Harvesting yeast from fermenting beer

[FrankyT]

So I brewed a kolsch with wyeast 1007 German ale, which has now been fermenting away (slowing down now) for the past 7 days.

But I just got the sudden urge to brew a small batch (1 gal) of another beer using the same yeast.

Obviously it's possible to pull out a small amount of fermenting beer and build up a starter but my question is about how much beer will I need to pull out to build it up? It's only a 1 gallon batch of 1.040-1.060 so the pitch rate will be pretty low

Thanks!

 

[Gnomebrewer]

If you still have krausen, skim some off and use it. Otherwise rack the beer to secondary and use a bit of the yeast cake. Mrmalty.com gives the quantities that you need (for a 1gal batch, my guess would be about 50-100mL of yeast caked or a bit more for krausen).

 

[rhys333]

If using fresh krausen, I'd estimate a scant ladle-full would be more than enough for a 1 gallon batch. Probably way more than you need honestly.