Impedance characterization of wheat starch at various water contents

Abstract

Wheat starch is used extensively in the food, entertainment, and papermaking industries. It is classified as a non-hazard material and an electrical insulator, but recent events manipulating dispersed grains found that electrical properties can vary in different environmental conditions. In order to make advancements in the safety of processes involving these materials, it is necessary to understand their electrical properties. Here, we study wheat starch grains in terms of the aggregation, dielectric resonant frequency, and moisture level. Samples are characterized by Scanning Electron Microscopy (SEM) and Impedance Spectroscopy (IS) using an interdigital planar electrode array. Results showed that the capacitance measurements increase with an increase in the water content (>27.4 Wt%) and therefore it enhances the ability to store an electric field in the polarization of the starch sample. Nyquist plots showed that the internal resistance of the medium decreases for larger grains at low and intermediate stimulus frequencies and at high moisture contents (21–27 Wt%). At high frequencies, the materials behave indistinguishably. We were also able to demonstrate the predictability of water content based on one samples IS measurements. We propose that by extracting the resonant frequencies from the measurements and identifying the resistance of the sample at those frequencies, we can use a regression to predict the water content of the mixture within 1.5 Wt%. Our results show that in the case of 15 − 50μm starch grains, there is a sharp increase in capacitance resulting from the addition of water (10–30 Wt%) which may increase risks of auto-ignition by approaching the Minimum Ignition Energy (MIE). This research offers useful information on how to use IS techniques for understanding both the starch properties and their moisture condition at the same time. This information will allow the industry to asses not only the quality but also the storage safety and handling conditions.