How to Pitch Yeast

Pitch yeast effectively using this guide covering a variety of pitching methods used to ensure your beer undergoes a successful fermentation.

| July 2019


You need to pitch an adequate amount of yeast to get your fermentation to start in a reasonable amount of time, proceed in an orderly fashion, and reach a reasonable final gravity (given the fermentability of your wort). A pitching rate of 1 million cells per milliliter per degree Plato is frequently cited as the standard rate for ales, although some sources give a lower rate. For a 5-gallon (19 L) batch of beer at 12°Plato (SG 1.048), this would be 228 billion cells. The optimal pitching rate for lagers is often given as twice this, although again lower rates can be found in the professional literature.

To accurately measure the amount of cells, you need a microscope, a special kind of slide called a hemacytometer (designed to count blood cells) and a vital stain (methylene blue). As most homebrewers do not have this equipment, most rely on pitching a given weight or volume of a yeast slurry, pitching yeast from a yeast starter of a given volume, or by pitching multiple packages of commercial yeast based on their cell counts.

For a 5-gallon (19 L) batch of moderate-strength ale, a long-standing rule of thumb has been to pitch a cup of yeast slurry. For homebrewers repitching yeast from the bottom of a fermentation bucket or carboy, this often works well because the density of yeast cells in the slurry immediately after fermentation is relatively low. This yeast sample will be liquid-like and colored with trub and hop debris that settled along with the yeast. If you harvest healthy yeast and let it settle overnight in your refrigerator, about one-third this volume (1⁄3 cup/80 mL) would be satisfactory. Yeast selected this way will be creamy to pasty in consistency. And, since the trub and hop debris will sediment in separate layers, it is relatively easy to use only yeast slurry, which will be off-white in color.

If you are making a yeast starter, you can estimate the amount of cells you will raise from a given volume of starter wort. The density of yeast in a well-aerated yeast starter would vary depending on yeast strain and other variables, but 50 million cells/mL to 100 million cells/mL is not an unreasonable estimate. If you calculate the total number of cells you need to pitch, simply divide this number by the density of your yeast starter to yield the size of the yeast starter (in milliliters). Or, see the table on page 43 for starter sizes for three different pitching rates over various original gravities from 8° to 16°Plato. This website also has a calculator that suggests a suitable yeast starter volume for a given volume and gravity of wort.

A rule of thumb BYO has used in the past is that, for moderate-strength ales, a 2-quart (2 L) yeast starter is optimal. Mr. Malty returns a value of half of this (for yeast starters initially aerated with oxygen), indicating that those calculations are based on slightly different assumptions. In reality, yeast density varies depending on yeast strain, aeration of the medium, nutrient availability, and other things. If you aren’t counting your yeast, you are relying on assumptions you can’t test. In practice, however, beer is fairly forgiving; if you raise a healthy yeast starter and are within the ballpark of the optimal pitching rate, your beer will likely be fine.



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