B-->A Allomorphic transition in native starch and amylose spherocrystals monitored by in situ synchrotron X-ray diffraction.

The B-->A phase transition in native starch granules and spherocrystals prepared from DP 20-40 synthetic amylose chains was investigated in situ at intermediate moisture content (20-30%) by wide-angle synchrotron X-ray scattering, using a temperature-controlled pressure cell. The transition in native starch was monitored at hydrostatic pressures of 1.6-11.0 MPa and occurred in a temperature range of 90-110 degrees C. The transition temperature increased with increasing amylose content and the transition was incomplete in amylose-rich starch. The B-->A transition in highly crystalline amylose spherocrystals was monitored at pressures between 2.0 and 28.5 MPa. The transition temperature was higher than in native starch, ranging from 125 to 135 degrees C. At 2.0 MPa, after conversion, the hydrated spherocrystals melted at 185 degrees C. Surprisingly, at the same pressure, in excess water, the spherocrystals did not solubilize but converted to allomorph A at 100 degrees C and melted at 160 degrees C. For all samples, the transition occurred in a matter of minutes and a higher pressure decreased the transition temperature. For the first time, thermal expansion coefficients were estimated for A- and B-amylose at intermediate moisture. A strong thermal anisotropy was observed for A-amylose, the expansion being higher along the b-axis than along the a-axis of the monoclinic unit cell. This anisotropy was attributed to the fact that, in the b-direction, amylose double helices lie at the same height along the chain axis while, in the a-direction, they are more closely packed in a zigzag fashion.