Eat as varied a diet as your pocket and tastebuds will allow. The truth is that much is not understood about the way in which foods interact with each other and with our bodies. So just vary it all up and forget about it. Picture: AP

London - A miniature sensor that changes colour when milk goes off has been invented by scientists. The tags, which cost just a fraction of a cent each, could be used to monitor the freshness of other foods, drinks and medicines.

Smaller than a fingernail, they contain a gel made out of a mixture of metals and chemicals, including tiny pieces of gold, a silver compound and vitamin C.

The gel starts out red but changes colour over time as its ingredients react with each other.

The rate of the colour change corresponds with the speed of growth of the food poisoning bug E coli at different temperatures.

When the sensor turns green, this means the bacteria have grown enough for the milk to be off. As the tag sits on the outside of the carton, there is no need to open it.

The patches can be tweaked to suit other bugs and other food products.

Their inventor, Dr Chao Zhang, of Peking University in Beijing, said: “The tag can be customised for a wealth of perishable products. This is what we think is the real breakthrough.”

Zhang, who wants to enter into discussions with food companies, added: “It is really inexpensive and safe and can be widely programmed to mimic almost all ambient-temperature deterioration processes in foods.”

The sensors were unveiled at the conference of the American Chemical Society.

The patch is programmed to match the growth rate of E coli in milk but could be tweaked to suit other bugs and other products. – Daily Mail