ABSTRACT
Polarization second harmonic generation (SHG) is described as a label-free, non-invasive and rapid method for the structural analysis of individual starch granules. SHG is a nonlinear optical process that is inherently generated in non-centrosymmetric materials, such as starch granules. SHG microscopy permits intrinsic optical sectioning making it ideally suited for studying structure inside individual granules without physical sectioning. Both the intensity and polarization response of the SHG signal are governed by the structure of the material, and characterized by the nonlinear optical susceptibility tensor. During polarization SHG microscopy, optical measurements of the tensor component values are rapidly performed for each position of the laser focal spot within starch granules by measuring the relation between the laser polarization and the polarization of the SHG signal. Mapping of these tensor components reveals structural features in the sample including crystalline concentration, average orientations of crystals in each pixel as well as a parameter describing quantitatively the structural order. Results from different polarization resolved SHG starch studies are presented including a comparison of starches from potato and maize varieties, followed by the effect of hydration on starch structure. Results from crystalline maltodextrin particles are presented and compared with the results from native starch granules.
