How do you know how much yeast you have?

[rockdemon]

Im getting very interrested in re-using and also harvesting yeast from my favourite commercial beers. It seems like a better and more fun method than bying new yeast every batch. It seems fairly easy but, how do you know how much yeast you have with this method? If i take the yeast from the bottom of, lets say 4 duvel bottles and make a starter, how will i know if this is a sufficient amount of cells? I really like this schwarzenegger guy, he makes it look so simple. It even looks like he makes the starter in the pot he boiled the wort in http://youtu.be/EVBIGf3Y4sg

 

[lthendricks8]

Depends on how big you make your starter. In a ~1.040 SG solution, yeast will reproduce until they achieve a certain concentration. This number is (somewhat, kind of, maybe, not really, but good enough for all practical purposes) a constant, assuming we're all starting with the 100 billion or so cells present in a WL/Wyeast pack - so if you're pitching in to a bigger beer, you need a bigger starter volume (which will mean a higher number of yeast cells).

If you knew the OG of the beer you wanted to brew with the dregs you want to propagate, you could estimate the size of the starter you needed using the Mr. Malty calculator or a similar tool and pitch the dregs of even one bottle in to a starter of that size (or a smaller size and then step that starter up, etc.). The yeast will then bud until they achieve that stable concentration. You can, of course, use more than one bottle - this does change the growth factor (i.e., the number of times you expect each yeast cell to bud), and also is a good precaution in case for whatever reason there are no viable yeast cells left in the one bottle you pitched in the "one bottle" scenario.