General Overview
A typical banana contains about 90 calories and is an excellent source of potassium, vitamin B6 and vitamin C. India is the world's top banana-producing country, with the United States and the European Union being the main buyers.
Browning
Unripened bananas are a light green color that begin to yellow as they ripen. A temperature above 57 degree Fahrenheit accounts for the ripening of the bananas and subsequent color change. The warmer the temperature, the sooner bananas will ripen and get darker in color. Thus, refrigerated bananas will darken much more slowly. Bananas left outside will begin to turn brown within a few days. This occurs because bananas contain an enzyme called polyphenol oxidase that reacts with oxygen and coats the banana with a sort of rust that accounts for the dark brown color. This enzyme is also found in apples, potatoes and pears.
If you want to put your already browned bananas to good use, check out this recipe for banana chips.
Slowing the Reaction
The reaction of the enzyme with oxygen can be prevented by treating the fruit with an acid such as lemon juice that counteracts the oxidizing process. Another way to prevent the browning of the banana is to heat it, which causes the enzyme to inactivate. Finally, one of the best ways is to reduce the amount of oxygen that comes into contact with the fruit by vacuum-packing it in an airtight container. Adding certain sulfuric preservatives can also slow the process of oxidation. For more tips on how to keep your veggies, and fruits fresh longer, go here.