a hot-stage was used to observe phase transition under shearless conditions during heating and isothermally at different temperatures. Phase transition under shear conditions was investigated using a rheoscope – a controlled rate/controlled stress rheometer with a built-in high-resolution video camera. An increase in starch granule diameter, disappearance of birefringence and granule disappearing were used to describe the phase transition. The diameter growth rate and final accretion ratio sequence of starch granules during heating under shearless conditions was waxy > maize > G50 > G80, which correspond with amylose/amylopectin ratio. The growth rate of granular diameter under shear conditions was controlled by two factors: swelling and dissolving. Starch granules were destroyed under shear conditions, but some could still be identified under shearless conditions up to about 100 °C. Granules totally disappeared in a very short time after gelatinization under shear stress conditions. Increased amylose content led to a lower granule diameter growth rate and accretion ratio, and a higher temperature before birefringence disappeared. It was found that the higher the melting temperature of the amylose–lipid complex, the higher the gelatinization temperature, and the amylose detected by differential scanning calorimetry in the high-amylose starches is proposed as an explanation of the results observed by microscope.