Updated: Jul 27, 2020
This article references AleMind - a brewing calculator we developed. Since the original publication of this article we have taken AleMind off the market for some improvements. We'll let you know when it is avaiable again!
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!
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 - 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. 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 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 - 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 - The hemocytometer grid has a 5x5 grid of large squares, each with another 4x4 grid of small squares inside. For simplicity, only count cells in the 4 large corner squares and the very center large square. 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.
If you have AleMind Level 2 - you can use it for Steps 6 & 7. For more details on how to use these functions, check out the Help Guide
If you do not have AleMind - you can still check out this post for the formulas used in calculating these values. Just scroll down to the "Cell Count" and "Pitch Rate" sections and do the calculations on your own.
Step 6 - Using the "Cell Count" function in AleMind (Level 2), input the total yeast slurry volume, dilution values, total cells counted, and your grid multiplier. Click calculate to get the density in Cells/mL, and Total Cells. You have completed the cell count! Now you can calculate the volume of the yeast slurry to pitch for the desired pitch rate.
Step 7 - If you have just calculated the cell count in AleMind, click "Use to Calculate Pitch Volume" to automatically insert the Cells/mL value into the Pitch Rate calculator. Select Ale or Lager, input the volume, and original gravity, and click calculate to get the correct volume of yeast slurry to pitch.
Cell Count and Pitch Rate Without a Microscope
Step 1 - 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.
Step 2 - Using the Pitch Rate function of AleMind, select Ale or Lager, input the volume, and original gravity. Input your density estimate in Slurry Cells/mL. Click calculate to get the correct volume of yeast slurry to pitch.