"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," explained Chao Zhang from Peking University in Beijing, China.

Use of the tags could potentially solve the problem of knowing how fresh packaged, perishable foods remain over time, he added. The real advantage, Zhang said, is that even when manufacturers, grocery-store owners and consumers do not know if the food has been unduly exposed to higher temperatures, which could cause unexpected spoilage, the tag still gives a reliable indication of the quality of the product.

The tags, which are about the size of a kernel of corn, would appear in various colour codes on packaging. Red, or reddish orange, would mean fresh. Over time, the tag changes its colour to orange, yellow and later green, which indicates the food is spoiled.

The colours signify a range between 100 percent fresh and 100 percent spoiled. 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," Zhang explained. The tags could also be customised for a variety of other foods and beverages.

The tags contain tiny metallic nanorods that, at different stages and phases, can have a variety of colours - red, orange, yellow, green, blue and violet, Zhang said at the national meeting and exposition of the American Chemical Society (ACS) recently.


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