Overbuild slurry cell count

[InspectorJon]

I'm researching starters and trying to be a little more analytical regarding cell counts. I've never really had trouble getting beer to ferment and it normally takes off pretty fast using the guess and by golly yeast estimation method but I'd like to try and dial it in. I have a few unique strains I have harvested and propagated successfully. I have been using the overbuild starter method but I am never really sure how much yeast is there. Brewer's Friend yeast calculator assumes 1 billion cells/ml of slurry. (Who knows where they get that number?) Brew United's calculator requires an original cell count (default is 100 billion cells). How can I know or at least have a reasonable estimate of how many cells are in my starting slurry?

Most of my brews have a starting gravity of 1.060 - 1.080. I make 3-4 gallon batches. I normally make a 1.5 liter starter beginning with about 30ml of dense slurry. Once it is finished I pour off about 8 oz into a sterile mason jar and let that settle in the refrigerator. That settles out to 30 ml of dense slurry in the bottom of the jar which goes in the next starter. Starting with clear 1.040 starter wort with little or no trub I assume the slurry is mostly yeast. Is there any way to estimate the number of cells in that slurry without a microscope?

 

[Brewsit]

You could try some yeast rinsing (not washing) to at least get a good idea of how much is likely viable yeast. When you put the slurry in the sanitized water and shake it, you will see three distinct layers, or at least two - one that settles quickly (dead yeast and trub) and another that is a different color that is likely mostly good yeast. If you separate this upper layer, you can estimate based on that volume. I believe there are some resources out there for estimating yeast quantities based on lightly settled or densely settled yeast that could help.

 

[BigRedBrewer]

For an ale, the general rule of thumb is to pitch 750,000 to 1,000,000 cells per mL per degree Plato. For a lager shoot for 1.5M.

So if you have a 1.040 beer (10 Plato) and your volume is 4 gallons (15,141 mL) and you were pitching at a rate of 1 million cells, you would want to pitch 151.5 Billion cells. A typical pack of yeast is never enough but a 2 Liter starter is almost always too much unless your gravity is super high.

 

[BigRedBrewer]

Also, if you use the brewers friend calculator it is going to get you very close to the calculated amount of yeast. I was using a microscope and hemocytometer for a while and my experience was that the calculator was always within about 5 to 10% of what I actually counted. For that reason I've just started trusting the calculator and skipping the hassle of cell counting.

 

[day_trippr]

I also have relied on the BF yeast calculator for over-building starters, then apportioning the net with the unpitched portion used for the next cycle.

Perhaps naively, I do the math and come up with a cell count for the saved portion based on the calculated net, and use that with its birth date to seed the next cycle in the calculator. This has worked very well indeed. I have kept strains running for two years straight with all the pitched batches finishing "on time".

I recently switched to the BrewUnited Homebrew Dad calculator as it simplifies the over-build math even more, but the strategy is the same...

Cheers!

 

[InspectorJon]

The calculators are all based on a presumed number of yeast cells one starts with. How would I determine the number of cells in the 30 ml of dense yeast slurry I start with without a microscope? How much dense slurry is in a 100 billion cell vial? I’m thinking of the amount that would be in the bottom of the vial if it stood in the refrigerator for a couple weeks. I’ve never bought commercial liquid yeast.

 

[BigRedBrewer]

The calculators are all based on a presumed number of yeast cells one starts with. How would I determine the number of cells in the 30 ml of dense yeast slurry I start with without a microscope? How much dense slurry is in a 100 billion cell vial? I’m thinking of the amount that would be in the bottom of the vial if it stood in the refrigerator for a couple weeks. I’ve never bought commercial liquid yeast.

I just assume the manufactures are being fairly accurate when they state the number of cells per pack. So a 100 billion cell white labs pitch should be 100 billion cells when it leaves the lab. The calculator then takes that 100 billion cells and applies a fairly predictable death rate based on how old the pack is assuming it wasn't subjected to extreme heat or cold.

There are ways to estimate cell count based on dense yeast slurry per ml. Here's an old thread about that:

https://www.homebrewtalk.com/threads/calculated-yeast-count-vs-ml-slurry-from-starter.569037/
I do believe its a fairly inaccurate way to count yeast and if you trust the brewers friend calculator you will be good to go. The yeast book recommends 750k cells/ml/P but I always shoot for 1 million just in case the initial cell count is off. The only way to be accurate is by doing your own cell counts. If you do that you should also do your dilutions by weight not volume because it is more accurate. You can get all the equipment for about $350. Kinda pricy but it's really the best method. Even doing that, its tricky on the homebrew level to get a uniform sample out of your starter to do the counts, especially with a highly flocculant strain that likes to stick together.