Thin-film calorimetric gas sensors for hydrogen peroxide monitoring in aseptic food processes
The sterilisation of the packaging material is the essential step in aseptic food processes to ensure safely packed products, which are microbiologically stable throughout their shelf life. Today, gaseous hydrogen peroxide (H2O2) in the range of several volume percent and at elevated temperature is...
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|Zusammenfassung:||The sterilisation of the packaging material is the essential step in aseptic food processes to ensure safely packed products, which are microbiologically stable throughout their shelf life. Today, gaseous hydrogen peroxide (H2O2) in the range of several volume percent and at elevated temperature is the preferred sterilant due to its strong microbicidal efficiency and its decomposition in environment-friendly products, namely water vapour and oxygen. In order to obtain a high degree of sterility, the initial H2O2 concentration has to be high enough and uniformly distributed over the package's inner surface. To ensure that the packaging surface is thoroughly treated by H2O2, a gas sensor is required that detects the present H2O2 concentration on selected locations of the package's surface while it is sterilised. The present thesis describes the realisation and characterisation of thin-film gas sensors based on an “on chip” differential set-up for monitoring the H2O2 gas concentration during the sterilisation of the packaging material. The differential set-up contains a catalytically active sensor segment, where H2O2 decomposes in an exothermic reaction causing a temperature increase towards a passive sensor segment, where a surface reaction is inhibited. In a first sensor arrangement, thin-film thermopiles have been fabricated on a single silicon chip, respectively, and their response behaviour has been characterised in H2O2 atmosphere. In a further arrangement, thin-film resistances have been built up as temperature-sensitive transducer platform on a silicon chip. On this platform, different catalytically active materials – platinum black, palladium and manganese oxide – have been tested with regard to their response against H2O2, wherein all of them showed a linear response characteristic, but manganese oxide posseses the highest sensitivity. Furthermore, three temperature-stable polymeric materials – fluorinated ethylene propylene, perfluoralkoxy and epoxy-based SU-8 photoresist – have been tested for the encapsulation of the sensor surface in terms of their chemical inertness against H2O2. Therein, all of them have shown a high resistivity against H2O2 underlining their suitability for sensor passivation. Within the frame of this work, the sensor set-up has further been realised on a thin polyimide foil because of its high temperature endurance, its chemical stability and particularly, its low thermal conductivity allowing an improved thermal separation of the active and passive sensor segment. As a result, the sensitivity of the polyimide-based sensor was strongly increased compared to the concentration-dependent response of the silicon-based sensors. Microbiological experiments with bacterial spores of Bacillus atrophaeus have demonstrated that the microbicidal effectiveness of the sterilisation process depends on the present H2O2 concentration in first order as well as on the contact time between the item that has to be sterilised and the gaseous H2O2. By means of sensor measurements conducted at the same time, a correlation model between the microbial inactivation kinetics and the sensor response was established that allows to use the sensor not only for concentration measurements, but also for the quantification and control of the degree of the package's sterility. In order to determine the present H2O2 concentration spatially resolved over the package surface during the short sterilisation cycle, a wireless sensor electronic based on the industrial ZigBee standard was developed. The sensor electronic contains a remote unit, which is connected to one of the calorimetric gas sensors fixed on a test package, and an external base unit connected to a laptop computer. For real-time measurements, a novel sensor read-out strategy was established, wherein the sensor response is measured within the short sterilisation time and correlated with both the present H2O2 concentration as well as the microbicidal effectiveness. As a result, this kind of “intelligent” package represents a novel instrumentation to monitor the package sterilisation in aseptic food processes under real-time conditions.|