Harvesting yeast from unused half vial

[perimeter]

I only pitched half of the vial of White Yeast California ale for my last brew because it was only two gallons. I immediately returned the other half to the fridge where it has been for a couple weeks. I read many pages on the yeast washing sticky about reusing yeast, my question is, can I propagate that yeast into more yeast so I don't have to buy new yeast?

 

[eastoak]

yes. make a half liter of wort and pitch the half vial. you could also use whatever yeast is in the bottom of the 2 gallon batch.

 

[perimeter]

Is there a way to do it without "contaminating" it with wort? Could I just add sugar water? How do the yeast companies do it without imparting any flavors?

 

[jhamilt]

Don't use sugar water, by propagating yeast in wort you sort of acclimate it to that environment. If you propagate the yeast by just feeding it simple sugars it won't be able to consume the more complex sugars found in beer. I'd suggest pale malt extract, I don't think you'll detect any changes to flavor.

 

[kingwood-kid]

The reason not to use sugar water is that it's nutritionally deficient. It supplies energy, but nothing else. You can get around this by adding a tiny amount of boiled flour. It's not an ideal solution, but it works. Feeding yeast nothing but sucrose doesn't cause them to lose the ability to ferment other sugars; it just causes them to temporarily turn off the genes responsible for making maltase. They'll turn them back on as needed. This will cause a minor delay, not a total failure.

I would follow eastoak's recommendation of harvesting the cake from your 2 gallon brew. It's already increased its population size to the point you want, and the yeast is much fresher than the vial in your fridge. Unless your 2 Gal batch was an imperial stout and your next batch is a blonde, you can skip washing the yeast if it seems like too much trouble. What settled out once before will settle out again.