Molecular and structural changes in starch and proteins induced by microwave treatment and their effect on pasting properties. A study on amaranth, buckwheat, quinoa, and sorghum in grain and flour form

Abstract

This study investigated the effects of microwave treatment (MWT) on the molecular structure and properties of starch and proteins, as well as on the pasting properties of flours. MWT was performed under uniform conditions (25 % moisture, 100 ◦C, 30 min) for amaranth, buckwheat, quinoa, and sorghum in two forms: grain and flour. MWT generally caused fragmentation of amylose and amylopectin, reducing amylopectin molar mass by up to 39 % and amylose long chains by up to 18 %, disrupting protein secondary structure (α-helix content up to 5 %), and altering morphology of starch granules and protein. All flours presented an orthorhombic crystal structure that was maintained after MWT. MWT changes were generally more pronounced in flour than in grain treatments, with amaranth showing the greatest change and buckwheat the least. Microwave-induced changes in pasting properties depended on intrinsic flour properties and structural modifications. Principal component analysis revealed a consistent change in all treated samples, but to varying degrees depending on botanical origin and treatment form. This study contributes to a deeper understanding of structural changes of starch and proteins by MWT and their contribution to altered pasting properties depending on whether the treatment is applied to grain or flour.