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First I'd like to apologize in advance, I'm loath to argue with people, 
but there are obvious errors in the below posting.

Firstly the industial strains of yeast used for beer,wine,bread and etc. 
are polyploidy.  I.e. they have multiple complete or partial copies of 
their genome.  And much like humans who suffer from extra copies of sex 
chromosomes, these yeast are sterile.  They cannot sexually reproduce. 
 And yes it is true that wild type S. cerevisiae do reproduce both ways. 
 But these are not available nor are they desireable for fermentation of 
food products.  This inability to observe their sexual state is why 
their  classification changes more often that some people change underwear.

Secondly I must reinterate that these strains will not bud under 
anaerobic conditions once they are depleted of stored energy reserves. 
They're grown in oxygen enrichment conditions that allow them to store 
up large amounts energy (I think its glycogen, but that seems wrong 
somehow) that they can use during fermentation and for budding.  Unless 
a little O2 is introduced they will eventually grind to a halt.  They 
are after all eukaryotic and require all the extra energy demands that 
come along with that status.  Prokaryotic organisms don't have these 
issues and can ferment forever.  Thats why one of these days I gonna 
experiment with using a nice gassy E. coli to produce CO2.

Thirdly, It really doesn't matter if you reuse all of the yeast cells 
because even if some are less active than others, it won't hurt anything 
if you have them there.  When dealing with spent yeast, more is better.
I'm going to try and re-lurk and prepare for my prelims.  

>In regard to yeast  "reproducing"; yeast reproduce sexually and asexually.  I could go in to all the details but suffice it to say that they won't reproduce sexually under anaerobic conditions, they are quite happy to bud however (asexual reproduction) and grow only somewhat slower under these conditions.  Believe me, this is what I do for a living.  The research lab I work in studies mitochondria and we have many yeast strains (Saccharomyces cerevisiae) that completely lack mitochondria, yet we are able to maintain them indefinately. THey will even mate, but not sporulate. 
>Secondly, I should clarify that the reason you don't want the sludge on the very bottom is that it contains deep stationary phase cells that will take longer to begin growing again.  Actually the layer of cells on the top of the bottom sludge is best if you don't have yeast that "float" in the medium very well.   
>What I do is swirl my flask just a bit to get the loose cells suspended, then I set aside about 1/2 cup of this and dump the rest.  you dont want to be keeping too many old cells IMO. eventually they will die. besides the cultures get nasty. 
>Also  I  affirm that bakers yeast are not related to any disease froming or tissue culture contaminating fungi. 
>I am a big fan of the yeast method of CO2 and would be happy to trade anyone some of my super yeast for cuttings. =)
>I also have great recipe for yeast media that you can make at home without lab grade chemicals. 
>Thanks for the opportunity to share my essoteric knowledge. 
>in Dallas