AC impedance and microstructure of rye starch exposed to water vapour

Purpose: The purpose of this paper was to record and analyse mass (m) changes of population of micro-granules of rye starch, effective alternative current (AC), (electric) complex impedance (EACI) of this system taking place during humidification as well as insight into microstructure evolution by X-ray diffraction (XRD). Design/methodology/approach: Mass changes of biopolymeric micro-granular sample occurring during its exposition to saturated water vapour at room temperature, was recorded in time by means of precise torsional balance equipped with special chamber. The same was done in case of microstructure by XRD method. Monitoring of EACI was performed by means of interdigit comb capacitor and precise RLC meter equipped with PC software. Specially designed and constructed measuring chamber was applied to control temperature and relative humidity (RH %) in ambient sample atmosphere. Electric measurements were carried out for 5 selected frequencies. Findings: Interdigit dielectric spectroscopy method turned out to be more sensitive technique to follow details of long lasting humidification than only mass changes recording. Correlation of changes of EACI with simultaneously occurring mass increase can be a way to describe the humidification stages processes involving water molecules adsorption and absorption by micro-granular biopolymeric sample. Frequency dependence of EACI gives insight into mechanism of water molecules inclusion, binding and immobilization on the starch granules surface as well as inside of its internal physical structure. Research limitations/implications: The recording duration of m(t) was limited to &sim104 s in case of humidification by balance resolution and character of the process. The whole range of measurements was limited to max &sim23 RH % of water uptake in order to prevent over molecular structure irreversible changes. The above limitations enable us to collect data for modelling reversible water uptake and connected EACI micro-granular biopolymeric population sample. Practical implications: EACI monitoring of humidification turned out to be much more selective than only gravitational measurement of mass change. Modelling purpose correlation of both can give new possibilities to modelling approach. Achieved data can contribute to better understanding of active adsorption and absorption centres in starch granules. Originality/value: For the first time practically important humidification process was monitored in statu nascendi, without disturbing geometry of starch granules population by means of EACI evolution record. It was enabled by application of interdigit comb capacitor as a sensing unit. This is the first report of four stages for EACI evolution as well as the first XRD direct record of granular starch reversal swelling.