The researchers started by building a device that generated an electrical current as it defrosted, connecting magnesium and gold electrodes through an electrolyte solution held in a plastic container. They tested the device with solutions of frozen edible electrolytes, including table salt and calcium-containing salts, and naturally electrolyte-rich foods, including a grape, melon and apple. As the solutions defrosted, they conducted current between -50 C and 0 C, which the researchers say could be fine-tuned, based on the amount and identity of the salt. Next, this device was connected to a colour-changing system, containing tin and gold electrodes and red cabbage juice, that produced an irreversible shift from reddish purple to blue when current was applied.
In the final step, the team put all of the parts together in a block of beeswax that held the temperature-activated and indicator solutions in separate chambers, and demonstrated that the self-powered device could be used for frozen food monitoring. The researchers say that their proof-of-concept sensor paves the way for edible materials to be used in inexpensive, safe technologies that alert customers to a frozen product’s storage history.
The authors acknowledge funding from the European Research Council, the European Union’s Horizon 2020 programme and the Sustainability Activity of Istituto Italiano di Tecnologia.