An ink that changes its colour when exposed to oxygen could help shoppers decide if their packaged food is fresh.

Oxygen is the enemy of fresh food because it causes food to degrade and bacteria need it to grow. Hence, much of today's packaged food comes in a protective atmosphere of gases such as nitrogen - with oxygen almost totally removed.

Andrew Mills and David Hazafy at University of Strathclyde, UK, have developed an irreversible solvent-based blue ink, which upon activation with UV light, loses all its colour and becomes oxygen sensitive; it will only gain its original colour upon exposure to oxygen.

The major advantages of the oxygen ink over most of the traditional methods for detecting oxygen are that it is cheap and easy to use, especially as it relies on a colour-change detectable by the human eye, said Mills. Solvent-based inks such as these are also easier to print on the common polymers used in food packaging, added Mills.

'An oxygen-sensitive ink, such as ours, could be used to show if the modified atmosphere remains intact, first at the packaging factory, then at the supermarket, ensuring faulty, damaged or tampered packages are not sold,' said Mills. The customer themselves will be able to pick up the food and tell instantly if it has be spoiled.

Sarah Corcoran