Dielectric Properties and Heating Properties of Aniseed Powder with Radio Frequency Heating

Abstract

Microbial contamination is currently a major safety issue in low moisture foods. Current sterilization and pasteurization practices, such as conventional thermal processing involving conductive heat transfer to inactivate pathogens, appear inadequate to address the contamination problem in low-moisture foods, which require long time heating, leading to quality deterioration with significant loss of colour and flavour. Radio Frequency (RF) heating has great potential in pasteurization of low moisture foods and offers the possibility to rapidly inactivate microorganisms while maintaining the food quality. Aniseed powder as one of typical low moisture foods possesses significant risk of microbial contamination. This study was to develop RF heating process to inactivate microorganisms in aniseed powder. The dielectric properties (i.e. dielectric constant (ε') and dielectric loss factor (ε'')) of aniseed powder, the influences of processing conditions on heating properties and the inactivation of microorganisms in aniseed powder were all studied in this research. Results showed that ε' and ε'' of aniseed powder decreased with an increase in frequency from 6.78 MHz to 47.46 MHz but then fluctuated between 300 and 2745 MHz. The overall change of ε' and ε'' ultimately exhibited a downward trend. Under a constant frequency treatment, ε' and ε'' of aniseed powder increased with an increase in moisture content, whereas the penetration depth decreased. Particle size showed no significant effects on the dielectric properties, heating rate and penetration depth. With an increase in RF processing time, the heating rate of the sample increased and adding the rolling-over operation step helped to improve the uniformity of RF heating. After processing for 100 s, the temperature of aniseed powder could increase to 60 °C, and the number of microorganism in aniseed powder was reduced by 6.18 logs when time extended to 150 s.