Tubular Radio-Frequency Heater

Radio frequency heating (RF-heating) with electric fields of 27.12 MHz is a novel technology to heat viscous and particle containing fluids uniformly and rapidly. The technology offers the potential to achieve significant better quality with pasteurised or sterilised foods.

The research project is aimed at a better scientific and technical understanding of the radio-frequency heating process and in particular at a better prediction of radio-frequency heating performance of certain food materials from their electric and rheologic properties. This will enable food processors to judge the feasibility of radio-frequency heating for their food products and to estimate possible quality achievements without the necessity of performing expensive and time consuming application tests.

The scientific goal of the project is a description of the complex fluid-mechanical and thermal processes inside the radio-frequency heater tube and the derivation of models that can be used to relate the radio-frequency heating performance of different important food systems to their physical properties like temperature dependent viscosity and temperature dependent electrical conductivity. These models will be used as a tool to predict the suitability of diverse food systems for radio-frequency heating, to determine limiting parameters for stable operation of the radio-frequency heating tube and to quantify its main dimensions like tube diameter and active tube length.

Heating experiments will be completed by microbiological measurements and by product quality judgements of selected food products in order to verify quality achievements of RF- heating in comparison to conventional heating processes.

Achievements

Several industrial and model food products like fruit preparations, baby food and sugar/starch solutions have been characterised with respect to rheological and electric properties. All tested products show high viscosity, non-Newtonian flow behaviour and would lead to laminar flow in the RF-heater tube under industrial operating conditions. Therefore, fluid dynamic investigation of the tubular heater has been confined to the laminar flow regime. The RF-heating of particles suspended in liquid has been investigated both experimentally and with model calculations. Of special interest are differences in heating characteristics of particle and liquid due to differences in their electric conductivity.

To model the RF-heating process numerically, a steady state model of the laminar flow of non-Newtonian liquids of known rheological properties has been constructed and compared, with excellent accordance, to velocity profile measurements in the heater tube. The flow model is used as a base for a thermal model that includes energy take up from the electric field and temperature equilibration by heat conduction. The thermal model is currently under experimental validation and will be used as a tool for process optimisation and scale-up.