Originally Posted by jackandstacey
Is it supposed to be hard. If not how do you get all former batch out? Can it only be used for the same type of beer again? I just kegged the first batch I ever made. There was a layer of white goo at the bottom. Did I do something wrong?
That layer of white goo is "trub". It's spent yeast, coagulated proteins, hops debris, etc. Sometimes it's fairly compactly packed, sometimes not. Some yeast strains will form a very tight yeast cake, some will not. Generally, the longer it sits, the more firm the yeast cake will be. I like to move the fermenter to where I'm racking the night before, if possible. That way the trub can resettle to the bottom overnight.
When I rack, I just siphon from above that yeast cake. Sucking up a little of the yeast won't harm a thing, as it'll just settle to the bottom again in the bottle or keg.
If you're reusing the yeast cake, the general instruction is to go from a lower gravity beer to a higher gravity beer, and lighter color/flavor to darker. You could easily put a big stout with an OG of 1.080 on a 1.040 cream ale yeast cake, for example. But I wouldn't do the reverse- you'd have darker color and flavors, as well as some stressed yeast.
I like to wash my yeast and save it for reusing. I have many jars of yeast in my fridge, saved from prior yeast cakes. We have a thread on "yeast washing" that can really help if you'd like to try that.