You can forget what your mother taught you about how to buy fruit.

A University of Arizona professor has invented a sticker that can tell consumers if a fruit or vegetable is ripe.

The stickers will be available to growers next year, and should make their way to supermarkets within two to three years, said their inventor, Mark Riley, a UA assistant professor of agricultural and biosystems engineering.

Growers and grocers throw out thousands of bushels of fruit each year because it ripened faster than it could get to market or be sold, Riley said. And consumers, with no simple way to tell whether fruit that looks good on the outside will taste good on the inside, often buy peaches, pears and melons they can't eat because they're under- or overripe.

"Right now, picking fruit is more of an art than it is a science," Riley said.

A marker on Riley's RediRipe stickers detects a chemical called ethylene gas, which is released by fruit or vegetables as they ripen, he said. As that happens, the sticker turns from white to blue. The more ethylene gas the fruit produces, the darker the blue, Riley said.

The color shift is not instantaneous once a sticker is attached. It takes about 24 to 48 hours, depending on how fast the fruit is ripening, Riley said.

And there are still bugs to be worked out: The stickers do not change color to reflect an over-ripe or rotten piece of fruit. Also, not all fruit produces enough ethylene to be detected by the the sticker, said Jim McFerson, manager of the Washington Tree Fruit Research Commission, a growers' research group that helped sponsor the research.

"There is still a lot of research to do," McFerson said.

Each sticker is expected to cost growers and grocers about a penny, Riley said.

The Arizona Board of Regents gave Riley permission to spend half his time in academia and half working on the sticker with his company, RediRipe, LLC, which he co-owns with New Mexico entrepreneur Robert Klein.

There is a patent pending for the stickers through the university, Riley said. When RediRipe goes to market, the university will keep the patent and the company will license the product.

Research on ethylene's use in fruit ripening began in the 1940s, and the gas is used to ripen fruits and vegetables in storage, Riley said. His stickers have been in the works for about four years; he has done multiple small field tests — including at an apple orchard in Willcox — and plans a much larger field test this fall in Washington.

The stickers could be used almost as a warning system, because ethylene production in one fruit can stir ripening in fruits nearby, Riley said. An indicator would be especially valuable to growers and grocers because the vast majority of fruit is now stored for long periods of time before being shipped to stores, Riley said.

Many consumers don't buy certain types of produce because they can't tell what's ripe just by looking, one grocer said.

"About the only thing people know how to purchase is a banana," said Jack Armstrong, senior produce buyer for Bashas' Supermarkets.

For consumers, the possibility of taking the guesswork out of produce purchases is at least worth giving the stickers a try, said Tucson resident Celeste Walker, who was shopping Tuesday at the Bashas' at East Camp Lowell Drive and North Swan Road.

"A piece of fruit might look good on the outside, but you don't know about it on the inside," Walker said.

Armstrong, who has worked in the fruit-buying business for 36 years, has seen some items that test for ethylene in stores before, but nothing that goes right on the fruit.

"This is a very viable product," Armstrong said. "All this product is doing is taking Mother Nature and letting her tell you what she's doing. The guesswork is gone."