Sensors monitor freshness of food

Shoppers may one day know the quality of fruit and vegetables in the supermarket with smart plastic food packages that use integrated sensors to detect ripeness.  

This is the ultimate goal of the EU-funded FlexSmell project lead by Switzerland’s Ecole Polytechnique Fédérale de Lausanne (EPFL). The team includes UK representation from the Universities of Manchester and Sheffield.

Principal investigator Danick Briand, team leader of EPFL’s SAMLAB (The Sensors, Actuators and Microsystems  Laboratory), is leading a consortium to integrate low-cost, energy efficient olfaction systems into plastic substrates that can be mass produced through roll to roll manufacturing methods. He envisioned the plastic could be used for packaging supermarket products or transporting produce.

Briand said the challenge will be ensuring the robustness of the system’s components, which will include sensors, electronics and an antenna capable of sending data wirelessly. 

These sensors will be used for measuring basic parameters such as temperature and humidity and more advanced markers that indicate produce quality, he said.

Briand said these include ethylene gas, an essential plant hormone that stimulates the ripening of fruit, the opening of flowers and the shedding of leaves. The gas, which is found in trace levels, represents the ripeness of fruit, he said.

The new smart packaging will also measure the amount of hexanol - an indicator of food deterioration in food - in the vapours emitted from foods.

Briand said there are three possible ways to measure these markers. The first technique, the capacitive principle, uses a capacitor chemical sensor composed of two electrodes separated by a dielectric material sensitive to a certain chemical species, he said.

The capacitive detection of chemical species relies on the change of dielectric constants or on the thickness of the dielectric layer due to swelling.

The other method, Briand said, is based on the resonating principle. When combined with a chemical sensitive layer, Briand said it becomes an extremely sensitive transducer. The sensitivity of the device is proportional to the square of its resonance frequency, he said. The addition of small foreign mass over the surface will shift its resonance frequency.

Briand said another means for detection uses the field-effect principle, which uses a transducer made of a transistor that operates at constant voltage. If a signal induced by a stimuli alters the potential distribution in the device or the semiconductor mobility, this will modify the output current.

The FlexSmell team envision all of this information could be sent wirelessly through RFID in food transportation. In supermarkets, Briand predicted this technology could one day lead to ‘smart perishable dates’ on product wrapping.

‘Individual devices that sense on plastic could happen soon for specific applications, but this technology integrated on a tag with RFID for food packaging and transportation is about five to ten years away,’ he said.