Yeast Slurry Comes Into Focus

The term “yeast slurry” is used by brewers to describe the pasty mixture of yeast and a liquid, usually beer. I will return to yeast slurries in a moment, but want to spend some time on different types of liquid/solid mixtures. When a compound like sucrose, or common table sugar, is dissolved in water, the sucrose crystals dissolve and the individual sucrose molecules “go into solution,” buoyed by hydrogen bonds between sucrose and water molecules. All solutions have a point of saturation (these are temperature-dependent) above which no more material can be dissolved. At 20 °C (68 °F), 2.04 grams of sucrose can be dissolved in 1 mL of water. Add more sugar and it simply settles to the bottom of the container; heat this container to 50 °C (120 °F), for example, and the solubility increases to 2.59 g sucrose/mL water. Common solutions include sugar, salt, “water” (most water contains all sorts of dissolved minerals), windshield washer fluid, etc.

Colloidal suspensions are another common class of liquids, and include blood, homogenized milk, paint, and writing inks. Colloids are different from solutions in that the compounds in a colloid are not dissolved, rather they are suspended and do not settle. The solids in colloidal suspensions tend to be relatively large molecules, and include things like proteins, micelles, and pigments. Although colloidal suspensions are typically stable, environmental changes, especially temperature and pH changes, can cause the suspended solids to settle. Filtered beers that remain clear for a long time period are called “colloidally stable” because chill haze in beer is a form of colloidal instability.

A slurry is a mixture of large particles and water. Slurries are not stable and the particles in the mixture settle relatively quickly. Common slurries include mixtures of water and rock/sand/clay, and water and single-cell organisms. Think concrete mix, clay solutions used in oil drilling, sandy mixtures panned for gold by miners, the living mixtures of microbes responsible for municipal waste water treatment, and the creamy mixture of goodness that can be harvested from a beer fermenter. So any mixture of brewing yeast and liquid, be it water, wort, or beer, is a yeast slurry.

In practice, the term yeast slurry is usually reserved to refer to the thick stuff that is harvested from a fermenter and used in subsequent batches. Whether yeast is bottom-cropped or top-cropped, yeast slurries are often stored for several days before use, and these mixtures are guaranteed to separate into a dense yeast layer and an aqueous layer. Since it is impractical to pour or pump this dense yeast layer, stored yeast is stirred before re-use. In fact, many commercial breweries store yeast slurries in agitated vessels that maintain slurry homogeneity during storage. This is especially important in preventing this thick yeast sediment from becoming hot, as yeast sediments can become metabolically active and produce heat that leads to changes in yeast vitality and viability.

To your point, we really know very little about this yeast slurry other than its very nature earns it the distinction of being called a slurry. Since the yeast slurry may be very thin or as thick as peanut butter, knowing something about the cell density is critical if the brewer wants to exercise control over pitching rate. Luckily, this is pretty easy to determine if you have a microscope, automatic cell counting machine, or graduated cylinder/centrifuge tube lying about the homebrewery. Just guessing that very few homebrewers have the former two devices, but inexpensive graduated measuring devices are great things to have for a variety of brewing tasks, like estimating slurry concentration.

In general, the cell density of sedimented yeast slurry varies little by yeast strain. This allows for a general relationship between sediment volume and cell density to be developed in the form of a standard curve. The graph found below was developed using data from the White Labs website and can easily be used to estimate the cell density of a yeast slurry.

Here is how to use this curve:
1. Put 25 mL of yeast slurry sample into a 25 mL graduated cylinder.
2. Place the sample in a refrigerator for 24 hours.
3. Measure the sediment volume (9 mL assumed in this example).
4. Calculate cell density:
a. Multiply sediment volume by 4 to determine % solids.
i. 9 mL x 4 = 36% solids.
b. Use standard curve equation to estimate density.
i. Cell Density ~ (0.0254 x 36) – 0.0096
ii. Cell Density ~ 0.90 Billion cells/mL
5. Determine how many yeast cells are needed for the brew. This example assumes 1 million cells/°Plato/mL wort, a wort density of 13 °Plato, and 17 liters (17,000 mL) of wort. I find this easier to read if cell density is expressed in billion cells per liter, and volume is expressed in liters; both are shown in this example.
a. Number of yeast cells = 1 million cells/mL/°Plato x 13 °Plato x 17,000 mL of wort
Alternative units
Number of yeast cells = 1 billion cells/liter/°Plato x 13 °Plato x 17 liters of wort
b. Number of yeast cells = 221 billion cells
6. Determine how much slurry is needed using the information above.
a. Slurry requirement = Yeast cells needed ÷ Cell Density
b. Slurry requirement = 221 billion cells ÷ 0.90 billion cells/mL
c. Slurry requirement = 246 mL

This method provides an estimated cell density that does not account for yeast viability (living versus dead cells) or yeast vitality (yeast health). Years ago when I was a student at UC-Davis I was compulsive about doing cell counts before pitching and began to see a trend after about 100 cell counts … yeah, we brewed a lot! The trend was that the cell density of slurries did not vary much if the slurry was collected from the bottom of a fermenter, and the amount of ale slurry (lager yeast is usually thinner) needed to pitch 20 liters of wort was always about 250 mL. In US terms, this is about a cup of slurry per 5 gallons. My point here is that once you get comfortable with a yeast strain you may begin to get a good feel for pitching volume requirements that don’t require cell density assessments of each batch of yeast.

And for those who don’t have the patience to wait a day to determine cell density, clinical centrifuges can be picked up on Ebay for under a hundred bucks. I suggest looking for the types that hold 25 mL centrifuge tubes. Ten minutes in the centrifuge will drop the yeast out of suspension pretty darn quickly. You may have slightly different results, but yeast cells are not super compressible since they are filled with liquid, so the differences between the settling and centrifuge methods are not huge.

One final word about stuff. I have used metric units with no conversions to tablespoons or pints because microbiologists communicate using International Standard of Units (SI units). Although I did not use scientific notation in my math, I did use it in my calculations. I strongly encourage all brewers to use the metric system because it is easy to use and is universally spoken, and if you haven’t used scientific notation in a while you will also find it very handy.

Response by Ashton Lewis.