The Department of Electric and Electronic Engineering recently developed a system to sterilise a fluid using only microwaves. This continuous flow system was designed to be used for the sterilisation of biological growth media but would be just as effective for any fluid. The proof of concept design can be further developed into a commercial product for either large scale continuous process lines or as a small lab appliance that can quickly sterilise any size batch of growth media.
The concept of sterilisation can also be applied to other fields such as the food industry in order to sterilise milk or fruit juices. A remarkably interesting application would be where this technology is applied to manufacturing lines where water is used to cool down equipment such as saws or drills. In these situations, the unsterilized water can form biofilms within the water pipes which cause blockages and results in the equipment not being cooled properly. If the water is sterilised before it enters the pipes the biofilms will not form, and the equipment will last longer.
Electromagnetic waves (Microwaves) in the frequency range of between 300 MHz and 300 GHz were discovered in 1921 and commercialised around 1940. These electromagnetic waves are most commonly known for their use in household microwave ovens. Today microwaves are commonly used in many heating processes because of its ability to rapidly increase temperature and in many industries such as communication, food processing and pharmaceutical production.
A master’s student, Adelaide Oberholzer, investigated this continuous flow sterilisation system, under supervision of Prof Johann De Swardt. The system focussed on liquid media and had a coiled pipe inside the multimode microwave cavity. This research demonstrated the effective use of conventional microwave components and parts as far as possible and the system was tested in a laboratory environment which yielded successful results.
1 Introduction and background
In studies and processes relating to microorganisms, it is essential to be able to grow only a specific type of microorganism. For this, a nutrient-rich broth, called growth media, is used. The growth media must be sterile to ensure there are no contaminations in the outcomes of tests or products being produced. However, current sterilisation methods only allow batch sterilisation of the media.
2 Project development
The BIOPEP Peptide Research Group lead by Prof. Marina Rautenbach from the Department of Biochemistry at the University of Stellenbosch is developing a new antibiotic. As part of the production line for this new antibiotic, a continuous flow sterilisation system is required for the growth media used. The aim is for the continuous flow sterilisation system to use only microwaves as a source of heat for the sterilisation of the biological growth media. The output power of the magnetron must, therefore, be controlled in order to control the temperatures to which the media will be heated.
A study was conducted 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 that was implemented using Matlab and two Arduinos as microprocessors. Because there are different types of microorganisms, it is important to test for different types as they may react differently to external stimuli.
The final system was built using a modified domestic microwave oven with a coiled pipe to allow the media to be pumped through the cavity. The developed anode current control method was employed to control the microwave output power and the sterilisation tests were conducted on contaminated growth media, which proved that the system is successful as a method of sterilisation.
The developed microwave system was successfully used in sterilisation tests conducted on different types of microorganisms at high concentrations. All tests that were done above 70°C completely sterilised the media without causing any visible damage to it. It was also shown that although microwave ovens are not very effective in terms of power, with a 58.5% efficiency, microwaves as a means of sterilisation is more energy-efficient than the autoclave currently used in the Department.
 Full research available at AE Oberholzer Development of a Continuous Flow Sterilisation System Using Microwaves (2017) <https://scholar.sun.ac.za/handle/10019.1/100985>.