I’ve made my first yeast starter from harvested yeast and I’m wondering how I can tell if it is viable. Should it be bubbling wildly or only a little bit? Should I see a growth in the actual volume of what’s in my flask or do the cells just multiply without making much of a visible difference? Thanks for any help.
If you leave it alone, you should see a krausen form. If you are shaking it constantly, you will probably just see a few bubbles forming on the surface. If you’re using a stir plate, you will not see much sign at all. If you made 1.030+ wort and added viable yeast, you will have growth. Put it in the fridge a couple of days before your brew and the cake will be bigger than when you started. If the starter is small enough, 1L or less, you can pitch all of it into a 5 gal batch. Any bigger than that, I always decant first.
Thanks, it looks like I have viable yeast. The cake at least doubled in size in a day. I shook it up and poured a bunch of it into a half gallon test batch just to see how it turns out. I didn’t use any sanitary techniques so for my next batch I’ll be a good boy and should be able to start harvesting permanently.
You can make one purchase of yeast last through 120 batches of beer by harvesting. If each pack of yeast was $5, that’s a savings of $600!! Just be sure when you’re harvesting that you are sanitary so you can save the yeast. Harvested yeast will keep for a long, long time. I just brewed up a schwarzbier with harvested yeast from a lager I did months ago. The beer is long gone but the yeast remains 
Happy Harvesting
Cheers!
