Use-by dates could be replaced by a colour-change sticker that predicts off food

Say goodbye to ever accidentally ruining a lovely cup of tea with a dash of off milk.

Soon we won't have to sniff the carton or locate the printed use-by-date. Instead a small tag stuck to the side of food packaging will change colour to warn us that the food inside has gone off, without us even having to unscrew the lid.

Food goes off because bacteria and fungus feeds on it and leaves waste products which are often toxic. Ingesting both the bacteria and its products can be harmful to humans.

According to the American Chemical Association, this new innovation is a cheap, simple piece of gel that can be stuck to food packaging. It works by responding to time and changes in temperature in a similar way to the food inside to predict bacteria growth in food.

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So when your to goes blue, don't even think about putting it in your tea, just chuck that milk out.

Researchers say it could be used on food, medications and other perishable products and can be programmed to respond to different triggers.

Here's the science bit: 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.

Chao Zhang, Ph.D., the lead researcher of the study explains: “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.”

The study was supported by National Natural Science Foundation of China, Hong Kong Research Grants Council and National Basic Research Program of China.