how can an un-ripe banana cange from un-sweet to sweet?

Unripe bananas have about 25% starch and only 1% sugar. Natural enzyme action converts this high starch content to sugar, so ripe bananas have a 20% sugar content.
http://www.foodreference.com/html/fbananas.html
Bananas contain about 74% water, 23% carbohydrates, 1% proteins, 0.5% fat, and 2.6% fiber (these values vary between different banana cultivars, degree of ripeness and growing conditions). In an unripe banana the carbohydrates are mostly starches. In the process of ripenening the starches are converted to sugars; a fully ripe banana has only 1-2% starch.
As the starch in green bananas is hard to digest, they must be cooked to be used by the body. As bananas ripen, the starch turns to sugar. Ripe bananas have the same energy value as green ones, but it is in sugar. Energy in ripe bananas is easily used by the body without cooking.
Ethylene gas can be applied to bananas to start the ripening process and to assure evenness of ripening. Bananas also produce ethylene gas naturally. During the ripening process, pulp temperatures should range from 58 to 64 degrees F, relative humidity should be controlled, and there should be adequate air circulation to ensure high quality fruit.
Bananas are harvested green. Unlike vine-ripened tomatoes, vine-ripened bananas don't taste very good. If bananas turn yellow on the plant the starch in the banana never turns to sugar and they become bland with a cottony texture.
The darkening of ripening bananas, proceeding from yellow, to brown, to black, is mainly due to large amounts of serotonin (an important human neurotransmitter), which is produced from tryptophan in banana peels. While this property would seem to implicate bananas as a natural antidepressant, such is not the case.
Upon ingestion, serotonin is immediately broken down by enzymes in the stomach (particularly monoamine oxidase). Due to its high melting point (213 deg C), serotonin is unsuitable for smoking and decomposes into toxic gases (carbon and nitrogen oxides) during combustion. Additionally, it cannot cross the blood-brain barrier.
I hope this can help your understanding.

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place the banana to a box which contain ethane
this is the chemical method to make the un-ripe banana to ripe

2007-10-31 23:08:25 補充：
ethene 才對　ｓｏｒｒｙ