Toward a fully edible sensor showing if frozen food has previously thawed

Standing in the frozen food aisle, it’s nearly impossible to tell if that Salisbury steak has been thawed and refrozen — a process with potentially harmful consequences. So, researchers report in ACS sensors have designed a food-grade appliance from edible materials, including table salt, red cabbage and beeswax, let you know. The proof-of-concept sensor provides a color reading when heated above a specific temperature, which can be set from -58 F to 32 F.

Keeping food cold while it is being transported and stored is essential to maintain taste and quality, reduce the risk of food poisoning and minimize waste. Although researchers have developed devices that warn manufacturers when cold items are exposed to undesirable temperatures, they only indicate changes above freezing. To make a sensor for frozen products, one solution could be to use materials with electrical properties that change when melted. It would also be ideal if such changes could produce a signal, such as a visible color change. In addition, an edible electronic device, using only food and consumable components, would be the safest way to control food. So Ivan Ilic, Mario Caironi and colleagues set out to develop the first fully edible, self-powered temperature sensor with a visible color indicator for use with frozen foods.

The researchers started by building a device that generated an electric current during thawing, connecting magnesium and gold electrodes via an electrolyte solution in a plastic container. They tested the device with solutions of frozen edible electrolytes, including table salt and calcium-containing salts, and natural electrolyte-rich foods, including a grape, melon and apple. As the solutions thawed, they ran power between -58 F and 32 F, which the researchers say can be adjusted based on the amount and identity of the salt. This device was then connected to a color-changing system, which contained tin and gold electrodes and red cabbage juice, which caused an irreversible shift from red-purple to blue when current was applied.

In the final step, the team placed all the parts together in a block of beeswax that kept the temperature-activated and indicator solutions in separate chambers, demonstrating that the self-powered device can be used for monitoring frozen food. The researchers say their proof-of-concept sensor is paving the way for the use of edible materials in low-cost, safe technologies that alert customers to a frozen product’s storage history.