Does viability really matter?

[BaldApe]

Hear me out. I'm not saying that it doesn't matter if you have yeast that's mostly dead.

What I want to know is, if I have a certain number of vigorous cells, and I grow them up to a number I need, do I really care about the viability of the original source?

Specifically, I have some saved yeast from about 6 weeks ago. I don't expect wonderful viability. But even if many of the cells are dead (I do have a hemocytometer and microscope), can't I start with what I have and wind up just fine?

 

[Hoppy2bmerry]

Yes just get it going in a starter so you get the count up to pitching rates.

 

[IslandLizard]

That! ^

A harvested slurry that's been refrigerated for 6 weeks should be quite viable, at least 50-70% viable, definitely not mostly dead, unless it was abused.
You can likely pitch some of it as is. Or, depending on estimated viable cell count, put an ounce (or half an ounce) of it in a 1-2 liter starter, you should end up with 200-300 billion viable cells, ready to go.

Mr. Malty
BrewUnited's Yeast Calculator

 

[Carolina_Matt]

I'm not an expert on yeast - I just started re-using yeast a couple months ago. My process is to pour 16 ounces of the slurry into a mason jar, then use that for two future batches by directly pitching it into the wort. I don't mess with starters. I really have no idea how many cells there are, etc.

Anyway, last Friday I pitched 8 ounces that had been in the fridge for 6-7 weeks. Based on the bubbling, fermentation was a little weird. There was very little bubbling the next morning. By day two there were bubbles every 5 seconds. By day three, it was done. I figured the yeast was dead and I waited too long. But I took a sample and it was at 1.010-1.012 so I guess it did its job. I'm interested to taste it next weekend and see how it is.