Originally Posted by mtarselli
Always throw away the bottom and top unless you are purposely trying to grow mutants to.change flocculation rates. The.middle yeast is your.normal healthy yeast. But remember you will change your attenuation as well and these strains should.only be.used 5-10 generations of cropping before you need a new starter
Man, I really hate that book, Yeast. So much bad information in it. At least it's got people thinking about yeast in a more detailed fashion.
Here, that advice isn't helpful. Taking the middle layer. Is applicable to conical fermenters. Presumably, the op only harvested the yeast once from a bucket or carboy and then he explains he shook the jar up. The harvesting and shaking event would redistribute any stratification of yeast based on flocculation tendencies. Further, it takes a lot to get DNA to mutate in Eukaryotes. So, unless the op is introducing chemical agents or blasting his beer with ionizing radiation he's unlikely to see any proliferating mutations. And even then he'd likely need to plate them and grow those colonies independently.
Now he could be unintentionally selecting for certain traits that could generate genetic drift but that's still unlikely to happen in any reasonable amount of time.
This is not to say that there aren't issues with over pitching washed yeast for several batches but these issues aren't because of mutations.