Calculating Yeast Cells in Slurry

[trustyrusty]

Hi I was listening to a Podcast the other day about pitch rate.

I have collect yeast from via yeast washing method, and if you can imagine a 600 ml water bottle I have about 25% of yeast/slurry once it has dropped and packed. How much of that is yeast? Or is it all part of the calculation.

I have used this amount in batch and seems fine but I think the pitch is quite important so I want make sure that I have a reasonable amount. or the right amount.

I have had a look on brewers friend and I am not sure if the slurry is calculated as all yeast? If I look closely there is a layer of whiter part in the middle which is the good stuff...

How can I measure the Gravity, that is in the bottle and does want to contaminate but if I measure before it goes in the bottle is it not going to change after a while while in the fridge.

I have also just collected all the slurry with a little beer left and and stored that, no problems...
Just like getting yeast from a bottle ...

cheers

 

[Vini2ton]

I know what you mean. Some of this stuff about counts and shit can just take the fun away. There is so many variables about whats in the yeasty trubby gloop you'd need to be a fuckin' bio-chemist to get your head around it. I tend to only wash yeast if I'm going to keep it in the fridge for a few months.When restarting, generally I have about an inch (oldfart) under water in a ball jar which I step up 500ml to 1lt. Ultimately we always want to improve our brewing processes but short of using a microscope and counting, I just go by the maxim of my dear old grand-dad, "keep it clean and give em heaps!".

 

[jibba02]

Depends on lots of things!
Take out the guesswork and purchase a microscope.

 

[jibba02]

Providing your beer was treated correctly. Eg. Propper pitching rate and controlled Temps. You should have enough rinsed yeast for 2 or 3 brews

 

[kaiserben]

There are about 4.5 billion yeast cells in 1 milliliter of yeast solids (solids with no excess liquid). According to Fix, in a slurry, only about 25% of the mass is yeast solids. Of course, if there is a lot of trub in there, you have an even lower percentage of yeast solids. The bad thing is that you can't tell how viable that yeast is, unless you have the equipment to properly test and count it. So this is where it gets a little bit like black magic. There are a number of factors that affect the viability of a given pitch of yeast. How old is the yeast? How stressful was their last fermentation? Have they had the proper environment and nutrients for successful reproduction or are they too scarred and tired to go on?

When the yeast is fresh and healthy off an previous batch, viability is maybe around 90%+. It goes down from there fairly quickly without proper storage and it also really depends on the strain of yeast. Unless you're going to get into testing viability, you're going to need to make some educated guesses and keep good notes on the results. This is where being a yeast psychic really helps. Start in a range of 80 to 90% viability and you probably won't be too far off. Use the Pitching Rate CalculatorTM to help figure out how much of that yeast you need. If your yeast viability is much lower than 90%, you should probably toss the yeast. If you really want to use it, you might consider pitching it in some starter wort to get the still viable cells active. When they're in solution, decant that active part of the starter into another vessel, hopefully leaving the dead cells behind.

Source = MrMalty

So, basically, unless you're doing proper cell counts under a microscope, it's all guess-work, but at least it's educated guess-work.

 

[trustyrusty]

Thanks

So 500 ml wort/slurry/water, about 25% slurry = 125 ml

25% slurry useful, ie good yeast = 31.25ml. Working on 1 KG = 1 Lt

There are roughly 30 grams? let says 2/3 are healthy to start

so about 20 grams. Yeast pack is 11g - so pretty close....

Not exact science but in the right ball park?

Microscope wont help me because I cannot count and don't know what to look at...

 

[kaiserben]

That figure of 25% is dependent on thickness of your slurry/wort/water solution (if you go into the "Repitching from slurry" tab in MrMalty you'll see you can choose "Yeast Concentration" depending on the thickness of your slurry). It's way too subjective IMO.

From what I understand a better way would be to let the yeast drop out and estimate the amount of slurry in the bottom of your vessel (not including wort/water). There's a useful slurry amount diagram I've seen on the net somewhere that I can't find now (it shows several vials of yeast slurry and estimates cell counts - if anyone knows what I'm talking about could they link to it?).

Let's say you put 100ml of slurry/water into a 100ml vial. You let it settle out and there's 20ml of compact solids at the bottom. You'd have 20 x 4.5bn cells = 90bn cells. And from there your number of cells would degrade by roughly 20% per month from when it had stopped fermenting.

Just on a side note, looking down a microscope and counting live/dead cells is surprisingly easy, but microscopes aren't cheap and it's extra work where you'd still be using estimates (by extrapolating a small amount of data to estimate an overall cell count).

 

[kaiserben]

Here's is the thing I was trying to find (mentioned above).

Source = Wyeast (LINK)

Estimates of cell counts can be made using percent yeast solids of the slurry. Percentage of yeast solids per volume of slurry can be estimated by allowing a sample to sediment under refrigeration and estimating the percent solids. Generally 40-60% yeast solids will correlate to 1.2 billion cells per ml. This will vary with the yeast strain. By using this method with every brew, a brewer can achieve consistent pitch rates batch to batch resulting in a more consistent product. The following picture relates sedimentation to cell count.




Once the brewer has determined the desired pitch rate and cell density of the slurry the brewer can collect the appropriate quantity of yeast. Slurry can be harvested based on volume or weight. 1L (1 quart) of yeast slurry (40% yeast solids) weighs approximately 1.1 Kg (2.4 lbs.).