Robert Klein’s sticker turns color as an apple ripens.

Robert Klein’s sticker turns color as an apple ripens.

A thumbnail-sized white sticker that turns blue as the fruit to which it’s affixed ripens is one step closer to reaching apple growers and packing houses. The Redi Ripe sticker underwent field tests this summer and fall in a Wenatchee, Washington, apple orchard used for research. Developed by a retired psychologist and a University of Arizona agriculture professor, the product is expected to be available commercially next year.

“We want to have it ready for the next apple harvest, although it won’t be used in mass quantities,” said Dr. Robert Klein, cofounder of Redi Ripe. “But there will be enough for Washington apple growers.”

Guessing game

Klein’s sticker is being tested as a maturity indicator for growers, but ultimately he hopes to see a similar type of sticker used on apples and other fruit and vegetables in supermarket bins to eliminate, or at least diminish, shoppers’ uncertainty about the ripeness of the fruit they buy. He was compelled to invent the sticker out of his own frustration with the guessing game of selecting ripe fruit at the store, he said.

“I would go into a grocery store, pick up an apple or a peach or a melon,” Klein stated in an e-mail, “bang it on the counter, smell it, squeeze it, maybe even take a nibble out of it…to try and determine whether it was ripe, close to ripe, ready to eat.

“I figured, ‘There’s got to be an easier way to do this.’”

Klein believes the stickers could eventually work on peaches, pears, tomatoes, and perhaps flowers as well.

He was in Wenatchee in August to inaugurate a field trial at a research center run by the U.S. Department of Agriculture. Conducting the research is Dr. Jim Mattheis, a plant physiologist with the Agricultural Research Service. The Washington Tree Fruit Research Commission has funded much of Redi Ripe’s research for the last four years. Klein said Redi Ripe also has received funds from the Small Business Innovation Research program of the USDA Cooperative State Research, Education and Extension Service.


The stickers work by detecting ethylene, the gas released by the fruit during the ripening process. Ethylene is produced as simple sugars in fruit break down.

University of Arizona scientist Mark Riley, cofounder of Redi Ripe, said as the apples ripen and the release of ethylene increases, the stickers change color from yellow to blue over a period of about 24 to 48 hours. The intensity of the blue color increases with the quantity of ethylene emitted.

Klein said the Washington trials, which focused initially on Gala apples, are aimed at ensuring “that visible and readable color changes on the outer surface of the sensor sticker correlate with measurable amounts of ethylene release.

“This is the first time we correlated the sticker color change with other indicators

[in the field],” he said.


The weather and many other imponderables have to be considered, Klein said. Ethylene release from the apples was not what they had expected. “The color [changing] aspect is working, but it doesn’t start off with a big bang as far as ethylene production.”

Mattheis said he was doing the normal analysis for ripeness that growers and processors would do. “We’re trying to establish the relationship between the sticker chemistry and the maturation process.”

After testing the sticker on Gala, Mattheis planned to move on to Red and Golden Delicious, Braeburn, and Fuji. The trials were scheduled to be finished in October.

McFerson said it’s important to test the stickers in the field as well as the lab. “Although it might seem simple, you have to test all the components, the adhesive that secures it to the fruit, the chemicals, safety, the color, the workers. That needs to be done in the lab, but it’s a more complicated situation in the field.”