Yeast Viability - How is this calculated?

[mthelm85]

I was looking in to cell counts per volume of slurry and have run in to a problem when using the Mr. Malty Calculator and the info I found at Wyeast's website about yeast harvesting.

According to the Wyeast site a given volume of slurry that contains about 40% yeast solids will contain roughly 1.2 billion cells per ml. Based on a pitching rate of 1 millions cells/ml/*P that would mean that for a 1.059 beer you would need to pitch 171 ml of slurry.

If you use the Mr. Malty Calculator and enter in OG 1.059, Volume 5 gallons, Harvest Date 30 days ago, and then adjust yeast concentration to 1.2 billion/ml and Non-Yeast Percentage to zero it says that you need 363 ml of slurry - more than double the amount that the previous calculation yielded.

However, if you tell Mr. Malty that the yeast harvest date is yesterday, it says you just need 174 ml of slurry - less than 2% difference from the original calculation.

That leads to my questions - anybody have a standard formula for the rate that the viability of yeast declines? Harvest date I assumed is the date that you harvested the yeast from your carboy but what if I have a slant with just a few small colonies? If I make a starter from a slant how viable should I consider that yeast to be?

 

[insurgus]

I'll skip the rest of the question for someone else, as you'd have to do a plate count with a hemocytometer to really know for sure.

Your slants are all going to depend. It will depend on how sterile you were creating your slants and if you used sterile procedures to inoculate. You will have to step up your starter from a slant. Typically 50ml -> 250ml -> 500ml -> 2L is a good safe bet to make sure your yeast is able to out compete any other bacteria in your wort.

If you make highly hopped wort for your starter, sterilize it in a pressure cooker and inoculate your starters with sterile techniques, step up your starters with the same... then I see no reason you'll ever have a problem with viability.

I run roughly 45 different yeast strains both frozen and slanted and have yet to have anyone notice the difference between a smack pack and my collection.

 

[mthelm85]

I'll skip the rest of the question for someone else, as you'd have to do a plate count with a hemocytometer to really know for sure.

Your slants are all going to depend. It will depend on how sterile you were creating your slants and if you used sterile procedures to inoculate. You will have to step up your starter from a slant. Typically 50ml -> 250ml -> 500ml -> 2L is a good safe bet to make sure your yeast is able to out compete any other bacteria in your wort.

If you make highly hopped wort for your starter, sterilize it in a pressure cooker and inoculate your starters with sterile techniques, step up your starters with the same... then I see no reason you'll ever have a problem with viability.

I run roughly 45 different yeast strains both frozen and slanted and have yet to have anyone notice the difference between a smack pack and my collection.

So do you have a ball park estimate of how many viable cells you're pitching? That's what I'm after. The info from the Wyeast site seems to be a decent way to get a ball park estimate of cells/ml because all you have to do is chill your starter in a flask and you can then see the % of yeast solids and then use the Wyeast info to estimate how many cells you have per ml but then you aren't taking viability into account like the Mr. Malty Calculator does.

I would think though that a fresh starter made from a slant would be even more viable than yeast harvested after a full fermentation because it hasn't been stressed as much but I don't really know.

I just didn't realize the difference was so huge until I plugged the numbers into the Mr. Malty Calculator and saw that, when taking viability into account, the result was that I should pitch more than double the amount of slurry.

I guess I'm just going to assume that a starter from a slant is really viable and if I have issues with under-pitching, I'll know to tweak the numbers a bit.

 

[Dr Malt]

This might be a bit much to wade through, but check this out. It might help.
Yeast Propagation and Maintenance: Principles and Practices | Maltose Falcons

Dr Malt

 

[theredben]

Don't forget that there is no magic number for yeast pitch counts. All the numbers you find in the literature are just rules of thumb based on previous experience. Find a system that works for you, and stick with it!