Yeast: The Practical Guide to Beer Fermentation (Brewing Elements)

by Chris White

While knowing the exact count helps, consistency is more important.

Cells to pitch = (1 million) x (milliliters of wort) x (degrees Plato of the wort)

We prefer slightly lower rates for ales (0.75 million) and slightly higher for lagers (1.5 million).

Keep in mind these suggested rates are for repitching harvested yeast, because that is what brewers are doing most of the time. When pitching a fresh, laboratory culture grown with aeration and good nutrition, a brewer can use up to a 50 percent lower pitching rate.

Let us run through an example of calculating the pitching rate for 12 °P ale wort. Since it is an ale wort, we will use a rate of 0.75. Multiply your pitching rate (0.75) by the specific gravity of the wort in Plato (12) to determine how many million cells you want per milliliter of wort. In this example you want 9 million cells per milliliter. Cells per milliliter (cells/ml) becomes your standard unit of measure. You then multiply that number (9 million cells/ml) by the volume of wort (in milliliters), to determine the total number of cells to pitch. If this is a 5.3-gallon (20 L) batch of homebrew: (pitching rate) x (milliliters of wort) x (degrees Plato of the wort) =

cells needed (750,000) x (20,000) x (12) = 180,000,000,000 In this example, you would need 180 billion cells to pitch your homebrew batch at a rate of 0.75 million. What if it was a 10-hectoliter commercial batch instead? (750,000) x (1,000,000) x (12) = 9,000,000,000,000 Now you need to measure out the appropriate amount of yeast. Typically, yeast slurries are in the range of 1 billion to 3 billion cells per milliliter, but it depends on how they were collected. If you have done a cell count, then you will have a good idea of the density; otherwise you will need to estimate.

Keep in mind that yeast harvested from fermentation often has more nonyeast material in it than lab-propagated yeast, so you will need to allow for that in your calculations.

Failure to rehydrate dry yeast properly will result in the death of approximately half the cells.