Yes, We Have
You and Your Students!
Vicki Cobb, Education World Science Editor
How can you speed up (and slow down) the way bananas ripen?
7 very green bananas
1 very ripe banana
2 small brown paper bags (lunch-size bags work best)
a resealable plastic bag
Setting the Scene
A banana is a living system that changes or ripens over time. The ripening process is accompanied by changes in color, texture, odor, and, of course, taste. Whenever easily observable changes occur, you can be sure you have a good subject for scientific investigation.
Begin the activity by asking your students how they can tell an unripe banana from a ripe one.
Is ripening fruit a self-contained system (is it built into the banana itself), or is the rate of the change affected by the environment?
Do different atmospheres, produced by different packaging, affect the rate of ripening?
Those are the questions this activity will set out to answer.
Set up this activity as a class project; or students might work in small groups to carry it out.
Set up the bananas in their environments as follows:
Put two green bananas in a paper bag. Fold over the top of the bag several times to seal out the air. Mark the contents on the bag.
Put one green banana and the very ripe banana in the other paper bag. Fold over the top of the bag in the same way. Mark its contents on the bag.
Put two green bananas in a zippered plastic bag. Seal the bag closed.
Wrap one green banana in several layers of plastic wrap. Be certain it is sealed tightly at each end.
Leave one green banana exposed to the air.
Leave the fruit alone for four or five days to ripen. Do not open any of the bags during that period. On the fifth or sixth day, examine all the fruit.
Which fruit is still green?
Which is most yellow?
Which is most brown?
You can eat the ripe bananas or put them back and allow the experiment to continue for another day or two.
Behind the Scenes
Ripening fruit "breathes," or respires. That means that it takes up oxygen and gives off carbon dioxide. Oxygen is essential for the chemical reactions involved in ripening.
In addition, ripening fruit gives off another gas -- called ethylene. Not only is ethylene a product of ripening fruit, in some mysterious way it also stimulates the further ripening of the fruit. For that reason it has been called the "ripening hormone." (Hormones are chemicals produced by living things that stimulate cellular changes.)
Paper bags tend to keep in ethylene, but they are porous enough to allow oxygen (and ethylene) to pass through. As a result,
the green banana in the paper bag with the ripe banana should ripen most quickly. It has both oxygen and extra ethylene from the ripe banana.
the green bananas in the paper bag should ripen faster than the bananas in the plastic bag because they have more oxygen.
the banana exposed to the air has an unlimited supply of oxygen, so it will turn brown most quickly. You will notice that the side of this banana that rests on the counter will ripen more quickly than the other sides, because it has the most intimate contact with its own ethylene.
the banana that is tightly wrapped in plastic has no oxygen supply and should ripen most slowly. However, if all the air is not sealed out of the package, this banana will ripen first. It will have oxygen and will be in contact with its own ethylene.
Now you can see why supermarkets sometimes sell bananas sealed in plastic.
How does this experiment explain how one rotten apple spoils the barrel?
Article By Vicki Cobb
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