Cell Count vs Biomass

[schematix]

I was wondering if anyone knows if pitching rates can be expressed in terms of yeast biomass (which i can easily measure on a kitchen scale) instead of cell counts (no microscope, yet).

Using a 2-step starter and a stir plate, i created about 900g from 1 smack pack. I have 0 clue how many cells i have. I am going to pitch 3/4 of that into 10G of 1.056 lager. Fingers crossed.

 

[illmatija]

The problem is estimating yeast density of the slurry.

However, if you know both the volume and the mass of the slurry, in theory you might be able to estimate cell count.

According to the Yeast book, every yeast cell weight around 8e-11 grams and given this plus some other parameters, yeast slurry with density of 2 billion cells per ml will weigh around 1.02 grams per ml.

Now, all this probably can't be taken for granted because it prolly further depends on the yeast strain and other factors like water density (which would be 1 g/ml for RO water).

Furthermore, given that the yeast slurry density is so close to water density, calculating cell count from volume and mass would be highly difficuly on a homebrew scale given that the result will highly fluctuate with small changes in input variables (volume, mass), and is therefore probably better suited on a commercial scale where higher precision can be achieved.

I have the exact formulas that would calculate yeast cell count given the mass and volume of a slurry, but I have to get back home to share it first.

 

[joshesmusica]

I'm no expert, and am really here for the responses from others, but I have to agree with what illmatija says. it's logical. we don't have such precise control on the homebrew scale. if we have a little more water this time than last time, it will make the cell count off. or especially after harvesting the yeast, we now have not only water to account for in the calculation but any amounts of trub and protein in there. not to mention that the little amount of water from the harvest has some alcohol in it. so this isn't going to really be a very precise way of measuring how much yeast you have (unless, as mentioned, you had all kinds of awesome lab equipment).

now i'm not saying a few hundred million cells will be that detrimental at our scale, but if our aim is to just get close, we might as well just use one of the online calculators. most of them are skeptics anyways, which means if we go by them we will almost always be slightly over-pitching.

but then again, if you just like being able to run the numbers yourself, i'm not trying to rain on your parade. all the more power to you.

 

[cannman]

I was wondering if anyone knows if pitching rates can be expressed in terms of yeast biomass (which i can easily measure on a kitchen scale) instead of cell counts (no microscope, yet).



Using a 2-step starter and a stir plate, i created about 900g from 1 smack pack. I have 0 clue how many cells i have. I am going to pitch 3/4 of that into 10G of 1.056 lager. Fingers crossed.

I'm not sure if the resolution of the average kitchen scale would give you the information necessary. Outside of the laboratory I sometime have to battle with household static when dealing with really fine measurements and that force alone might convince you of a situation that really isn't so. With yeast and starters, omg that could be billions of cells!

I think we have enough information from other factors that we can estimate with sufficient accuracy for the application the number of cell via control of the fermentables (starter og or g/L with laboratory data), attenuation analysis, temperature, and of course observation.

Ha, reminds me of studying Redshift to obtain this week's lotto numbers.

Good luck!

 

[schematix]

I guess the crux of my question is actually is there a difference between 2 yeast cells that weigh 8e-11, or 4 that weigh 4e-11?, or even 1 that weighs 1.6e-10? Is it the number of cells or the mass of yeast that is critical? I know most brewing literature indicates cells so there is probably a reason....

 

[illmatija]

I am interested in knowing the answer to your question.

But the problem is that measuring exact yeast-only mass still seems hard, at least according to the formula that I use.

Inputs:
M, V = mass, volume

Outputs:
My, Nc = yeast mass, count

Parameters:
Dw = water density (default 1 g/ml)
Dy = yeast density (default 1.087 g/ml)
mc = yeast cell mass (default 8e-11 g)

Intermediate variables:
Vw, Vy = water, yeast density

Formulas start:
Vw * Dw + Vy * Dy = M
Vw + Vy = V

Formulas end:
My = (M - V * Dw) / (1 - Dw / Dy)
Nc = My / mc

--

In the formula for My, both the divisor and divident are gonna be relatively small numbers, thus the result varies highly on the input parameters and parameters.

Play with these and see if it makes sense for you to use them. What volume do you get for 900g you have?

 

[schematix]

Play with these and see if it makes sense for you to use them. What volume do you get for 900g you have?

Density observed was about 1g per ml. Graduations in a 2L erlenmeyer are fairly course so that's about best i could do. I decanted better than 95% of the starter beer.

Quick correction. I actually had 650g total. I pitched 485g of it (rest is going to be used for another clean starter). I'd have to imagine this is more than 485 billion cells though as it hasn't even been 24 hours yet and kraussen is forming. I pitched at 40F and let it free ride to 50, now holding +/- 1.

Ideal pitching rate for this beer is 1.5 million cells / ml / degree plato. OG was 1.056 (14P). Volume was 11G = 41639ml -> 1.5*41639*14 = ~875B.