Development of microwave cool plasma for microbial inactivation in food applications

Plasma is a growing field of non-thermal processing research for microbial inactivation and disinfection of food and food contact surfaces. An extensive review of the literature revealed that plasma can be generated from many forms of electromagnetic waves ranging from DC to microwaves under vacuum or atmospheric conditions. Part of this review catalogued a wide variety of techniques for plasma generation to serve as a reference source on this new processing technology. The thesis set out to investigate microwave cool plasma equipment design to establish a simulation model, using Computer Simulation Technology (CST) Microwave Studio software, to assist in fabrication and further modification of laboratory and larger scale prototype low pressure microwave plasma devices. Also the effects of operational conditions with these equipment designs were determined by measuring plasma density using a Langmuir probe and surface temperatures by fibre optic methods, at defined coordinates within treatment chambers of the devices. These results were used to confirm the simulation data. Using defined conditions, investigations were conducted on the inactivation of vegetative cells of bacterial food pathogens, Salmonella and Listeria and spores of food spoilage bacteria Geobacillus and Bacillus using low temperature plasma. The temperature ranges of the surface to be sterilised were determined to be less than 45 degrees C. The thesis attempted to determine the mechanism of plasma inactivation by excluding the exposure of charged particles (using charged surfaces to attract or repel plasma particles) and UV radiation (using a quartz filter) and measuring inactivation of bacterial spores, in the presence and absence of these modifying treatments. Given the design configuration of equipment and operational conditions the investigations determined that UVC was the factor responsible for microbial inactivation effects. Other species such as electrons, ions, radicals and also the other ranges of UV (UVA and UVB) appeared to play no part in the process of microbial inactivation in the relatively short time periods investigated, where UVC acted to inactivate microorganisms. Microbial inactivation effects of up to ~ 4 log reduction (as determined by viable count) were achieved for bacterial spores using both the laboratory scale and one metre plasma units. The presence of added organic load of dried milk solids to spore preparations moderated the inactivating effects of plasma. Nonetheless an approximately 2 log inactivation of Salmonella and Listeria adsorbed to the surfaces of almonds could still be achieved in a period of 60 s exposure. The work conducted in the thesis provides the first steps towards scaling up the design configuration of the one metre plasma unit to provide an array that can expose a larger area to plasma treatment.