Development of a continuous flow sterilisation system using microwaves
Date
2017-03
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
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.
AFRIKAANSE OPSOMMING: Hierdie projek fokus op die ontwikkeling van ʽn kontinue vloei sisteem wat mikrogolwe gebruik om biologiese groei media te steriliseer. Die temperatuur van die vloeistof moet beheer word en dus moet die uittree drywing van die magnetron beheer word. Verskillende drywingsbeheer metodes was ondersoek en daar was gevind dat die beste metode sal wees om die magnetron se anode stroom te beheer terwyl die katode spanning konstant gehou word. Hierdie beheer is geïmplementeer deur twee huishoudelike mikrogolfoond hoë-spannings transformators aan dieselfde magnetron te koppel. Die anode transformator se intree spanning word deur ʽn triac baan beheer wat so die anode spanning, en dus die anode stroom, beheer. Daar was ondersoek ingestel op verskillende maniere om die drywing te vergelyk met ʽn veilige meetbare spanning of stroom. Die finale ontwerp gebruik ‘n stroommeter om die intree stroom van die anode transformator te meet. Die verband tussen hierdie stroom en die uittree drywing is gedokumenteer. PT100 weerstand temperatuur probes word in die finale sisteem gebruik om die intree en uittree temperature te meet. Die vloei deur die PTFE spoel binne die mikrogolfoond word gehandhaaf deur ʽn klein peristaltiese pomp. PTFE word gebruik omdat dit nie geaffekteer word deur mikrogolwe nie en dit is chemies onkreatief. Die verhouding tussen die maksimum bereikbare temperatuur en verskillende drywingsvlakke was gedokumenteer en gebruik in die ontwikkeling van die beheerstelsel. Die beheerstelsel word geïmplimenteer deur gebruik te maak van twee Arduinos en ʽn Matlab program. Die finale sisteem was geskuif na die Biochemie departement waar die finale sterilisasie toetse gedoen was. Daar bestaan verskillende tipes mikroörganisme en dit is dus belangrik om verskillende tipes te toets omdat hulle drasties anders kan reageer in prosesse. In hierdie studie was beide gram positiewe en gram negatiewe bakterieë getoets sowel as gis. Die spesifieke gram positiewe bakteria was Micrococcus Luteus, stam: NCTC 8340. Vir gram negatiewe bakteria was Escherichia Coli, stam: DH5α wat ‘n pGKCherry plasmied bevat gebruik. Laastens vir gis was Saccharomyces Cerevisiae gebruik. Alle toetse was by 90 °C uittree temperatuur en ʽn vloei-tempo van 3.5 l/h getoets. Al die toetse was ook in duplikaat gedoen om die resultate te bevestig. Die eerste toets was M. Luteus teen ‘n konsentrasie van 103 selle per ml. Steriliteit was bereik in hierdie toets en daar was besluit om die konsentrasie op te stoot na 106 selle per ml vir al drie tipes mikroörganismes. Steriliteit was in al drie gevalle bereik. Die invloed van temperatuur op die sterilisasie proses was ondersoek deur ʽn konsentrasie van 106 selle per ml van M. Luteus teen die volgende uittree temperature te toets: 70 °C, 50 °C en 37 °C. Die resultate het gewys dat slegs die 70 °C toets steriliteit behaal het. Die gevolgtrekking van die toetse is dat ʽn kontinue vloei sisteem was slegs mikrogolwe gebruik suksesvol is as ʽn sterilisasie proses.
AFRIKAANSE OPSOMMING: Hierdie projek fokus op die ontwikkeling van ʽn kontinue vloei sisteem wat mikrogolwe gebruik om biologiese groei media te steriliseer. Die temperatuur van die vloeistof moet beheer word en dus moet die uittree drywing van die magnetron beheer word. Verskillende drywingsbeheer metodes was ondersoek en daar was gevind dat die beste metode sal wees om die magnetron se anode stroom te beheer terwyl die katode spanning konstant gehou word. Hierdie beheer is geïmplementeer deur twee huishoudelike mikrogolfoond hoë-spannings transformators aan dieselfde magnetron te koppel. Die anode transformator se intree spanning word deur ʽn triac baan beheer wat so die anode spanning, en dus die anode stroom, beheer. Daar was ondersoek ingestel op verskillende maniere om die drywing te vergelyk met ʽn veilige meetbare spanning of stroom. Die finale ontwerp gebruik ‘n stroommeter om die intree stroom van die anode transformator te meet. Die verband tussen hierdie stroom en die uittree drywing is gedokumenteer. PT100 weerstand temperatuur probes word in die finale sisteem gebruik om die intree en uittree temperature te meet. Die vloei deur die PTFE spoel binne die mikrogolfoond word gehandhaaf deur ʽn klein peristaltiese pomp. PTFE word gebruik omdat dit nie geaffekteer word deur mikrogolwe nie en dit is chemies onkreatief. Die verhouding tussen die maksimum bereikbare temperatuur en verskillende drywingsvlakke was gedokumenteer en gebruik in die ontwikkeling van die beheerstelsel. Die beheerstelsel word geïmplimenteer deur gebruik te maak van twee Arduinos en ʽn Matlab program. Die finale sisteem was geskuif na die Biochemie departement waar die finale sterilisasie toetse gedoen was. Daar bestaan verskillende tipes mikroörganisme en dit is dus belangrik om verskillende tipes te toets omdat hulle drasties anders kan reageer in prosesse. In hierdie studie was beide gram positiewe en gram negatiewe bakterieë getoets sowel as gis. Die spesifieke gram positiewe bakteria was Micrococcus Luteus, stam: NCTC 8340. Vir gram negatiewe bakteria was Escherichia Coli, stam: DH5α wat ‘n pGKCherry plasmied bevat gebruik. Laastens vir gis was Saccharomyces Cerevisiae gebruik. Alle toetse was by 90 °C uittree temperatuur en ʽn vloei-tempo van 3.5 l/h getoets. Al die toetse was ook in duplikaat gedoen om die resultate te bevestig. Die eerste toets was M. Luteus teen ‘n konsentrasie van 103 selle per ml. Steriliteit was bereik in hierdie toets en daar was besluit om die konsentrasie op te stoot na 106 selle per ml vir al drie tipes mikroörganismes. Steriliteit was in al drie gevalle bereik. Die invloed van temperatuur op die sterilisasie proses was ondersoek deur ʽn konsentrasie van 106 selle per ml van M. Luteus teen die volgende uittree temperature te toets: 70 °C, 50 °C en 37 °C. Die resultate het gewys dat slegs die 70 °C toets steriliteit behaal het. Die gevolgtrekking van die toetse is dat ʽn kontinue vloei sisteem was slegs mikrogolwe gebruik suksesvol is as ʽn sterilisasie proses.
Description
Thesis (MEng)--Stellenbosch University, 2017.
Keywords
UCTD, Microwaves, Sterilization equipment industry, Continuous-flow culture (Microbiology)