Ripe as read

Supermarket customers may no longer need to squeeze and unintentionally bruise fruit as labels with new intelligent ink that detects ethylene could check the ripeness for them.

Prof Andrew Mills at Strathclyde University, is developing the ink to detect the ripening agent in a project that follows the creation of a water-based, oxygen-detecting ink.

Mills, the principal investigator, said: ‘Ethylene is released by some fruit and vegetables, particularly stone fruits such as peaches and passion fruit, as they ripen. It is a ripening hormone, so as it is released it builds up and will make the fruit ripen even faster.

‘Many shops are promoting ready-to-eat fruit that is ripe enough, so if you could have stickers on a packet of fruit or on the fruit skin itself that would actually tell you the state of the fruit, which would be useful.’

The technology will be similar to an ethylene sensor developed at the Fraunhofer Institute for Physical Measurement Techniques in Freiberg, Germany.

The main differences however, are that instead of using intelligent or functional ink, Fraunhofer’s detector comprises an infrared radiator and was developed for use in warehouses of fruit wholesalers.

‘If you are thinking of a sensor, suddenly you are thinking of a costly instrument,’ said Mills. ‘The nice thing about an ink is that it is so inexpensive — we are looking at labels that you could stick on fruit, so the inexpense would be its major advantage.’ Fraunhofer’s device costs about £788.

‘Also, it makes it accessible to the customer, who can just look at the sticker instead of squeezing the fruit.’

Mills plans to develop ink that will show a ‘dramatic and striking’ colour change to indicate the level of ripeness. For example, a dark red would indicate a very ripe fruit and a pale yellow would indicate one that is unripe. While the ink might go through varying shades of colours in between, it is the extremes that the researchers are most interested in.

He likes the idea of a small square of ink indicating the level of ethylene in a fruit package but other suggestions include a functional ink to be invisibly printed in the bar code that appears when swiped at the supermarket checkout.

‘The levels of ethylene you typically find in food packages is about 0.1 parts per million (ppm). Once you get levels much above that then the ripening has begun and once the process starts, the levels of ethylene shoot up.

‘So it may be that we do not really have to pick up on that very low level. It may be perfectly fine to know when it is up at 10ppm or 100ppm and by that time you know that all the fruit is ripe. It only needs to provide a rough guide of where it is,’ said Mills.

Like the oxygen-detecting ink, the ethylene sensors will be made using a UV light-absorbing semiconductor, which the researchers hope to ‘switch on’ with a burst of UVB light. The ink remains stable in ambient light, which contains UVA but no UVB light. It only starts to sense the gases when the fruit supplier activates the light in a controlled manner, enabling them to know exactly at which point the fruit was unripe.

Mills and his team also intend to make changes to the existing oxygen indicators by making them solvent-based rather than water-based, and intends to do the same with the ethylene detectors.

‘Water-based ink is OK but not very good for printing on packages, whereas solvent-based inks will usually print on most of the polymers that are used in packaging,’ said Mills.

‘Also, we want to print on the inside of packages so there must not be any question of this ink rubbing off.’

Harmless solvents, such as ethanol and acetone, could be used in the inks. Already used in the packaging of sweets, they evaporate as soon as they have been warmed up in the printing process.

The Strathclyde team will work with industry partner Sun Chemical, an ink manufacturer that will help with issues such as the suitability of ink for printing on an industrial scale.