Hi Ed,
Thank you for the email. Sorry to take so long in getting back to you.
I received a similar question at the same time as yours (I assume in
regards to the same discussion), so this email will be copied to another
brewer.
Eukaryotic apoptosis (programmed cell death) is still a very active
field of research mostly within animal systems. The current research
continues to shed light on the function of yeast aging, senescence, and
autolysis.
That being said, yeast autolysis has long been studied by the brewing
industry because of it's detrimental affects on the flavor and
appearance of beer. Autolysis is caused by the action of intracellular
hydrolytic enzymes including proteinases and glucanases. Factors
including , available food source (conditions of starvation), yeast
health (stressed or low viability) temperature (higher), alcohol
concentration (higher), osmotic pressure (higher), and strain selection
all contribute to the onset of autolysis. However for most beers, these
conditions are fairly standard and you should expect similar levels of
autolysis. Therefore, cell density (amount of yeast in beer able to
autolyze) and time become the most important factors in controlling or
minimizing autolysis.
Generally, good yeast handling and fermentation practices eliminate any
problems associated with autolysis. However, as time (yeast in contact
with beer) increases, autolysis will set in. The rate will of autolysis
will depend on the above mentioned factors (temperature, alcohol
concentration, etc.) but in most situations, finished beer can sit in
the primary for a couple of weeks before it becomes detectable. An
increase in pH indicates autolysis is occurring.
Autolysis is generally not a concern with bottle conditioning because
the cell density is at a very low level (5-10E 5 cells per ml. versus
30-100E6 in the fermenter).
I hope that this helps.
Cheers,
Greg