Evaluation of methods for determining food surface temperature in the presence of low-pressure cool plasma

Abstract

Cool plasma technology is being developed for decontamination and modification of food surfaces. Microwave cool plasmas generated at low pressures, between 0.1 and 2 mbar, are generally believed to induce only weak heating of food surfaces, with effective temperatures below 60 °C, and, therefore, to be suitable for the treatment of thermo-labile materials. Measurement techniques suitable for determining the temperature of product surfaces undergoing plasma treatment are essential to confirm that the temperature is acceptably low, and for effective process control. The harsh environment in the plasma treatment chamber, including the low pressure and the presence of electromagnetic fields, electrons and other reactive species, such as ions, excited molecules and radicals, poses a challenge to establishing reliable and reproducible techniques for surface temperature measurement. This paper compares the performance of fiber-optic probes, thermocouples (with and without sheaths), resistance thermometers, Thermochrons (temperature loggers) and temperature indicators (color changing strips and tapes) under low-pressure microwave cool plasma conditions. Both the reliability of the temperature measurement and the response time are considered. It is found that the fiber-optic probe is the only method that both has an acceptably fast response time and is not susceptible to interference from the plasma environment. Cool plasma technology is being investigated and developed for decontamination of food surfaces. The technology is being considered as very promising alternative to chemical and/or heat processing, particularly for the decontamination of fresh produce and dry food products, such as seeds and nuts, but also powders. The low temperatures on food surfaces, with effective temperatures below 60 °C, make the process suitable for the treatment of thermo-labile materials. Measuring the temperature of product surfaces undergoing plasma treatment is essential to both confirm that the temperature is acceptably low, and for effective process control. The harsh environment in cool plasma, including the presence of electromagnetic fields, electrons and other reactive species, however, poses a challenge to establishing reliable and reproducible techniques for surface temperature measurement. For industrial uptake of the technology not only the process validation from a food safety perspective is important, but also the capability of fast and accurate temperature sensing. The findings of this study allow food and equipment manufacturers to choose the most appropriate temperature measurement system for temperature control in an industrial low pressure cool plasma process.