How to tell when your milk really spoils, without having to smell it

Scientists in China have developed a way to tell if that carton of milk is really bad without the smell test.

They came up with what they’re calling smart tags, small gel-based tabs that stick to containers of food and change color when something has expired. The tags were tested using E. coli and milk, and changed color when the milk had spoiled.

“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 Beijing, told CBS news.

The small tabs are made of metallic nanorods (tiny particles of metal), vitamin C, acetic and lactic acids and agar. The nanorods change color, mimicking the length of time microbes grow on foods. They also react to changes in temperature that may affect a product’s freshness.

The tabs start red, then change to different shades of orange, yellow and green. Red means the food is fresh, green means it’s headed for the trash.

According to the American Chemical Society -- the tags were recently unveiled at its national meeting in Dallas -- the tags will cost pennies and can be customized for a variety of containers and substances, including medicine.

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