It may be easy to know when milk is rotten, but there are some spoiled foods that are indistinguishable from their fresh counterparts. This is especially true for foods contaminated by bacteria such as E. Coli and Salmonella. Only once you’re battling food poisoning will you know that they were contaminated.

On the flip side, people tend to throw out food because the “best by” or “sell by” date has passed, despite the food still being safe to eat. In fact, Freethink reports that a full third of food produced in the US is wasted due to confusion over labeling.

In search of a solution, researchers from MIT developed a sensor that pierces food and then changes color if the food is contaminated. The sensor is about the size of a postage stamp and is made out of a silk protein extracted from moth cocoons into a microneedle mold. The benefit of silk is non-toxic and edible. The visible backside of the sensor has the letters E and C printed on them, representing E. Coli and pH. These letters are printed in a special bioink that changes color when the sensor detects contamination.

To test their sensor’s ability to catch food contamination, the researchers injected filets of raw fish with either E. coli, Salmonella, or a contaminant-free liquid. About 16 hours later, the “E” changed from blue to red on the fish containing E. coli, indicating that the bacteria had been correctly identified. The sensor on the Salmonella sample remained blue, meaning it was able to discern between the two bacteria. After 24 hours, the letters on all of the fish samples changed to red, indicating that they’d spoiled.

According to MIT, this is faster than other sensors that have been made to detect spoilage. Next, the researchers want to cut down on the amount of time it takes their sensor to sound its warning. On top of that, the researchers plan to test the sensor’s ability to catch other bacteria, such as Salmonella, and to work on other types of food, including produce.

If this sensor manages to make its way onto everyday food packaging, it could put a serious dent in the amount of food being wasted each year and in the number of cases of foodborne illness.