ESTIMATING YEAST DENSITY

[Lele]

Hi everybody and greeting from Italy
Do you know what's the cell density (billion cells per millimeter) of a yeast that has been harvested from another fermentation?
(Sorry for my English )

 

[MoonBrewer]

Ciao! It may be challenging to estimate density from a harvested yeast cake since there will be trub and hop matter mixed in with the yeast cake after fermentation. You can separate the yeast by washing or other method to be more accurate.

From the book "Yeast" by White and Zainasheff, a 'pure' yeast slurry that has been refrigerated and the yeast has compacted in bottom of the vessel, the slurry is around 8 billion cells/ml.

The book also states that a yeast suspension with less than 1 million cells/ml is not visibly cloudy, it will be clear liquid. More than 1 million cells/ml the liquid will be cloudy. So, you could dilute a sample until it is just barely clear, then calculate the original density based on the amount of water added to dilute the sample.

 

[Lele]

From the book "Yeast" by White and Zainasheff, a 'pure' yeast slurry that has been refrigerated and the yeast has compacted in bottom of the vessel, the slurry is around 8 billion cells/ml.

Thanks for the information
After what you wrote, I checked the book and there's is written: “Once that happens, the yeast (excluding the liquid above it) is at a very high density, somewhere around 8 billion cells/ml.”

But after there is written also: “At the beginning of fermentation, cell density is around 5 million to 15 million per milliliter, and it is around 25 million to 60 million per milliliter at the end. Once you harvest the yeast from either the top or bottom, you will most likely have between 0.8 billion and 2 billion cells per milliliter.”

Apparently, the number of the cells in a new vial is 8 billion cells/ml, but if you harvest the yeast, it's only 0.8 billion and 2 billion cells per milliliter (maybe because the conditions of the fermentation aren't ideal).

Did I understand correctly?

 

[bierhaus15]

Having harvested and counted hundreds of yeast slurries, from both pro and home brewer fermentations, I've never seen amounts that high; unless in pure propagation where the yeast density is like peanut butter. Normal amount are very much in the 0.5-2.0 billion cells/ml range.

For bottom cropped yeast with minimal liquid, I'd estimate around 1.0-2.0 billion cells/ml, and top cropped around 0.5-0.75 billion cells.

 

[Lele]

For bottom cropped yeast with minimal liquid, I'd estimate around 1.0-2.0 billion cells/ml, and top cropped around 0.5-0.75 billion cells.

So top cropped yeast has less cells than bottom one...interesting information. Thanks.
But I think that in bottom cropped yeast there are more deadh cells, aren't there?

And just one more question: ever in White's book, I've read that, after one month, only 50% of cells are still alive. Is that correct, in your experience?

 

[jfolks]

I always assume 1 B/ml. You can adjust based on liquid to yeast ratio (per this Wyeast chart). https://goo.gl/images/ipGdj9

 

[bierhaus15]

So top cropped yeast has less cells than bottom one...interesting information. Thanks.
But I think that in bottom cropped yeast there are more deadh cells, aren't there?

And just one more question: ever in White's book, I've read that, after one month, only 50% of cells are still alive. Is that correct, in your experience?

Yes, bottom cropped tends to have more dead cells and non-yeast material, compared to a good top crop. I've also frequently seen yeast viability as low as 60% in Whitelabs/Wyeast packages a few months old, but still within the best before. For saved washed yeast using water, that number goes way down... 30-40% viable after 2 or more months. Not even worth saving yeast if leaving for that long, IMO.