Worldwide, nearly a third of all food is wasted. Much of it is still safe to eat, but consumers throw it away because it’s close to or beyond its printed expiration date. That waste could be mitigated if food were packaged with a sensor that monitored its spoilage in real time. But such a device would need to be low cost, easy to produce, and unambiguous to read. Toward that end, Firat Güder of Imperial College London in the UK and his colleagues have designed a paper sensor that detects the gases emitted during food decomposition.
At typical levels of humidity, a paper surface becomes coated with a thin layer of water, which absorbs water-soluble gases from the environment (right panel of the figure). The gas molecules contribute positive and negative ions that change the water’s electrical conductivity by an amount proportional to the gases’ concentration. To access that electrical information, Güder and his team drew electrodes on paper (left panel) with commercial carbon ink and a ballpoint pen. The device’s conductivity was sensitive to the concentration of ammonia and trimethylamine (TMA), two water-soluble gases associated with food spoilage.
Güder and his colleagues used the sensor to monitor the spoilage of a chicken breast and codfish at room temperature and codfish in the fridge. As the meats decomposed, they produced ammonia, TMA, and dimethylamine, and the sensor’s response increased by up to a factor of 10.
The team integrated the paper sensor into a commercial near-field-communication tag so it could talk to a smartphone. Below a threshold amount of ammonia, the sensor responded to the smartphone; above the threshold, the tag was unresponsive. In a practical application of the sensor, a manufacturer would set the threshold to an appropriate concentration for the product, and the consumer would check the freshness with their phone.
The sensor does have a couple of drawbacks. Its response depends strongly on humidity and doesn’t distinguish among species of water-soluble gases. But the humidity is stable in some environments, such as packaged food, or can be monitored separately, and chemical additives are able to tune water’s sensitivity for specific gases. (G. Barandun et al., ACS Sens., 2019, doi:10.1021/acssensors.9b00555; thumbnail photo credit: Lance Cheung/USDA.