Fruitbowl Science

Bananas and ethylene

This time of year I am reminded about a good recipe for Banana bread that I make.  Sometimes Infamous buys a bit too many ‘naners at the warehouse store and ends up with some overripe ones.  These are fragrant, soft, sweet and perfect for bread but a bit too ripe for fresh eating.  This would be a good time to discuss ripeness and how a plant   hormone affects everything from harvest, storage, delivery, and shelf life.

The main hormone we are talking about here is Ethylene.  It is a gas which is a bit unusual for a hormone.  Ethylene’s molecular structure is quite simple. It has 2 carbons hooked together with a double bond and a couple of hydrogens on each end.  As a gas, it is emitted by ripening fruit.  Usually, hormones and a lot of other things in biologic systems have a feedback mechanism which detects when a lot of the hormone is around and tells the organism to slow or quit making it for a while.  Ethylene is strange in that it not only has no feedback mechanism, it actually induces its own synthesis.  This has the effect of making more, which has the effect of making even more.  If this sounds like a runaway freight train, you should see my bananas. 

The plant begins to make a little bit of ethylene to do several things to its fruit.  It softens it, sweetens it, changes its color and makes it aromatic.  And then things go horribly wrong.  Because, you see, it just won’t stop!!  The first ripe fruit sends all these chemical signals (as a gas) next door to another fruit which makes that one ripen.  All of a sudden, there are ripe fruits all over the plant and all over the grove and all over the hillside and, well, you get the picture. 

Besides encouraging synchronous ripening in fruits, ethylene has many uses by man.  Orchardists have used ethylene for years to ripen under-ripe fruits.  They can pick the fruit while it is firm and slightly under-ripe and ship it that way.  The tougher consistency makes a better product since it does not bruise as easily.  Then, when the fruits are either enroute or near the destination, they can be ripened by ethylene gas to look like tree ripened fruit. 

There is a slight catch to doing that trick though.  Although many plants can take stored starches, for example, and make them turn into sugars when exposed to ethylene, there is a limit to how much stored starch there is in a detached fruit.  When most of that starch has been cleaved into nice tasty sugars, the fruit cannot get any more sugar from the tree.  If the fruit wants to really ramp it up and make the most delicious sugary fruit possible, it can only do that by getting extra sugar from the branch it is growing on.  This is why, in certain fruits like apples, you can taste the difference in tree ripened versus crate ripened fruit. 

Back to the bananas.  Some fruit has the ability to store tremendous amounts of carbohydrate as starch.  Bananas were originally a rather non-sweet starchy fruit.  If you have ever cooked Plantains, you have tasted an old-fashioned banana.  People have selected and bred and selected some varieties of banana to make it the sugary fruit we know today.  A banana can store so much starch, that it can make a lot of sugar when the ethylene train gets rolling.  That is why, for this fruit, they ripen as well in your fruit bowl as they do on the tree.  A banana does not need the tree to supplement its sugar content.  That is a convenient thing for a fruit that gets very soft and mushy when it ripens.  They would not ship very well in the mush stage. 

If you ever get fruits that you need to ripen, put them with a ripe fruit.  The ethylene will do the trick.  Alternatively, if you have fruits you would like to slow down, give them cool temperatures and good ventilation.  Get the ethylene out of the container.   Or you could make my banana bread recipe…

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1 Comment

  1. Theron said,

    December 29, 2009 at 1:14 PM

    Very entertaining and well written.


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