Microscopical study of the destructuring of waxy maize and smooth pea starches by shear and heat at low hydration

Smooth pea and waxy maize starches were processed under controlled temperature (< 160 degreesC) conditions at low moisture content (30%) using a rheometer with pre-shearing, to simulate extrusion. Specific mechanical energy was measured and remained lower than 10(3)J/g. Starch modification was analysed at different structural levels by differential scanning calorimetry (DSC), X-ray diffractometry, light microscopy and at the granular level by transmission electron microscopy (TEM). Thermal inputs induced crystallite melting and internal disorganisation of granular structure, whereas mechanical input (SME > 10(2)J/g) induced granule fragmentation. Under shearing and heating, the concomitant destructuring mechanisms led to a higher rate of modification. Fragmentation itself induced only partial loss of crystallinity (< 50%). Small fragments finally melted due to a local temperature increase contributed by interparticular friction. Susceptibility to fragmentation was related to the mechanical properties of starch granules associated with their architecture and botanical origin. (C) 2001 Academic Press.