Dielectric properties of soybean protein isolate dispersions as a function of concentration, temperature and pH

Dielectric properties of commercial soy protein isolate (SPI) dispersions were measured over the frequency range of 200-2500 MHz by the open-ended coaxial probe method using a network analyzer as a function of concentration (5, 10 and 15 g/100 g water), temperature (20-90 °C) and pH (4.5, 6.6 and 10). Results indicated that the dielectric constant (ε′) decreased with temperature (except at 90 °C) and frequency while increased with concentration. The loss factor (ε″) increased with frequency and concentration; however, temperature showed mixed effect. Both ε′ and ε″ data were related to frequency using a polynomial model. The significant change in ε′ and ε″ at 90 °C was a result of protein denaturation which was identified by differential scanning calorimetry (DSC). Change in the association/dissociation behavior of SPI dispersions due to electrostatic attraction/repulsion among protein molecules on heating as a function of pH was assumed to be responsible for the significant increase in dielectric parameters. Penetration depth (Dp) was estimated under various conditions and it decreased with an increase in frequency, concentration, temperature and pH. The minimum Dp was found at alkaline pH at a temperature of 90 °C and frequency of 2450 MHz.