Crystallite orientation maps in starch granules from polarized Raman spectroscopy (PRS) data.

In this work, polarized Raman spectroscopy (PRS) was used to determine orientation maps of crystallites present in Phajus grandifolius starch granules based on the anisotropic response of the glycosidic Raman band at 865cm(-1). The response of this band was preliminarily evaluated using model A-amylose crystals as standard. The A-amylose crystals oriented "in plane" showed a maximal intensity ratio of ∼3.0 for bands 865/1343cm(-1) when the polarization of the laser was along the chain axis of the crystal, i.e., parallel to the axis of the amylose double helices, and a minimal intensity ratio of ∼0.25 when perpendicular. The orientation maps of Phajus grandifolius starch granules showed two distinct regions: one isotropic and the other with a highly anisotropic response. The origin of the difference might be changes in both organization/concentration and orientation of the crystallites across the starch granules.

A-type crystals from dilute solutions of short amylose chains.

Model A-type amylose single crystals were prepared by recrystallizing dextrins from acid-hydrolyzed native starch and narrow fractions of short chains of enzymatically synthesized amylose, in dilute water/acetone solutions. In most cases, spindle-shaped crystals with a sharp, round or flat apical end were formed, organized in rosettes or fan-like assemblies. The morphology and crystal size were shown to strongly depend on the average degree of polymerization (DP), distribution width (DW), and degree of branching of the chains. The largest and most clearly faceted single crystals were prepared using fractions of synthetic amylose. Typically, 5-10 µm long crystals were obtained from fractions with 17 ≤ DP ≤ 20 and DW ≤ 8. Chains with DP > 40 and a high polydispersity formed ill-defined networks of smaller crystallites. Fractions of branched and more polydisperse limit dextrins yielded crystals smaller than those obtained from narrow fractions of synthetic amylose. The morphological analysis of faceted single crystals combined with electron diffraction data confirmed that the double helices were oriented along the long dimension of the crystal and packed into lamellae with a parallelogram cross section defined by the a and b directions of the monoclinic unit cell of A-amylose. The lamellae are stacked along the c-axis that is oriented parallel but opposite to the growth direction of the crystal.

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.