A tag the size of a kernel of corn could tell consumers whether a carton of milk has turned sour or package of vegetables has spoiled without opening the container, according to researchers.

Food spoilage tags

Researchers say their "time–temperature indicators" can be programed to track the temperature history of a food or beverage. (American Chemical Society/YouTube)

They presented findings at the 247th National Meeting & Exposition of the American Chemical Society on Monday in Dallas, showing the tags, when attached to packaging, change colour over time, tracking the “temperature history” of a product.

"This tag, which has a gel-like consistency, is really inexpensive and safe and can be widely programmed to mimic almost all ambient-temperature deterioration processes in foods," said Chao Zhang from Peking University in Beijing.

The tags use metallic nanorods that indicate when food has been unduly exposed to higher temperatures, which could cause unexpected spoilage.

“In our configuration, red, or reddish orange, would mean fresh,” Zhang said. “Over time, the tag changes its colour to orange, yellow and later green, which indicates the food is spoiled.”

More reliable

"The colours signify a range between 100 per cent fresh and 100 per cent spoiled," the American Chemical Society (ACS) said in a report on the research.

"For example, if the label says that the product should remain fresh for 14 days under refrigeration, but the tag is now orange, it means that the product is only roughly half as fresh. In that case, the consumer would know the product is edible for only another seven days if kept refrigerated," the ACS said

The suggestion is that the tags could more more reliable than expiration dates if food is left exposed to higher temperatures before it's sold.

The tags take into account temperature fluctuations that would spoil a product before its expiration date, the ACS said in a video presentation on the technology.

The researchers started with milk, but say the tags can be customized for a range of foods and beverages, and even medicine. They’re also cheap, costing only a fifth of a penny each.