Continuous radio-frequency heating for pasteurization and sterilization

The continuous pasteurization or sterilization of particle-containing foods in aseptic processes is carried out with conventional tube heat exchangers or scraped surface heat exchangers. Drawbacks of such methods are the slow temperature equilibration processes and the long treatment times that are necessary until the most thermally inert parts of the food have been subjected to the required temperature for the required time. This leads to overexposure, and is often accompanied by detrimental changes to the texture, taste, color and nutri-physiological quality of the food. By more rapid and more uniform heating of temperature-sensitive foods, considerable quality improvement could be realized.

 

To address this, the Fraunhofer Institute is studying continuous radio-frequency heating for pasteurization and sterilization of viscous, particle-containing foods under mild conditions. The heat generation in radio frequency (RF) heating arises from the excitation of electrical (ion) currents and the subsequent generation of heat in the food by the electric field.

 

The RF heater under test is a cylindrical heater tube made of insulating material. On the outer wall of the tube there are two opposing profiled electrodes connected to the RF voltage (27.12 MHz). The electrical RF field develops between the electrodes and penetrates the tube wall and food transverse to the direction of flow. The interaction of the electrical RF field with the electrically conducting food in the tube section between the electrodes leads to heating. The RF heater allows extremely rapid heating of products with heating rates of up to 5°C/s. In applications with a strictly laminar product flow, heating rates of 2°C/s are realistic. For tubular flow with plug-flow characteristics, ideal uniform heating of products is even possible at very high heating rates. For strictly laminar tubular flow of highly viscous foods there is a considerable temperature excess on the tube wall due to the very long residence times of parts of the product near the tube wall. By reducing the RF power, these excess temperatures can be limited. Mild mixing effects through bends in the tube and via suspended particles lead to further improvement in the temperature distribution. Suspended particles heat up evenly in the RF field and as fast as the surrounding liquid. The long temperature equilibration times that are necessary in conventional heaters to achieve satisfactory heating of the particles can hence be considerably shortened or can even be fully omitted. Run away heating during the RF heating did not occur.

 

The operating behavior of the prototype is very stable. The RF heater tubes are electrically and mechanically robust. The pressure and temperature requirements of a sterilizing process are fulfilled without problems. RF generators and power regulators are also based on proven industrial technology. To further aid process optimization and plant scale-up, the first fluid dynamics models and thermal models of the heater tube have been developed. These models take into account the temperature-dependent rheological and electrical properties of the foods.

The main environmental advantage of the technology is reduced energy consumption. The researchers have identified potential to speed up the processing time by a factor of ten thus reducing process energy .

Technology contact:

Thomas Pfeiffer
Tel: +49 8161 491-424
Email: thomas.pfeiffer@ivv.fraunhofer.de