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I'm building up a starter from an old 750ml starter of lager yeast I've had in the fridge for 6 months. I first made a 2l starter from the yeast sediment. This took about 8 hours to get started (on a stirplate) and then fermented out in about 30 or so more hours. I then refrigerated overnight, decanted again and made a second 2l starter, which had a krausen within an hour, and is still fermenting now.

The beer will have an OG around 1.051, the yeast is Wyeast 2042 danish lager, and I will be pitching cold and fermenting around 9c.

My difficulty is that I have no way to estimate the original number of cells. My plan is to do a third stage 2l starter, but I'm worried that I might still be under-pitching. Should I do a fourth stage just in case?

This site should have an answer for you, it also has a calculator so you could re-trace your steps based on age to figure it out

http://www.mrmalty.com/pitching.php#s5" onclick="window.open(this.href);return false;

http://www.mrmalty.com/calc/calc.html" onclick="window.open(this.href);return false;

Only potential issue I see, if you are doing the starters at room temp instead of lager temp, you will be selecting for the hogher-temp yeast. Not a good this when you're building it up to use at lower, lager temps.

Brian: Thanks - I've been using the calculator but I hadn't seen the re-pitching info.

I've just discovered that according to the calculator, I would need 31% viability to ferment a 1/2 gallon batch, and since my starter was about 1/2 a gallon I essentially did that. I'm thinking that maybe it would be OK to assume that I started with ~31 billion cells...

[EDIT] I did my starter at room temp - when I switch fermentation type to "ale" Mrmalty says 14 billion.

Liam: I'm sure that would happen eventually - I wonder how many batches it takes? I did my starters for the first batch with this yeast at room temperature and it went fine.

FWIW I've always done my lager yeast starters at room temp. On another note, i'm on an english ale kick tonight so your peculiar is up next

http://www.brewersfriend.com" onclick="window.open(this.href);return false;

On their pitching rate calculator they have a slurry section where you can sort of estimate how big of a starter you need. I do fine it hit and miss though, since I don't know the density of my slurry.

Chris: Thanks, that looks like another good site, and I've never seen it before.

I guess that in the end, although there are great tools for estimating the increase of cells per liter of starter and the decline of viability over time, it will always come down to guessing the original density. I'll probably feel more comfortable with it after I've been successful at it once or twice.

Brian: I hope you enjoyed it!