Molecular basis of changes in pasting and rheological properties of starches physically modified by microwave radiation

This study investigates the effects of controlled microwave treatment (100 °C, 25% moisture, 30 min) on the molecular and rheological properties of pure starches (normal and waxy maize, normal and waxy rice, wheat, potato and tapioca). Structural changes were assessed through high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC/PAD) to resolve amylopectin chain-length distributions, and size-exclusion chromatography with multi-angle light scattering, differential refractive index, and viscometric multi-detection (SEC/MALS-dRI-Visco) to determine amylose and amylopectin molecular parameters for both debranched and whole molecules. The impact of these changes on the pasting properties of starches and the viscoelasticity of their gels was established. Microwave treatment induced source-specific responses. Waxy starches showed increases in very short amylopectin chains and molecular degradation, with minimal rheological impact due to their low viscometric profile. Potato starch experienced the most pronounced rheological modifications despite minimal molecular degradation, suggesting supramolecular reorganisation, supported by an increase in amylopectin intrinsic viscosity. Wheat and tapioca starches exhibited moderate structural changes and enhanced gel stability, attributed to an increase of fractions comparable in size to long amylopectin chains. Rice starches displayed similar pasting and rheological responses, having amylopectin fine structure a predominant role, while the absence of amylose in waxy rice favoured greater amylopectin hydrolysis. Correlation analyses linked amylose short/medium chains to parameters related to stronger and firmer gels. In contrast, short amylopectin chains correlated with weaker gels. These findings offer mechanistic insight into the potential of controlled microwave processing to tailor starch functionality in food systems.