Chinese researchers have unveiled "smart tags" that change color as food spoils.

Described as "about the size of a corn kernel," the tags stick to food containers and go through the color spectrum as the food gradually deteriorates, CBS News reported. People will know if milk or other items have gone bad without even opening the container. The tabs are also inexpensive to make, costing about a fifth of a penny each. 

The tabs, which were showcased on Monday at the American Chemical Society's National Meeting in Dallas, were placed on a container of milk and tested through exposure to varying temperatures and bacteria.

"We successfully synchronized, at multiple temperatures, the chemical evolution process in the smart tag with microbial growth processes in the milk," lead researcher Dr. Chao Zhang, a scientist at Peking University in Beijjing, said in a statement, as quoted by CBS News.

The system not only shows when food is completely spoiled, but it also lets people know when it's halfway there. For example, a carton of milk expected to last 14 days would likely have a week of freshness left if it is halfway through the color spectrum.

The smart tags work through gold and silver nanorods, which are tiny and inexpensive and interact with vitamin C, acetic and lactic acids, and agar, causing their color to shift.

Going from red when completely fresh to yellow and green when spoiled, the tags change as a silver layer forms on the gold nanorods.

"Therefore, as the silver layer thickens over time, the tag color evolves from the initial red to orange, yellow, and green and even blue and violet," Zhang explained.

The tabs reflect both the food's natural deterioration in relation to its expiration date and any damage caused by fluctuating temperatures.

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