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Old 06-19-2013, 11:54 AM   #1
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Default How do I know how much yeast slurry to use?

I've been trying to get into washing and re-using my yeast, but there's still one aspect of it that confuses me: How do I know how much yeast is in that Mason jar in the fridge?

A few weeks ago, I brewed an Oktoberfest using WLP820 (Oktoberfest/Marzen) yeast. After 9 days fermenting, I did a 4 day d-rest, then cold-crashed it. A week later, I racked it to a keg to lager (and get it off the yeast). I washed and saved the yeast in 3 Mason jars. That was 2 weeks ago.

I would like to re-use this yeast to do a Rauchbier. I would like to just decant the clear liquid, warm the slurry to room temperature, and pitch it (as opposed to doing a starter, then cold-crashing and decanting). My problem is, I have no idea how much yeast I have in these jars. I mean, I can see it obviously, and I could measure it by volume, but given that it's been in the fridge for 2 weeks, what's its viability? How much of it is trub? Heck, even if I know the volume, how do I know how many cells that represents without knowing the density of the slurry/yeast cake?

I'd be willing to do a starter if it would result in a clear answer. I basically need 400 billion cells. If I pitch all 3 jars (the entire yeast cake collected from the Oktoberfest batch), I'm pretty sure that'd be an overpitch, but that's still preferable to underpitching, isn't it? My current plan is to just pitch 2 jars, but that's still just a wild guess.

How can I improve my process here and bring some sort of accuracy to my yeast cell counts when re-using yeast like this?

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Old 06-19-2013, 12:01 PM   #2
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Why not use a starter? That'd solve all your questions.

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Old 06-19-2013, 12:26 PM   #3
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Would it, though?

Wouldn't I still have to know how much yeast I was pitching into the starter in order to have any idea how much I'll get out of it?

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Old 06-19-2013, 12:28 PM   #4
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Mrmalty.com has a "Repitching from Slurry" tab that may help you out.

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Old 06-19-2013, 12:33 PM   #5
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what size mason jars? this is a cold, slow fermenting yeast so i'd use lots of yeast, not the whole cake though. there are yeast calculators that will give you an estimate of how much yeast is in the slurry, it's a lot of yeast. the whole point of saving yeast for me is to avoid making a starter and 2 weeks in the fridge is nothing, i've used jars of yeast that were months old and they started fermenting without unusual lag times.

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Old 06-19-2013, 01:01 PM   #6
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Without a microscope, you really are never going to know.

What you can do is estimate. Calculators can do that. You can also do that with your own knowledge.

eg if you are used to making starters, you should know about how much space x number of cells take up in your flask. If you use a calculator to get you in the ballpark and then rely on your experience with your equipment, you should be able to get relatively close to your target.

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Old 06-19-2013, 01:07 PM   #7
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Here's a calculator I came up with.

But as Billl says, there are lots of estimates in there and you don't really know.
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Old 06-20-2013, 01:28 AM   #8
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Quote:
Originally Posted by kombat View Post
I've been trying to get into washing and re-using my yeast, but there's still one aspect of it that confuses me: How do I know how much yeast is in that Mason jar in the fridge?

..... I washed and saved the yeast in 3 Mason jars. That was 2 weeks ago.

I would like to re-use this yeast to do a Rauchbier. I would like to just decant the clear liquid, warm the slurry to room temperature, and pitch it (as opposed to doing a starter, then cold-crashing and decanting). My problem is, I have no idea how much yeast I have in these jars. I mean, I can see it obviously, and I could measure it by volume, but given that it's been in the fridge for 2 weeks, what's its viability? How much of it is trub? Heck, even if I know the volume, how do I know how many cells that represents without knowing the density of the slurry/yeast cake?

I'd be willing to do a starter if it would result in a clear answer. I basically need 400 billion cells. If I pitch all 3 jars (the entire yeast cake collected from the Oktoberfest batch), I'm pretty sure that'd be an overpitch, but that's still preferable to underpitching, isn't it? My current plan is to just pitch 2 jars, but that's still just a wild guess.

How can I improve my process here and bring some sort of accuracy to my yeast cell counts when re-using yeast like this?
You can estimate the volume of yeast you have in a jar by putting same amount of liquid in another like jar and then measuring it.
Depending on how effectively you have rinsed the yeast it is likely you will have around 2 - 3 bn cells per ml of compacted yeast in the jar.

So for example if you washed the yeast very well and had 50ml of yeast then you could estimate you have around 50 x 3bn = 150bn cells.
If you were less careful and the sample shows some discoloration from trub then go for a lesser density of around 2bn per ml (50ml x 2bn = 100bn cells)

Now how many of those cells are viable depends on age since fermentation was completed. When you input the harvest date for the yeast into Yeastcalc it will estimate how many viable cells you have and guide you to making a starter if required.
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Old 06-20-2013, 01:59 AM   #9
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Originally Posted by el_caro View Post
If you were less careful and the sample shows some discoloration from trub then go for a lesser density of around 2bn per ml (50ml x 2bn = 100bn cells)
Or if you can tell that you have roughly 50% yeast and 50% trub, then just multiply that times the 2-3 billion cells/ml to tell you how much yeast you have.

So if you have 100 ml total, but you can tell it's only 50% yeast, then you'd have 50 ml of yeast.
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Old 06-20-2013, 12:45 PM   #10
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Quote:
Originally Posted by mtnagel View Post
Or if you can tell that you have roughly 50% yeast and 50% trub, then just multiply that times the 2-3 billion cells/ml to tell you how much yeast you have.

So if you have 100 ml total, but you can tell it's only 50% yeast, then you'd have 50 ml of yeast.
Wyeast has an explanation about harvesting yeast on their website. This is basically what they say, too. They say that if you have 40-60% solids, then there are ~1.2 billion cells per ml.

But I assume that is with fresh yeast, and it degrades over time. They also say to use it within 2 weeks, and I know others say it will keep longer. Yeastcalc includes the date in its calculations.

Seems like you should underestimate the viability, since it seems generally worse to underpitch than to overpitch?
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