Updated: Apr 14
I won't lie - cell counting was one of the more difficult and intimidating skills I learned when I decided to kick up my "brew lab" game. But after reading numerous reputable texts on the importance of proper pitch rates, I embraced it as an inevitable necessity if I was to take my brewing from good to great. Before I continue, cell counting is NOT a practice reserved for "pro" brewers; however, it is a tedious and arguable unnecessary step for the average homebrewer. Take that as you will - it's not necessary, but it will be a contributing factor in kicking your beer up to that next level. If you're a scientifically inclined brewer, I encourage you to keep reading!
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Most, if not all credit goes to Chris White and Jamil Zainasheff for their brilliant work in "Yeast", a well known text from the Brewing Element Series. I reference Yeast constantly as if it were a textbook and contribute my foundational knowledge of fermentation and laboratory practices to this book. I highly recommend picking up a copy! While much of what I know came from this book, I have developed my own rhythm and habits that have worked well for me. The processes described below are a combination of the steps outlined in Yeast as well as my own experiences.
WHAT IS A CELL COUNT?
A cell count is a way of determining exactly how many yeast cells you have in a particular volume of slurry or liquid. Cell counts are commonly done when re-pitching a yeast slurry, or when pitching a starter to a new batch of beer. A sample of the slurry is taken, counted, and after a little arithmetic, a brewer can determine how many yeast cells they have available, or the density of cells in the slurry. Both can be used to determine the proper pitch volume.
WHY DOING A CELL COUNT IS IMPORTANT
It is possible to pitch too many or too few yeast cells to your batch of beer. Too few cells negatively affects flavor - while too many cells negatively affects yeast health of the next generation. Knowing how many cells you have allows you to pitch the proper amount. The number of cells you pitch is determined by the pitch rate of the beer. White and Zainasheff recommend pitch rates of 750,000 cells/mL of wort/°P for ales, and 1,500,000 cells/mL of wort/°P for lagers.
CELL COUNT WITH A MICROSCOPE AND HEMOCYTOMETER
The yeast you harvest from the fermenter (or the starter) is your "slurry" - this is what you will be counting and then (re)pitching at the correct rate to another batch of wort. Keeping good sanitation practices is vital at this point, since anything that finds its way into the slurry will end up in your next batch.
STEP 1 - Get total volume
Note the total volume in mL of yeast slurry you have on hand
STEP 2 - Dilute a sample of the slurry in water
This depends on the density of your slurry, but typically you will want to dilute a small sample to make counting easier. Do this by adding 2 mL of yeast slurry to 200 mL of water. This gives you a dilution factor of 100. This dilution factor will be used in the formula below. The diluted liquid will not be pitched into the beer so there is no reason to keep it sanitary.
The pictures below show a yeast slurry that has not been diluted (left) compared to a slurry that has been diluted (right).
STEP 3 - Add drop to hemocytometer
Add a drop of the diluted liquid to the hemocytometer and carefully cover it with the thin glass square that came with the hemocytometer. This spreads the liquid out evenly over the grid.
STEP 4 - Put hemocytometer on microscope
Place the hemocytometer on the microscope and take a look using a magnification of 4X to get an overall view of the grid. If any air bubbles are in the grid, repeat Step 3.
STEP 5 - Count cells
The hemocytometer grid has a 5x5 grid of large squares (25 total). This is the counting area. Each large square has another 4x4 grid of small squares inside.
For simplicity, only count cells in 5 of the 25 large squares (I use the 4 corners and the very center). Since you are only counting 5 out of 25 (i.e. 1/5th) of the large squares, the grid multiplier will be 5. This grid multiplier value will be used in the formula below.
If a cell is touching the left or bottom border, count it. If a cell is touching the top or right border, don't count it.
I like to draw out a 5x5 grid on a piece of paper to keep track of sub-totals.
Once you have counted all the cells in 5 of the 25 large squares, add them together. From the image above, 18 + 34 + 23 + 23 + 28 = 128. This is your counted cells value. This value will be used in the formula below.
STEP 6 - Calculate the slurry density
Yeast Cells/mL = Counted Cells x Grid Multiplier x Dilution Factor x 10,000
Dilution Factor = 100
Grid Multiplier = 5
Counted Cells = 128
Yeast Cells/mL = (128) x (5) x (100) x (10,000)
Yeast Cells/mL = 640,000,000
STEP 7 - Calculate pitch volume
Once you have calculated (or estimated) the density (cells/mL) of the slurry, you can calculate the volume of slurry to pitch.
First, determine your pitch rate. White and Zainasheff recommend pitch rates of:
750,000 cells/mL of wort/°P
1,500,000 cells/mL of wort/°P
Next, determine the total cells needed:
Total Cells Needed = Pitch Rate x mL of Wort x °Plato of Wort
For Example: If you brew a 5 gallon batch of blonde ale with an OG of 12°P
Pitch Rate = 750,000 cells/mL/°P
mL of Wort = (5 gallons) x (3785 mL/gallon) = 18,925 mL
°P of Wort = 12
Total Cells Needed = (750,000) x (18,925) x (12)
Total Cells Needed = 170,325,000,000
Last, calculate the pitch volume:
Pitch Volume = Total Cells Needed / Slurry Density
Pitch Volume = (170,325,000,000) / (640,000,000)
Pitch Volume = 266 mL
And there you have it! For a 5 gallon batch of blonde ale with an OG of 12°P, you will pitch 266 mL of the yeast slurry.
CELL COUNT AND PITCH RATE WITHOUT A MICROSCOPE
STEP 1 - Estimate cell density
Without a microscope, you will need to estimate the cell density of the slurry. According to White and Zainasheff, harvested yeast typically has a density of 800,000,000 - 2,000,000,000 cells/mL.
That's a fairly large range, so do your best to estimate. Or you can just use the average of the range (i.e. 1,400,000,000 cells/mL).
STEP 2 - Calculate pitch volume
Once you've estimated the slurry density, use this value to calculate the pitch volume. Follow Step 7 from above using your estimated value.