Pasteurisation of Rooibos tea leaves using microwave radiation and the development of associated hardware

Van der Merwe, Bernard Johannes (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Rooibos tea is a herbal tea that is gaining popularity worldwide. In order for it to be safe for consumption by consumers, the tea leaves are pasteurised with steam in commercial processing plants. The viability of using microwave radiation to pasteurise Rooibos tea leaves was investigated in this research project. Hardware associated with these microwave systems, in the form of microwave applicators, a directional coupler and a prototype power detector, was also developed. This project showed that microwave radiation may be an alternative way to process Rooibos tea leaves and may produce a higher quality product. Control circuitry that controlled the output power of the microwave generator (magnetron) was developed. It used a TRIAC to control the phase angle of the supply voltage to the transformer that supplied power to the magnetron. Two special applicators were developed in order to conduct pasteurisation experiments on the Rooibos tea leaves. An air-cooling applicator cooled down a sample of Rooibos tea leaves while it was being irradiated. This was to test whether the so-called non-thermal, or microwave specific, effects of microwave radiation was able to pasteurise tea leaves at ambient temperatures. The other applicator was a travelling wave applicator that heated samples evenly. The temperature of the sample was measured and a control system was developed that could heat a sample to a desired temperature. Tests were conducted in order to determine the viability of microwave pasteurisation of Rooibos tea leaves. Sterilised Rooibos tea leaves, which were inoculated with bacteria, were used. The tests showed that Gram-negative Escherichia coli was eliminated from the samples after being irradiated for 2.5 minutes at 90 °𝐶. Gram-positive bacteria (Lactococcus lactis) was also tested. After a 5 minute period of being irradiated at 100 °𝐶, a four-fold reduction in bacteria was observed. Microwave pasteurisation at ambient temperatures was tested. The tests showed no significant reduction in bacteria and it was concluded that any microwave specific effects were insignificant at low temperatures. In order to measure the power output of the microwave system, a directional coupler and power detector was developed. The directional coupler used an innovative design to achieve a constant coupling of 35 𝑑𝐵 and a directivity greater than 20 𝑑𝐵 over the microwave oven frequencies of 2.4 − 2.5 𝐺𝐻𝑧. The power detector was a prototype that showed that a thermistor-based design can produce an output that was linearly proportional to the input power to the power detector. It was concluded that microwave radiation appears to be a viable method of pasteurising Rooibos tea leaves, but that more testing is required using unsterilised Rooibos tea leaves. The hardware that was developed worked well. The prototype power detector can be developed further by using high frequency thermistors.

AFRIKAANSE OPSOMMING: Rooibos tee is 'n kruietee wat tans wêreld-wyd meer bekend raak. Om te verseker dat die tee veilig is vir verbruikers word die tee blare gepasteuriseer met stoom in kommersiële fabrieke. Die moontlikheid van die gebruik van mikrogolfstraling vir pasteurisering van Rooibos tee blare is in hierdie navorsingsprojek ondersoek. Hardeware wat verband hou met hierdie mikrogolf stelsels, in die vorm van mikrogolf toedieners, 'n direksionele-koppelaar en 'n prototipe drywingsdetektor, is ook ontwikkel. Hierdie projek het getoon dat mikrogolfstraling 'n alternatiewe manier kan wees om Rooibos tee blare te verwerk en kan moontlik 'n hoër kwaliteit produk lewer. ʼn Beheerstroombaan is ontwikkel wat die uittree drywing van die mikrogolf-bron (magnetron) beheer. Dit het 'n TRIAC gebruik om die fase hoek van die toevoerspanning aan die magnetron se transformator te beheer. Twee spesiale toedieners is ontwikkel om pasteurisasie eksperimente op die Rooibos tee blare uit te oefen. 'n Lugverkoeling toediener het 'n monster van Rooibos tee blare afgekoel terwyl dit bestraal is. Dit was om te toets of die sogenaamde nie-termiese, of mikrogolf-spesifieke, effekte van mikrogolfstraling in staat was om tee blare te pasteurisasie teen kamertemperatuur. Die ander toediener was 'n golfleier toediener wat die monsters eweredig verhit het. Die temperatuur van die monster is gemeet en 'n beheerstelsel is ontwikkel wat 'n monster tot 'n gestelde temperatuur kon verhit. Toetse is gedoen om die moontlikheid van mikrogolf-pasteurisasie van Rooibos tee blare te bepaal. Gesteriliseerde Rooibos tee blare, wat met bakterieë ingeënt is, is gebruik. Die toetse het getoon dat alle Gram-negatiewe Escherichia coli selle dood was nadat dit bestraal is vir 2.5 minute teen 90 °𝐶. Gram-positiewe bakterieë (Lactococcus lactis) is ook getoets. Na 5 minute se bestraling teen 100 °𝐶 is 'n viervoudige afname in bakterieë waargeneem. Mikrogolf-pasteurisasie teen kamertemperatuur is getoets. Die toetse het geen beduidende afname in bakterieë getoon nie en die aanname is dus gemaak dat enige mikrogolf-spesifieke effekte geen impak gehad het teen lae temperature nie. Om die uittree drywing van die mikrogolf stelsel te meet is 'n direksionele-koppelaar en ʼn drywingsdetektor ontwikkel. Die direksionele-koppelaar het 'n innoverende ontwerp gebruik om 'n konstante koppeling van 35 𝑑𝐵 en 'n direktiwiteit groter as 20 𝑑𝐵 oor die mikrogolf-oondfrekwensies van 2.4 − 2.5 𝐺𝐻𝑧. Die drywingsdetektor was 'n prototipe wat getoon het dat 'n termistor gebaseerde ontwerp 'n uittree kan produseer wat lineêr eweredig is aan die intree drywing. Daar is tot die gevolgtrekking gekom dat mikrogolfstraling 'n moontlike metode van pasteurisasie van Rooibos tee blare blyk te wees, maar dat meer toetse vereis word met die gebruik van rou Rooibos tee blare. Die hardeware wat ontwikkel is, het goed gewerk. Die prototipe drywingsdetektor kan verder ontwikkel word deur die gebruik van hoë frekwensie termistors.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107927
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