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One of the best ways is to do it visually.
First, did you rinse the yeast? This helps to remove dead cells/trub and will give you a higher viability of collected yeast.
Visual estimation of cell density is based on the eye's fairly sharp threshold for observing turbidity. When viewed in a standard 13 x 100 mm tube, yeast suspensions of less than about 1,000,000 cells per ml are not visibly turbid. Above this threshold density they are visibly cloudy. By adjusting the number of cells in a suspension until just barely visible, you can obtain a suspension of known density (approximately 1,000,000 cells/ml) and then use the dilution factor to obtain the slurry concentration.
METHOD - BALLPARK CONCENTRATION
1) Take 1 mL of well-resuspended slurry, and add it to 9 mL water, mixing well. This is your 1:10 dilution.
2) Take 1 mL of 1:10 dilution, and add it to 9 mL water, mixing well. This is your 1:100 dilution.
3) You see where I am going with this... Just keep making dilutions until the suspension is not turbid. THIS is the dilution where you have ~1,000,000 cells/mL.
4) Calculate the cell density in slurry.
This step is easy.
cell density in slurry = (1,000,000 cells/mL) * (dilution factor)
Lets say the dilution you hit where the suspension is no longer cloudy is 1:100. That means:
cell density in slurry = (1,000,000 cells/mL) * (100) = 100,000,000 cells/mL
METHOD - ACCURATE CONCENTRATION
***** NOTE: If you have a turbid 1:10 dilution, and your 1:100 is not turbid, your ACTUAL point of no turbidity may be somewhere in between the two dilutions. To be most accurate, once the dilution is no longer visible (ex. 1:100), take the last turbid dilution (ex. 1:10) and do a 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8 and 1:9 dilution of that, giving a total dilution of 1:20, 1:30 ,1:40, 1:50, 1:60, 1:70, 1:80 and 1:90, respectively. Let's say the dilution where there is no longer turbidity is the 1:4 dilution (1:40 total dilution). Then:
cell density in slurry = (1,000,000 cells/mL) * (40) = 40,000,000 cells/mL
Hope this helps you. It works great for me. I am pretty close every time. I work in a lab and I have checked my dilutions using a hemocytometer and a microscope. I am usually within ~10% of 1,000,000 cells/mL on my non-turbid dilution, but I have very accurate graduated cylinders from work. If you do this technique, invest a small amount of money (like $20) on a nice 10 mL graduated cylinder and ~20-30 13x100 mm test tubes. The tubes and the graduated cylinder can both be washed and reused. ALSO, get a nice 100 mL graduated cylinder for measuring out the volume of slurry that you calculate you need for a given batch. Knowing the cell concentration within ~10% doesn't accomplish anything if, in the end, you don't have an accurate measurement of the volume slurry you are adding.