THE apples of autumn are as crisp and juicy as ever. But most will be staying that crisp and juicy, if less aromatic, months longer because of a gas called SmartFresh that growers and packers are using to slow ripening.

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.