Dielectric properties of thermosonically treated Agaricus bisporus slices during microwave vacuum drying and correlation with the water state

The dielectric properties of thermosonically treated Agaricus bisporus slices during microwave vacuum drying were determined, by using a network analyzer and an open-ended coaxial-line probe at microwave frequency (915 and 2450 MHz). The permittivity at 25 °C was also correlated to the water state determined by low field nuclear magnetic resonance (LF-NMR) analysis. Results showed that the dielectric constant ε' and loss factor ε″ of A. bisporus slices were affected significantly by the composition of water state in the sample. Thermosonic pretreatment enhanced the influence of semi-bound water (M22) and bound water (M21) on the permittivities in sample slices during microwave vacuum drying (MVD). The dependency of permittivities on the moisture content and temperature at 915 and 2450 MHz for thermosonically treated A. bisporus samples could be described by third-degree polynomial models, with R2 higher than 0.998. Infrared thermal imaging analysis confirmed that thermosonic pretreatment is a potential technology to promote the uniformity of A. bisporus slices during the MVD process.