Wednesday, March 16, 2005

Apple #48: Bananas

I got into a discussion today about how bananas ripen. In my experience, the peel of an unripe banana is very thick and the fruit is skinny. In a more ripe banana, especially ones that are going brown, the peel is very thin and the fruit is much thicker. Thus it was my theory that somehow, the white stuff on the inside of the peel becomes part of the banana. Whether this is true, and exactly how this happens was a mystery. So I decided to try to find out (see the comments for a reader's answer).

Along the way, I learned some other interesting things about bananas.

(Photo from Wikimedia)

  • The banana plant (not a tree) is really a type of herb. It is in the same family with orchids, lilies, and palms.
  • When the blossoms become bananas, these are called "fingers." The bananas grow in rows, or clusters, called "hands."
  • There are usually about 15-30 fingers on one hand. One banana stem can grow about 7-10 hands, or anywhere from 100 to 300 bananas.
  • Bananas are 74% water, 23% carbohydrates, 1% protein, and 0.5% fat.
  • Vinegar and wine can be made from fermented ripe bananas.
  • In Hawaii, under the most optimal conditions, banana harvests have been reported as high as 75,000 pounds per acre. In 1992, Hawaii produced 12.0 million pounds of bananas.

The Ripening Process

  • Most fruits, including bananas, ripen in essentially the same way. The key to ripening is the release of a gas called ethylene.
  • Ethylene is released when something traumatic happens to the fruit -- like when it's picked, or the skin is pierced, or when a fungus or bacteria attacks it.
  • When the ethylene is released, this "turns on" a bunch of enzymes in the fruit. These enzymes do all kinds of things, including helping to change starch into sugar.
  • All kinds of stuff happens, actually, once the enzymes go to work. Chlorophyll gets broken down. Sometimes new pigments are released so that the fruit changes color. Acids are broken down so that the flavor changes from sour to neutral. Pectin is broken down, which "unglues" the cells, making the fruit softer (when too much pectin breaks down, the fruit becomes pithy).

Bananas through the stages of ripening
(Photo from A Piece of Heaven)

Back to the Bananas

  • Bananas are picked green. They're shipped in their unripe condition in refrigerated trucks. Refrigeration stops the ripening process. This means the bananas won't go bad before they get to the store, and they'll resist bruising during the journey.
  • Once the bananas reach the distributor, they're put into a warehouse and gassed with ethylene. The refrigeration stops the ripening process, but this external ethylene tells the enzymes in the bananas to go ahead and do their thing.
  • Once you have the bananas home, you can also keep them longer by putting them in the refrigerator. The peel will darken, but the ripening process will be slowed.
  • If you want bananas to ripen faster, but them in a brown paper bag. This keeps the ethylene they produce right next to the bananas and accelerates all that enzyme activity.
  • Bananas produce enough ethylene on their own, you can use them to ripen other fruits. If you put a banana in a paper bag and add to it, say, an unripe pear, the ethylene that the banana releases will also tell the pear to get to ripening.

Ethylene and Other Fruits and Flowers
  • Because of ethylene, that old phrase about one bad apple spoiling the whole barrel is actually kind of true. One apple that is wormy or has got some kind of fungus on it is producing ethylene like mad. Hence, the rest of the apples in the barrel will ripen like crazy, too. Apples are one of the highest producers of ethylene of all fruits.
  • Oh, and you might want to keep your bananas and apples away from cut flowers. Cut flowers are very sensitive to ethylene, which is known as both the ripening hormone and the death hormone.

So actually, the peel does not become part of the banana, which was my original theory. I didn't find anything that discussed the effect of ripening on the peel, but I'm going to assume, based on the other things I found, that the fruit gets bigger because of everything the enzymes are doing to it, and the peel might get thinner for the same reason. I think it's safe to say that the peel does not turn into fruit. (see the comments to this entry for one reader's answer to this question.)

How Bananas Grow and How Bananas Ripen
University of Hawaii at Manoa, Knowledge Master database,
General Crop Information on Bananas
Eastern Connecticut State University, Plants and Human Affairs course,
Fruit Ripening
Catalytic Generators LLC,
Ethylene FAQs,
Ethylene Gas


  1. As the starches are broken down into sugars in the ripening banana, the osmotic pressure in the fruit rises. This draws water into the fruit and away from the peel. The fruit grows and the peel shrinks. Science!

  2. Wow, thanks for that explanation, oncobyte! I knew there had to be a reason why the peel gets thinner. And you're right, science sure does help sometimes (see


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