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Keeping a (color-coded) tab on spoiled food: Freshness tag device saves your stomach

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Spoiled food may soon be easy to spot, even in unopened containers, through the use of new color-coded tabs. 

Expiration dates are uncertain. There is also confusion over whether or not food can safely be consumed after the printed date. Because of this, many people will throw away any food that has reached its printed date, leading to large quantities of food being wasted. 

A new tab, developed by a team of researchers in China, turns different colors, indicating the freshness of the food inside a package. Chao Zhang of Peking University, lead author of the paper accompanying the invention. 

Time-temperature indicators (TTI's) have been used to keep a record of how long a food product has been exposed to temperatures where bacteria and other microorganisms could thrive. Current devices are hindered by high costs, and are not able to carry out a wide variety of tasks. 

"[Our] TTI can be specifically tailored and thus used to track perishables, dynamically mimic the deteriorative processes therein, and indicate product quality through sharp-contrast multicolor changes. The flexible programmability of our TTI, combined with its substantially low cost and low toxicity, promises a general applicability to each single packaged item of a plethora of perishable products," researchers wrote in the article announcing the results. 

Plasmonic nanocrystals are used as the main building block of the new tags. These new tags record the temperatures food has experienced, and a built-in processor models what is going on inside the package. The result is indicated by a series of dramatic color changes on the device. 

In the model designed by Zhang, the freshest foods are indicated by a red tag. This color changes to orange, then yellow, indicating a questionable product. When the indicator determines the food should be thrown away, it turns green, then blue.

Tiny gold nanorods are normally red, but can turn different colors. Zhang and his team found a way of directing those color changes by environmental conditions. 

"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," Zhang said

Medicine, as well as food, could benefit from use of these new freshness indicators. 

The paper was presented at the 247th National Meeting & Exposition of the American Chemical Society (ACS), in Dallas, Texas.  

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