SmartFresh is just starting to be used effectively on produce other than apples. Cantaloupes and bananas treated with it are already being sold.

Officials of the manufacturer, AgroFresh, as well as apple wholesalers, say that 55 to 60 percent of the apples sold in the United States are treated with SmartFresh, a synthetic gas introduced in 2002. The cost to growers is about a penny for every pound of apples, and the treatment is most likely harmless to humans, according to pesticide experts.

The gas blocks the ripening effect of ethylene, a natural plant hormone that makes fruit ripen and eventually decay. A six-month-old Jonagold treated with SmartFresh is as firm as one stored for two months under traditional methods, and a Red Delicious stays crunchy for three weeks after storage, instead of one, according to James Mattheis, a postharvest physiologist for the federal Agriculture Department in Wenatchee, Wash.

Since the 1960’s growers have kept apples firm in warehouses by reducing oxygen and raising carbon dioxide levels in what is called controlled atmosphere storage. That has allowed some varieties to be sold all year, although they don’t keep their full flavor and can go soft and mealy in stores and homes.

With SmartFresh, apples retain their pleasant acidic tang, stay crisp and juicy, and remain as sweet as conventionally stored apples. The treatment also keeps the skins of some varieties, including Honeycrisp and Cortland, from turning greasy.

So far so good. But SmartFresh also changes apples’ flavor balance. By suppressing ethylene it decreases the esters that give ripe apples their fruity aromas, though it does not affect the aldehydes that impart a fresh greenish fragrance.

The reduction in aroma is less of a problem for varieties like Fuji and Granny Smith that have little fragrance to begin with. But other varieties treated with SmartFresh can disappoint.

“For a variety like McIntosh that depends on its unique aroma, if you take it away, you’ve gutted the apple,” said Michael Janket, a grower and professed “apple snob” in Willington, Conn., who does not use SmartFresh.

Scientists have found, though, that treated apples can become more aromatic once they are out of storage several weeks and the chemical’s effects start to wear off.

Treatment with SmartFresh has proved tricky for produce like pears and avocados, which, unlike apples, must ripen off the tree. Bartlett pears, for example, will stay hard and green until they rot if given too high a dose or if they are not harvested at the right time before treatment. Scientists are trying to figure out how to keep the fruit firm and increase storage and shelf life, but still allow for proper ripening.

One success story appears to be that of the L&M Companies of Raleigh, N.C., which started importing cantaloupes treated with SmartFresh from Central America last winter. Fruit that is to be treated can be harvested riper and still stay firm and free from decay, and the company will greatly expand imports this year.

AgroFresh officials said a large national retailer, which they would not identify, was selling treated bananas, which stay free of brown spots on store shelves for four days instead of two. But some studies have found that treatment can make the fruit blotchy, so that it is yellow and green outside but ripe inside. Researchers have found that SmartFresh can help tomatoes stay firm, but so far only small commercial shipments have been made, from Mexico to the United States.

On fruit other than apples, SmartFresh is being used more widely abroad: on substantial quantities of Asian pears in Korea, on South African and Chilean avocados sent to Europe, on Chilean kiwis shipped to Europe and on small lots of plums in France and Chile. In addition, scientists are researching the effects of SmartFresh on dozens of other items, such as mangoes, cherimoyas, chayotes and broccoli.

Consumers may soon face more trade-offs, with some items offering better quality and shelf life, and others perhaps suffering compromises in aroma. SmartFresh will make long-distance shipping easier, helping China, for example, send fresh apples to the United States.

While other methods of managing ethylene — including the use of controlled atmosphere, ethylene scrubbers in storage rooms and an orchard spray called ReTain — are common in the apple industry, SmartFresh has proved more effective because it shuts down internal ripening processes for long periods.

SmartFresh is the trade name for 1-methylcyclopropene (1-MCP), a gas structurally similar to ethylene. Edward Sisler and Sylvia Blankenship, professors at North Carolina State, discovered its effect on ethylene in the early 1990’s and obtained a patent on its use as an inhibitor in 1996. A formulation has been used since 1999 to keep cut flowers and potted plants fresh.

In July 2002 AgroFresh, a subsidiary of Rohm & Haas in Spring House, Pa., gained approval from the Environmental Protection Agency to use SmartFresh on apples, melons, tomatoes, pears, avocados, mangoes, papayas, kiwis, peaches, nectarines, plums, apricots and persimmons. It has been approved for similar uses in 26 other countries. (Its use does not have to be reported on labels.)

AgroFresh technicians apply SmartFresh to apples soon after harvest by combining a powder and water in a conelike device in storage rooms, releasing 1-MCP gas. Treatment takes one day and leaves no detectable residue. As a side benefit it often replaces diphenylamine, a moderately toxic chemical approved for controlling scald, a postharvest disorder.

Susan Kegley, a senior scientist at the San Francisco-based Pesticide Action Network North America, which advocates reductions in pesticide use, reviewed documentation from New York State officials after they approved the chemical. That review, she said, suggested that SmartFresh “is likely to be very low-risk to consumers.”

Although some treated apples are sold soon after harvest, most are released in winter or later. Many growers say that some large grocery chains now require that apples they buy after midwinter be treated in storage, and that some shippers claim that fruit has been treated when it has not. Treated apples sometimes are sold at farm stands and farmers’ markets.

“SmartFresh has allowed us to offer wonderful apples late in the season,” said Joseph Nicholson Jr., president of Red Jacket Orchards in Geneva, N.Y. “Customers say, ‘I can’t believe how good the Empires are in April.’ ”

But a study from 2002 to 2005 led by Stephen R. Drake, then a research horticulturist at the federal Agriculture Department, found that consumers liked SmartFresh apples four months after harvest no better than fruit stored in controlled atmosphere.

So far AgroFresh has devised procedures for about two dozen apple varieties, with soft apples like Jonagold benefiting more than naturally hard varieties like Fuji and Pink Lady. Research into SmartFresh has boomed, particularly on how an apple’s crunch can be maintained without a loss of fragrance. “That’s the tightrope that everybody’s walking,” said John K. Fellman, a postharvest physiologist at Washington State University.

Scientists are still investigating the impact of SmartFresh on nutritional content, but so far it appears to maintain antioxidant and vitamin levels.

Although AgroFresh officials say that their goal is to improve the quality of apples, not prolong storage, newspapers in England last year reported complaints that apples treated with SmartFresh and warehoused for 12 months were competing in the market with freshly picked apples. But while the marketing season for varieties like Gala and Jonagold is extended by SmartFresh, others, like Red Delicious and Granny Smith, have been sold year round since the advent of controlled atmosphere. In fact, since the introduction of SmartFresh, the average age of an apple at sale has declined in the United States.

Whatever its merits, fruit treated with SmartFresh may not appeal to those who dislike industrial agriculture and prize seasonal produce. In spring and summer, those shoppers can choose fresher seasonal fruits, newer apples from New Zealand or Chile (although those may have been treated, too) and local summer apples. For now they can buy organic apples, but AgroFresh officials say they may apply to the National Organic Standards Board to allow SmartFresh fruit to be labeled organic.