Novel strategies for evaluating the effectiveness of aseptic sterilisation processes by means of a multi-sensor set-up

The aseptic processing and packaging of food, in combination with the possibilities of aseptic storage, transport and packaging recycling, represents one of the most sustainable technologies in modern food processing and contributes significantly to reducing the losses of fruits, vegetables and milk and to increase the availability of this food all over the world. In contrast to conventional food processing methods, product and container are continuously sterilised separately and brought together under sterile conditions. This allows for a gentler thermal treatment of the product, while maintaining the benefits of an extended shelf life and the storage without refrigeration. Besides the thermal treatment of the product, the containers are sterilised in order to inactivate potential pathogens and/or food spoilers on the surface of the packaging. Today, hydrogen peroxide (H2O2) applied in either gaseous or vapour phase, particularly because of its good handling and high environmental compatibility, represents one of the mostly used sterilising agents in the aseptic filling industry. Against this background it is actually surprising that the mechanisms of biocidal action of H2O2 are rather poorly understood. In order to characterise the effectiveness of the sterilisation by H2O2, test objects inoculated with an unnaturally high load of microorganisms are taken through the sterilisation process. In retrospect, these are examined for viable bacteria. It seems obvious that this is a very time consuming and costly procedure. This work focused on the development of novel strategies for determining the effectiveness of the sterilisation by gaseous H2O2 by means of a real-time monitoring system. Therefore, a handheld sensor system for the online monitoring of the H2O2 concentration, which represents an important factor of the sterilisation, has been developed in the first instance. It could be shown that this monitoring system largely meets the technical requirements for field use. The main strategy, however, aimed at identifying the influencing factors of the sterilisation by gaseous H2O2 – determining in detail the impact of each of these variables on the microbicidal action – and to make them detectable using predominantly commercial gas sensors. The variables of the sterilisation process include the H2O2 concentration, humidity, gas temperature, flow rate and time of exposure. Initially, different types of gas sensors, including a calorimetric type, different metal-oxide semiconductor (MOX) as well as an electrochemical gas sensor have been identified as potential candidates for monitoring the factors which are involved in the sterilisation. Furthermore, the inactivation kinetics of bacterial spores by gaseous H2O2 have been investigated in several series of microbiological tests using Bacillus atrophaeus (formerly Bacillus subtilis) spores. In a first approach, a correlation between the microbicidal effectiveness and the signal output of two commercial gas sensors via one parameter, namely the H2O2 concentration, was established. Nevertheless, since the sterilisation is not solely dependent on the H2O2 concentration, other factors had to be taken into account. The second approach meanwhile aimed no more to establish a parameter-dependent correlation, but rather a direct correlation between sensor response and microbicidal action. In a first work on this topic, the sensing characteristics of several sensor types have been analysed with respect to their sensitivity towards the influencing factors of the sterilisation process. These are the H2O2 concentration, the humidity, the gas temperature and the gas flow rate. In parallel, microbiological tests have been carried out. It could be shown that the sensors in test equally responded to the influencing factors of the sterilisation. Additionally, oriented on the concept of an electronic nose, the feasibility of determining the microbicidal effectiveness on the basis of chemical images obtained from the sensor output, by applying pattern recognition, has been demonstrated. In a second work on this topic, the same sensors had been used, but the field of examined parameters has been defined more precisely and the number of measurement points has been increased considerably in order to obtain a higher statistical significance of both, the sensory measurements and microbiological tests. Thus, it was possible to establish two models, which correlate the sensor responses of the calorimetric gas sensor and two different types of MOX sensors with the microbicidal action within a wide parameter field. Based on this, a multi-sensor system for monitoring the effectiveness of the sterilisation process by gaseous H2O2 is proposed. The content of this thesis concludes with a study on the sensing mechanisms of different types of metal oxides towards H2O2, since a vast number of sensors used in this project belong to the group of MOX sensors. Therefore, pure SnO2 and WO3 as well as Pt- and Pd-doped SnO2 films were deposited on a self-developed array structure. It could be demonstrated that the doping with catalytically active materials significantly increases the sensitivity of MOX sensors towards H2O2. In this way, even low concentrations of H2O2 down to the ppm range can be detected. Thus, these sensors can be adopted for other fields of application where the detection of rather low concentrations is required, e.g., in the exhaust system of aseptic sterilisation devices or for environmental control in the vicinity of H2O2 deploying processes.

Publication year:2014

Accessibility:Open