Magnetron output power controller used in the application of rooibos tea sterilisation

Van der Merwe, Jacobus Pieter (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: The objective of this project was to design a microwave sterilisation system that can be used to sterilise Rooibos tea. Rooibos tea is currently sterilised with a steam treatment, but this a ects the tea's colour and avour. Mi- crowave irradiation was investigated as an alternative form of sterilisation. Often microwave sterilisation tests are performed without the proper imple- mentation of microwave power control. Accurate microwave output power control was achieved by designing a custom magnetron power supply that uses a TRIAC to switch the mains voltage every half-cycle. A control system, that measures the applied and the re ected microwave power, was implemented to ensure a constant microwave power is delivered to the load. Calibrated mea- surements were taken of the system input power, microwave output power and applicator temperature. Adjustments were made to the microwave applicator to ensure that the dry tea did not burn. Microbiological tests were performed on the treated samples for Salmonella, E.Coli/Coliforms and aerobic plate counts (TVC's). The South African stan- dard for microbial safety of Rooibos tea was used as reference. The microwave sterilisation system worked well for Salmonella and TVC's, but E.Coli/Coliforms proved more resilient. A de nite batching problem was identi ed with the sterilisation of raw farm products that contain di erent concentrations of mi- croorganisms. This study provides a good foundation for any further research on the microwave sterilisation of Rooibos tea, as well as a range of hardware solutions for various other microwave power control applications.

AFRIKAANSE OPSOMMING: Die doel van hierdie projek was om' n mikrogolfsterilisasiestelsel vir Rooibostee te ontwerp. Tans word Rooibostee met stoom gesteriliseer, maar dit affekteer die kleur en geur van die produk. Mikrogolfbestraling was ondersoek as 'n al- ternatiewe sterilisasiemetode. Dit gebeur gereeld dat mikrogolfsterilisasietoetse gedoen word sonder dat be-hoorlike mikrogolfdrywingsbeheer toegepas word. Akkurate mikrogolfuittreedry- wingsbeheer was behaal deur 'n gespesialiseerde magnetronkragbron te on-twerp, wat gebruik gemaak het van 'n TRIAC om die intreespanning elke halfsiklus te skakel. 'n Beheerstelsel, wat die toegepaste en weerkaatste mikro- golfdrywing gemeet het, was ontwerp om te verseker dat die las teen 'n kon-stante mikrogolfdrywing bestraal word. Gekalibreerde metings was gemaak van die stelsel se intreedrywing, mikrogolfuittreedrywing en die temperatuur van die tee. Aanpassings moes gemaak word aan die mikrogolfoond om seker te maak dat die Rooibostee nie brand nie. Die mikrobiologiese toetse wat gedoen was op die behandelde teemonsters het getoets vir Salmonella, aerobiese plaattellings en E.Coli/Coliforms. Die Suid-Afrikaanse standaard van veiligheid vir mikroorganismes in Rooibostee was gebruik as verwysing. Die mikrogolfsterilisasiestelsel het goed gewerk vir Salmonella en aerobiese plaattellings, maar E.Coli/Coliforms was meer be-stand teen die behandeling. 'n Definitiewe probleem was geïdentifiseer waar verskillende monsters van 'n plaasproduk verskille in mikrobiologiese aktiwiteit getoon het. Hierdie studie bied 'n goeie grondslag vir verdere navorsing op die mikrogolfsterilisasie van Rooibos tee, asook 'n reeks hardeware-oplossings vir verskeie mikrogolfdrywingsbeheertoepassings.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105834
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