Re: Why do the yeast die???

I think this is a very interesting question, so I did a little research on it.

I suspected that there would be a number of different reasons ethanol would 
kill yeast (inefficiency of anaerobic respiration/interference with cell
membrane/interfere with enzyme functions). 

 Respiration is the release of energy, usually from glucose, in the tissues of
all animals, green plants, fungi and bacteria.

The most efficient form of respiration is aerobic respiration: this requires
oxygen. When oxygen is not available, some organisms can respire anaerobically
i.e. without air or oxygen. Yeast can respire in both ways. Yeast gets more
energy from aerobic respiration, but when it runs out of oxygen it does not die.
It can continue to respire anaerobically, but it does not get so much energy
from the sugar. Yeast produces ethanol (alcohol) when it respires anaerobically
and ultimately the ethanol will kill the yeast.


I got this information from:
http://www.purchon.co.uk/science/respire.html

Perhaps more important is that ethanol will destabilize the cell membrane of
yeast, which will eventually lead to lysis and death.  

For a very difficult to read article on that topic see:
Factors influencing deactivation of yeast cells exposed to ethanol.
                          J Appl Bacteriol. 1987 Aug;63(2):153-64.

If the yeast are producing the ethanol via fermentation, ethanol accumulating
inside the yeasts can also be toxic.  Presumably it interferes with enzyme
functions which are necessary for yeast growth.  Also, ionic compounds (salts)
are not soluble in ethanol.  High ethanol concentration could cause
precipitation of some cell salts.
Reference:
Influence of the rate of ethanol production and accumulation on the viability of
Saccharomyces cerevisiae in "rapid fermentation".
                          Appl Environ Microbiol. 1976 Feb;31(2):158-62.

Erin