MadSci Network: Botany
Query:

Re: how do bananas rot?

Date: Wed Mar 19 10:25:33 2003
Posted By: Joseph E. Armstrong, Faculty, Botany, Illinois State University
Area of science: Botany
ID: 1046195145.Bt
Message:

The short answer bananas rot like anything else.  It's a rotten world 
around us, and that's not a political commentary.  Decomposers, primarily 
fungi and bacteria, are a necessary part of ecosystems.  Organisms produce 
metabolic wastes, they shed aged, used parts, and ultimately they die.  
Rather than allow all these perfectly good organic molecules to pile up, 
decomposers are adapted to using these "wastes" for their raw materials 
and energy source. Decomposer fungi, molds and yeasts, have enzymes that 
can digest the molecules composing the banana.  Molds can produce millions 
of spores, and a constant rain of spores sprinkles down upon us and other 
living organisms, but nothing much happens as long as the organism is 
alive.  But when the organism dies, and is not using energy to maintain 
its cellular integrity, then the fungus has the opportunity to exploit 
this island of resources that it has landed upon.

The banana had done what it was designed to do, sort of.  The real 
function of a banana is to smell and taste of tropical fruity goodness to 
attract an animal seed disperser by appealing to their senses with smells 
and tastes that mean nutritious food.  The banana flesh is loaded with 
starch and sugar to reward and please the animal, and the large, hard 
seeds are dispersed either discarded as the animal eats around them, or 
after they pass through the GI tract.  Your bananas don't have large, hard 
seeds because they are a sterile mutant, produced by a clone of sexless 
plants. You can find the tiny, undeveloped abortive seeds if you look.  
But the banana doesn't know it has no viable seeds, no offspring, so when 
the fruit reaches maturity, the flesh softens by enzymatically loosening 
the cell wall material, starch is converted into sugar for a sweet taste, 
and a pleasing fruity odor is produced anyways.  The banana, like other 
fleshy fruit, is plant tissue whose function is to be consumed, and if not 
eaten by a seed disperser its ripe and ready for rapid decomposition by 
fungi and bacteria.

A more interesting question is why do food spoilage fungi make food look, 
smell, and feel icky, a technical term for unappetizing?  Fungi are in 
competition with big animals like ourselves for this food resource, and if 
fungi aren't obnoxious when present, they'd get eaten and digested by 
animals who didn't notice the mold.  So fungi that make their presence 
known and who make food look, smell, and taste bad will win the 
competition more often, leaving more offspring, than fungi that do not 
conspicuously mark their territory.  This is an example of how evolution 
works. Our senses are either repelled by moldy food or not, and the 
differing reproductive success of these molds (icky food molds leave more 
offspring more often)have shaped their biology to win the competition for 
islands of food by rendering the food unappetizing.  Since these islands 
of food are here and there, and short-lived, fungi must be capable of 
dispersing large numbers of offspring widely and quickly.  Of course such 
quick growth is important because the sooner the mold announces its 
presence the more likely it is to win the competition by avoiding 
consumption.  Whenever you throw out moldy food the fungus has won!  

There is a terrific video called "The Rotten World Around Us."  It has 
some wonderful time lapse photography of rotting fruit.  



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