Yeast slurry density

[Lisa Ruști]

Hi everyone,

I'm new in this field and I'm struggling to find out the density of a sample of cropped yeast before being diluted and stored in the yeast storage vessel. I weighted a graded cylinder of 1000 mL with cropped yeast slurry and I obtained a density of 0.5 g/mL, but I think the CO2 in the slurry also influence my analyse. So, how can I get rid of that CO2 from yeast?

 

[Donoroto]

Hello and welcome!

When brewers speak of yeast density, they are usually referring to how many yeast cells there are in a certain volume, like "2 billion cells per milliliter", with the actual count varying considerably by yeast strain. The actual density in grams per milliliter is not usually relevant.

Just from a practical point of view, I'd expect yeast slurry to be slightly more dense than water, so just over 1 g/ml. But I have no measurements to back that up. All I know is that my yeast sinks to the bottom of the jar of beer.

Removing dissolved CO2 from a liquid: Any excess can be removed by shaking it while not under pressure. The small amount of dissolved gas at normal atmosphere should be negligible. But if you need to remove even that, then pull a vacuum somewhat below atmosphere for some time, again know that agitation can reduce the degassing time to minutes instead of hours.

 

[Lisa Ruști]

Another idea came into my mind right now. If I count the number of cells from 1 g of yeast slurry and the number of cells from 1 mL of yeast, I gues I could find out how many grams do I have in 1 mL, knowing the cells number from both. What do you think?

 

[Mark Farrall]

If you're storing your slurry at fridge, or even room, temperature the amount of CO2 won't change the estimate for your slurry density. Assuming you've been releasing pressure on your slurry from time to time. I lost one last month because I forgot about it and it emptied itself into the sink when I opened it.

You're more likely to overpitch than underpitch with slurry. I just use a conservative estimate on the density and make sure the slurry is healthy. So if it's been in storage for a while I'll put it through a starter first and use some yeast nutrient in the boil, especially if I haven't created a starter.

If you do go down the counting route, some strains are up to four times smaller than other strains, and weight may be a better measure than counts unless you know the strain you're using well. I've listened to a few interesting podcasts on how to stain and count yeast for the type of question you're trying to answer. Though I've always balked at the equipment needed, so just used a conservative estimate instead.

 

[Josh Hughes]

The calculator on here is pretty good for how much to use. At least I’ve had good fermentations using it. If it’s old make a starter. If not use the cal hand you’ll be fine.

 

[Ward Chillington]

Plus 1's on Mark and Josh's assessment, get in the ballpark with slurry and you should be fine. While there is a huge amount of "science" in brewing but on homebrew level, "art" works pretty good too. Mind the age, poor off the old wort or supernatant (there's a science word!) and don't worry about over pitching and you'll be fine. I've been doing slurry reuse for about a year now and tried to follow some quantity recommendations and found I under pitched the batch and that was with kveik!

By the way, welcome to the forum!