Browsing by Author "Oberholzer, Adelaide Emily"

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    Development of a continuous flow sterilisation system using microwaves
    (Stellenbosch : Stellenbosch University, 2017-03) Oberholzer, Adelaide Emily; De Swardt, J. B.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: The focus of this project is to design a continuous flow system to be used for the sterilisation of biological growth media by using only microwaves. The output power of the magnetron has to be controlled in order to control the temperatures to which the media will be heated. A study was done on different methods of microwave power control and it was found that anode current control is the most suited method. The developed method controls the anode current of the magnetron while keeping the cathode at its constant voltage. Two high voltage transformers used in domestic microwave ovens were connected to the same magnetron. This allowed the anode transformer’s supply voltage to be changed by using a triac control circuit. This changes the anode voltage, and so the current, while keeping the cathode constantly heated. Methods of relating the microwave power to a safely measurable voltage or current were also investigated. In the final design, a Hall effect current sensor was used to measure the current on the primary side of the anode transformer and this current was related to the output power of the magnetron. Temperature sensors to be used for the inlet and exit fluid temperatures were also investigated and PT100 resistance temperature probes were used in the final design. A small peristaltic pump was used to pump the fluid through a PTFE coil inside the microwave cavity. PTFE was selected because it is not susceptible to microwaves and it is chemically non-reactive. A study was done on the relationship between microwave power and the maximum temperatures reached by the fluid for different flow rates. This was used to develop a control system which was implemented using Matlab and two Arduinos as microprocessors. The final system was moved to the Biochemistry Department to commence sterilisation tests. Because there are different types of microorganisms, it is important to test for different types as they may react differently to external stimuli. For this study, both gram negative and gram positive bacteria were tested as well as yeast. The specific gram positive bacteria used was Micrococcus Luteus, strain: NCTC 8340; for gram negative bacteria Escherichia Coli, strain: DH5α containing a pGKCherry plasmid, and for yeast Saccharomyces Cerevisiae was used. All tests were done in duplicate to confirm the results. The target exit temperature was 90 °C and the flow rate was 3.5 l/h. An initial test was done on M. Luteus with a concentration of 103 cells per ml. This batch was completely sterile and it was decided to increase the concentration to 106 cells per ml for all of the microorganisms tested. All three of these tests achieved sterility. It was then decided to reduce the temperature to see if this had an effect on the results. These tests were done with M. Luteus at a concentration of 106 cells per ml. The temperatures selected were 70 °C, 50 °C and 37 °C. Only the 70 °C batch achieved sterility. It is concluded from this project that continuous flow microwave sterilisation is possible and very effective.