Games Yeast Play

Okay, admit it. At least once when you were a kid, you grabbed the last piece of pizza and put it on your plate while you were still only half-finished with another piece. It seems that some fungi have figured out a more sophisticated way to place food off limits to others. There's a cool article (subscription required) in New Scientist (Dec. 23-Jan 5 2007, page 32) about the yeast used in brew-making. Like lots of critters, yeast feed on sugars:

Most organisms that generate energy from sugars use oxygen to break the molecules down into water and carbon dioxide.

The yeast in question, however, turns the sugar into alcohol. The problem with that is that the organism only gets 1/18th the energy it would get from the latter process as it would from the more common one. Why would evolutionary pressures allow for the adoptions of such an inefficient process? The answer to this riddle was figured out by John Aris of the University of Florida. It's ingenious.

First, alcohol is poisonous to most other microbes. The production of alcohol allows the yeast to fend off competition. Once it runs out of sugar, the yeast turns the ethanol into acetaldehyde. It uses available oxygen to break this down further to reclaim energy, according to the article.

[The yeast] turns sugars into a poison that kills off any rivals, and then feasts on the poison.

The trick for brewers is to make sure there's not enough oxygen around to complete this process. So seal those bottles of home-brew up tight. Now you know why.