The tag, which would appear on the packaging, also could be used to determine if medications and other perishable products were still active or fresh, they said.

This report on the color-changing food deterioration tags was presented today as part of the 247th National Meeting and Exposition of the American Chemical Society (ACS).

The meeting, attended by thousands of scientists, features more than 10,000 reports on new advances in science and other topics. It is being held at the Dallas Convention Center and area hotels.

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 lead researcher of the study. Use of the tags could potentially solve the problem of knowing how fresh packaged, perishable foods remain over time, he added. And a 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 researchers developed and tested the tags using E. coli in milk as a reference model. We successfully synchronized, at multiple temperatures, the chemical evolution process in the smart tag with microbial growth processes in the milk, according to Zhang. The tags could also be customized 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 colors: red, orange, yellow, green, blue and violet, Zhang explained. “The gold nanorods we used are inherently red, which dictates the initial tag color,” he said. “Silver chloride and vitamin C are also in the tags, reacting slowly and controllably. Over time, the metallic silver gradually deposits on each gold nanorod, forming a silver shell layer. That changes the particle's chemical composition and shape, so the tag color now would be different. 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.