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 temperat ure (< 160 degreesC) conditions at low moisture content (30%) using a rheom eter with pre-shearing, to simulate extrusion. Specific mechanical energy w as measured and remained lower than 10(3)J/g. Starch modification was analy sed at different structural levels by differential scanning calorimetry (DS C), X-ray diffractometry, light microscopy and at the granular level by tra nsmission electron microscopy (TEM). Thermal inputs induced crystallite mel ting and internal disorganisation of granular structure, whereas mechanical input (SME > 10(2)J/g) induced granule fragmentation. Under shearing and h eating, the concomitant destructuring mechanisms led to a higher rate of mo dification. Fragmentation itself induced only partial loss of crystallinity (< 50%). Small fragments finally melted due to a local temperature increas e 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.