Modern food production involves many complex processes that need to be controlled to ensure food safety and food quality.
The proliferation of national and international guidelines, ordinances and laws on food quality and safety means that stricter controls through governmental agencies and/or in-house quality assurance systems (Hazard Analysis Critical Control Point, HACCP-concept) are required. This frequently involves time-consuming and expensive intermediary steps, since individual production processes have to be monitored analytically in the control laboratory.Such controls generate additional financial burdens that disproportionately affect small and medium enterprises.
So-called on-line methods, integrated into the production process, save time and money, but are currently restricted to measuring the simplest chemical and physical parameters such as pH and temperature. More complex systems are under development, but are not yet reliable enough for commercial application.
In cooperation with the Fraunhofer Institute for Physical Measurement Techniques (IPM) a robust and cost effective gas sensor array for the rapid detection of volatile compounds should be elaborated in the scope of a Fraunhofer Research Project. The gas sensor array is intended for application in various areas of food industry:
- Incoming goods control
- Process control
- Storage and transport.
We used commercially available, inexpensive metal oxide sensors (Fig. 1), which are already used in waste gas and ventilation controls, and in fire detectors and combined them into a system that follows the principles of gas chromatography. Unlike alternative systems with a similar role (so-called electronic noses), this system uses a chromatographic separation column which splits a gas mixture and feeds the sensors with the individual substances (Fig. 2).
In the first project phase, the single components were selected according to the analytical requirements. In the second project phase, proof of principle was established by combining the components into a complete system.
Initial tests were carried out with standard substances, and our bespoke metal oxide sensors were tested against commercial sensor arrays. The four standard substances could be detected within six minutes (gas chromatography would normally take 45 minutes). Although the reproducibility of the sensor test was comparable to the reference analysis, the sensitivity was up to ten-fold higher.
Further experiments were carried out to address practical issues, such as the detection of bacterial contamination, which was achieved considerably faster by the application of volatile metabolites than using traditional microbiological techniques. In our approach, the volatile metabolites were identified using headspace gas chromatography. In a second step, the experimental approach was transferred to the sensor system. The results are presented in Fig. 3.