Fruit-lovers know what happens: If you place an apple next to a banana, it ripens faster than usual. The solution to this puzzling fact is the ethylene gas which every fruit gives off to a certain extent, and bananas more than most. Fruit wholesalers deliberately make use of this effect. They expose green bananas or tomatoes to ethylene to make them ripen faster, causing the fruit to develop the typical yellow or red color that customers expect. Conversely, the wholesalers keep the ethylene concentration in the warehouses low if the fruit needs to stay fresh for a long time. Fine feeling is required for both processes: If too much ethylene is pumped into the fruit warehouses, the bananas turn brown and overripe. If too much gas is extracted from the refrigerated halls, a large amount of cool air also escapes, unnecessarily wasting energy. A new sensor can now measure the ethylene concentration accurately at a low cost. “This sensor is much more compact and also much cheaper than traditional complex measuring systems. At roughly a thousand euros, it costs only about a tenth of the price,” says Dr. Jürgen Wöllenstein, team leader at the Fraunhofer Institute for Physical Measurement Techniques IPM in Freiburg. He and his team developed the sensor in collaboration with colleagues from the University of Barcelona. Its core components are an infrared radiator, similar to a heat radiator, which emits radiation of different wavelengths, and a filter that only allows radiation with a wavelength of 10.6 micrometers to pass through. The filter is necessary because ethylene absorbs radiation of this wavelength. The more ethylene is present in the air, the less radiation reaches the detector, which is likewise integrated in the sensor. This method of measuring gas concentration is already used for CO2. “The challenge with ethylene is its extremely long wavelength of 10.6 micrometers. We had to ensure that the radiation could travel a very long distance through the air, as this is the only way we can reliably measure the effect and thus the ethylene concentration,” explains Wöllenstein. The researchers deflect the radiation with gilded mirrors to make it cover a distance of over three meters inside a sensor the size of a box of cigarettes. The scientists have also optimized the infrared radiator to emit as much heat as possible at the appropriate wavelength. A prototype of the sensor already exists. In about two years from now, the sensor could be showing fruit wholesalers how far to turn on the ethylene tap or reduce the supply.
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