Water activity and dielectric properties of gels in the frequency range 200 MHz–6 GHz

Dielectric spectroscopy gives information on a molecule's chemical relations with its surroundings while water activity ( a w) is the thermodynamic measure linked with water bonding to the food matrix. Thus, it should be possible to access information on a w via dielectric measurements, which could be very interesting, in practice, regarding easiness of dielectric measurements. Forty-five model samples of animal gelatin gel of various water and NaCl contents ( a w from 0.91 to 1) were characterised in terms of a w and dielectric properties. To analyse the variations of water activity with water and salt contents, over a larger moisture content range, isothermal sorption curves were determined. This investigation provides correlations between a w and some dielectric properties, such as relaxation frequency. Results of this first investigation are encouraging but lead to some questions about influence of NaCl on dielectric measurements, the possibility of associating dielectric measurements with sorption isotherms and problems concerning measurements on real food products.