Bacterial inactivation using radial mode ultrasonic devices

• Main Author: Hunter, Graham I.
• Published: University of Glasgow 2008
• Subjects: 660.6; QR Microbiology : TJ Mechanical engineering and machinery
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.495321

Bacterial inactivation is vitally important in many industrial processes and in particular those manufacturing products for human consumption. The consequences of poor inactivation processes can result in severe illness and death. To prevent this, several techniques have been developed over the years to ensure products are free from harmful bacteria. In the vast majority of cases either thermal or chemical inactivation is used in liquid products. Pasteurisation and boiling have been accepted methods of rendering a product free of pathogenic bacteria for centuries, as has the addition of small amounts of chlorine to water. Unfortunately, the use of thermal and chemical methods has a detrimental effect on the nutritional value, taste and texture of the product. Also, bacteria are living organisms and as such are subject to evolution. In fact, the rate at which bacteria multiply probably makes them the fastest evolving organisms on the planet. This means that bacteria are likely to evolve resistance to inactivation methods over a period of time and new methods will be required to aid the inactivation of pathogens. The use of ultrasonic vibrations to cause cavitation and thus bacterial inactivation has been shown to be successful. This method causes physical inactivation of bacteria by disrupting the bacterial cell wall thus adding a new tool in the arsenal against pathogens. This project investigated the bacterial inactivation capacity of radial and probe type ultrasonic horns. Radial horns were designed using finite element analysis and validated using experimental modal analysis. Several radial horns were investigated, tuned to the R0 and R3 modes, while probe type horns tuned to a longitudinal mode. A finite element model has been created to estimate the acoustic fields created by these devices, where water is the load medium, and therefore where zones of cavitation could be expected to occur. This finite element model is validated against photographic and chemiluminescent visualisation studies of the cavitation field. A microbiological study was carried out to determine the bacterial efficiency of these devices in a static (non-flow) system. This study used the organisms Escherichia coli and Staphlococcus aureus as the test organisms as they are non-pathogenic, well understood, easy to handle, and are useful indicators of bacterial inactivation performance that have been extensively studied. Further to the positive results from these tests, a continuous flow radial horn system was designed and constructed. This was used to test the bacterial efficiency of the radial horn device in a continuous flow situation. Results on the effect of flow rate and ultrasonic amplitude on the bactericidal effect on the two bacterial organisms used are presented.