Bananas ripen on the counter. Is that true of any unripe fruit or vegetable that you buy?
For better or for worse, no. Fruits fall into two categories: “climacteric” fruits, which can ripen off the parent plant, and “nonclimacteric” fruits, which cannot. The difference lies in their responses to a gas called ethylene, which occurs naturally in plants but can also be introduced in an industrial setting. Climacteric fruits respond to ethylene by producing their own ethylene. This in turn triggers accelerated use of oxygen and release of carbon dioxide and enzymatic actions that lead to softening and, in many such fruits, the conversion of starches into sugar—in other words, ripening. Storing unripe climacteric fruits in a paper bag with a ripe fruit (which is giving off ethylene) can help speed ripening because the bag traps some ethylene, concentrating it in the air around the fruit. Nonclimacteric fruits produce very little ethylene after they are harvested, and they usually don't convert their starches to sugars. This means that once they are severed from the parent plant, they don't get any sweeter. Climacteric fruits like bananas, tomatoes, and kiwis can be shipped in an unripe, firm, and resilient state and then ripened with ethylene gas before being displayed in the grocery store.
THE BOTTOM LINE: Some fruits, called climacteric fruits, can ripen on the counter (or you can speed their ripening by placing them in a paper bag with a ripe fruit), while others, called non-climacteric fruits, must be picked and purchased at the peak of sweetness.
WILL RIPEN ON THE COUNTER
NO MORE RIPENING