zeg

Recipes 

Results vary depending on many things. Better sanitation and gentler treatment of yeast will allow for more. Some types of yeast hold up better than others. The type of beer you're brewing is also a factor, since some types rely more on the yeast's flavor (etc) properties than others. Finally, how sensitive you are to maintaining the same properties and avoiding off-flavors will affect this.

There are a few things that happen. Assuming you're handling the yeast well enough that they're not being stressed, the yeast themselves will keep being yeast essentially indefinitely. In that case, the main problems are mutation of your selected strain and contamination by bacteria and wild yeasts.

Mutation is encouraged by the selective pressure you apply by your technique. Probably the biggest (or at least easiest to understand) factor here is the flocculation behavior---depending on how and when you rinse the yeast, it's common to collect the most (or occasionally least) flocculant yeast from the batch and then propagate only those. This has more profound effects than just yeast haze, as the flocculation also affects other aspects of fermentation, such as attenuation ability.

Contamination is sort of obvious, I guess. Since you're just harvesting whatever happens to be floating in the beer, over time organisms other than your desired yeast strain will tend to increase in number. Eventually they'll be numerous enough to affect the results, either through an obvious infection or by more subtly changing the flavors you get.

If you're not being very good to your yeast, then you can have other problems as well.

SMc0724

Recipes 

Sorry, which calc sites?

Pickettj

Recipes 

So is it not possible to take a wyeast pack or white labs vial and raise your own yeast? Keeping the yeast out of beer until you've cultivated a seperate colony for brewing with? In other words, have a mother colony and only use the sisters(?) for brewing.

__________________
On deck: Honey Wheat

Fermenter 1: You'll Be Bock

Fermenter 2: Empty

Secondary: Empty

In The Botlle: Autumn Amber Ale

In the keg: Nutcase Brown Ale

Pickettj

Recipes 

If this isn't possible then how do they fill the smack packs and vials to begin with?

__________________
On deck: Honey Wheat

Fermenter 1: You'll Be Bock

Fermenter 2: Empty

Secondary: Empty

In The Botlle: Autumn Amber Ale

In the keg: Nutcase Brown Ale

zeg

Recipes 

Yes, and this is exactly what the big labs do (at least, in principle). Keeping a strain pure and resisting unwanted mutations for more than a few generations is more complicated than just feeding the yeast, though. At that point you need to be growing pure colonies from single cells and various other microbiological parlor tricks. The yeast really aren't interested in making good beer, they just want to reproduce and evolve, so it takes a lot of effort to keep them in line.

(Even with the mother/daughter colonies, eventually the mother colony will mutate or become contaminated, at which point you've got to very carefully select a few individual cells and then figure out which of them is the one you want to use for your new mother, since the others may be gnarly mutants.)

Jukas

Recipes 

http://www.mrmalty.com/calc/calc.html
http://www.yeastcalc.com/

Pickettj

Recipes 

So are we talking microscopes and petry dishes here?

__________________
On deck: Honey Wheat

Fermenter 1: You'll Be Bock

Fermenter 2: Empty

Secondary: Empty

In The Botlle: Autumn Amber Ale

In the keg: Nutcase Brown Ale

zeg

Recipes 

Petri dishes and slants. Microscopes are perhaps helpful, but I don't think required. Also, more critical sanitation requirements---you probably don't want to be doing this in your kitchen or brewing area. (I don't have experience with this, but I happen to have been reading about the techniques lately.)

SMc0724

Recipes 

thanks.

mrmalty site says 10% viable;
yeastcalc says 45% viable.

That is a big difference. Is this just a wild a** guess, or what?

zeg

Recipes 

Probably the answer is "sort of." The data may be taken under different conditions, and the results almost surely depend on the strain in question.

My suspicion (which is mostly a WAG as well) is that those calculators are ok for relatively fresh yeast and get worse as it ages. (Partly this is obvious---variations have more time to accumulate as time passes, so it's easy to say that at packaging time, the viability is X. Your prediction gets worse as it degrades.)

I would be rather surprised if the 10% number is correct given the number of people who apparently succeed using old yeast. Even so, if you're near or past the expiration date, it'd be prudent to make a small starter with low-gravity (1.020ish) wort and then pitch that into a real starter.