FREQUENCY AND TEMPERATURE-DEPENDENT DIELECTRIC PROPERTIES OF CASHEW NUT MILK FOR MICROWAVE PROCESSING

Plant-based alternatives to dairy are a rising trend, especially among consumers avoiding cow milk due to dietary restrictions. Much attention has been given to soy, however cashew nut, native to Brazil, is gaining popularity. Commonly known as cashew nut “milk” (CNM), these beverages are aqueous extracts from disintegration of cashew nuts in water. Thermal treatment is used to improve their physical and microbiological stability. Instead of conventional methods, microwave technology can be applied aiming a faster volumetric heating and higher sensory quality. Dielectric properties (DP) - dielectric constant (ɛ') and dielectric loss factor (ɛ'') - are key parameters for microwave heating, since they characterize the interaction between electromagnetic energy and the food material. The objective of this work was to study the DP and electrical conductivity of CNM as a function of temperature and field frequency. The CNM was prepared by soaking, grinding the cashew nuts in water at 40 °C, and then filtering and homogenizing the obtained extract. Using a network analyzer and the open-ended coaxial probe technique, the DP were determined over a frequency range of 200–3000 MHz at temperatures from 20 (room temperature) to 120°C (sterilization temperature). Values of ɛ' decreased with increasing of both frequency and temperature, which can be due to disturbances of the alignment of water dipole molecules with the alternating electric field. Regarding ɛ'', the observed behavior was related to the two mechanisms (ionic conduction and dipolar rotation) involved in microwave heating. Particularly at commercial frequencies of 915 and 2450 MHz, power penetration depths were calculated and polynomial correlations were adjusted to model the temperature dependence of the DP (R2≥0.98) and power penetration depth (R2≥0.98). Linear correlation with temperature was found for electrical conductivity (R2>0.99). The obtained results are useful for further studies on the design of CNM processes based on microwave energy.