Two New Zealand scientists have invented a label for fruit which changes colour as the fruit ripens.

The sensor label, believed to be world-first technology, is already being applied to pears.

It detects aromatic compounds given off by the fruit as it ripens, changing the label through a range of colours.

This allows shoppers to choose to buy pears that are ready to eat immediately or firmer fruit for eating in a few days.

Further research to develop sensor labels for summerfruit, kiwifruit, avocado and melons has been proposed by Hortresearch scientists Dr Keith Sharrock and Dr Ron Henzell.

The two researchers, whose invention is being marketed as "ripesense", say there is nothing else in the world that measures ripeness in this way.

Much of their five years' research was state-funded through the Foundation for Research, Science and Technology.

Initially the label would be inside the transparent punnet, and "read" the ripeness of all four pears, but eventually it was hoped to create individual labels for individual pieces of fruit, such as avocado or melon.

Another Government funding agency, Technology New Zealand, also invested $96,000 in the commercial development of the labels.

Supermarket exectutive Stuart Johnston of Progressive Enterprises Ltd said the new label was trialed in three Woolworths and Foodtown supermarkets in Auckland in September.

The new technology is now being trialed on green anjou pears in Portland supermarkets in the United States.

Dr Sharrock said a key reason for developing the sensor labels was the difficulty shoppers had determining fruit ripeness.

Pears, unlike apples, needed to soften before they achieved their maximum flavour and shoppers often squeezed and damaged the fruit as they made their selection.

The clamshell pack, moulded to the shape of the pears, was developed to trap the aroma necessary for the sensor to function, but it also protected them from crushing and bruising, which made it easier for retailers to sell tender, juicy, ripe fruit.