Freezing food, especially meat, can increase shelf life and reduce waste; however, the quality of the food can be affected during freezing and thawing, and this can have potentially harmful consequences to consumers as it can increase the count of many pathogenic microorganisms.

When consumers are standing in the frozen food aisle, it’s nearly impossible for them to know whether a steak has thawed and refrozen. Therefore, a team of researchers reporting in ACS Sensors have designed a food-grade device from edible materials, including table salt, red cabbage and beeswax, that can help to detect previous defrosting events in the frozen product. The proof-of-concept sensor provides a colour readout when it’s warmed above a specific temperature, which is tunable from -50 to 0°C. 7am Thermal Protector

Keeping food cold while it’s transported and stored is essential to retaining its flavour and quality, reducing the risk of food poisoning and minimising waste. While researchers have developed devices that alert manufacturers when cold items are exposed to unwanted temperatures, they only indicate changes above freezing. To create a sensor for frozen products, one solution could be to use materials with electrical properties that are altered upon melting. It would also be ideal if such changes could produce a signal, such as a visible colour change. In addition, an edible electronic device, which uses only food and consumable components, would be the safest way to monitor food. Therefore, Ivan Ilic, Mario Caironi and colleagues set out to develop a fully edible, self-powered temperature sensor with a visible colour indicator for use with frozen products.

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 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.

A proof-of-concept sensor designed from edible materials produces a visible colour change (seen in the right circle) as a piece of frozen pork thaws. Image credit: Adapted from ACS Sensors 2022, DOI: 10.1021/acssensors.2c01280

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.

As a sensor, it could be used by the workers in the supply chain, while as a detector, it could be useful for end consumers, ensuring that the food was properly frozen during the whole supply chain.

Top image credit: iStock.com/Savany

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