Dokument

Summary:
Package sterilization is an essential step during aseptic packaging of food, pharmaceuticals or medical instruments to prevent microbiological contamination of the product. In food industries, the main objective is to produce consumer-safe and long-term stable food products. In recent years, the favored method to sterilize package material is by use of gaseous hydrogen peroxide (H2O2) at concentrations up to 10% v/v and elevated temperatures up to 300 °C. These process parameters enable a fast and effective, in chain sterilization of packages prior to filling with sterile products. Monitoring of this sensitive process is performed by predefined machine settings and laborious microbiological challenge tests, with earliest results after 72 hours. In previous works different sensors to monitor the packaging sterilization process have been developed, but till now there is no commercial system available to continuously monitor the final gas concentration or the microbial sterilization efficacy online within the package. In the present work, as a first approach the sensing principle of a calorimetric H2O2 gas sensor has been studied in more detail. The sensor is based on a differential set-up of one catalytically activated and one passivated temperature-sensing element. Surface characterizations have been performed to reveal the chemical reaction of H2O2 at the applied catalyst manganese(IV) oxide (MnO2). The surface characterization depicted a transition of the manganese oxidation state. Moreover, the treatment with H2O2 eliminates the polymeric layer on top of the catalyst, which has been applied as polymer matrix to attach the catalyst onto the sensing element. The calorimetric gas sensor has been further described by analytical expressions in order to evaluate the theoretical temperature rise. Thereby, different sensor scenarios (steady-state process, gas diffusion process and convective gas flow) have been described by the sensor's thermochemistry and physical transport mechanisms. These theoretical assumptions have been accompanied by surface and thermal characterizations of polymers applied as passivation materials. The characterizations demonstrate the suitability of the three investigated polymers (SU-8 photoresist, Teflon derivatives PFA and FEP), to act as a passivation against gaseous H2O2. As second approach of this work, a novel biosensor has been developed. This biosensor is based on interdigitated electrodes (IDE) on which a standardized test organism is immobilized. This test microorganism, spores of Bacillus atrophaeus, is commonly applied in industrial microbiological challenge tests to evaluate the efficacy of sterilization processes. Impedance measurements are applied to characterize the microbiological samples at the sensor surface before and after the gaseous H2O2 sterilization process. Thereby, a remaining change in impedance and phase has been observed. Numerical simulation tools have been employed to analyze the sensor signal, and to gather material parameters of the spores. Finally, the impedimetric and calorimetric sensor have been combined to serve as a miniaturized sensor system to analyze the efficacy of the gaseous sterilization process.

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