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Defining Yeast Slurries and Dealing With An Overcarbonated Keg: Mr Wizard

Q
I just finished reading your reply to a question on re-using yeast. I’m about ready to get started doing so, most of the process is clear to me. One exception; what exactly is meant by yeast slurry?

I’ve read about yeast slurry at least a hundred times, if not more. So far all I know is it’s a mixture of yeast and liquid. The questions are what proportion of yeast to liquid constitutes a slurry and how can I measure it? In all the references to yeast management I’ve read, I can’t think of anywhere slurry is actually defined, hence my question.
Steve Stanley
Aurora, Colorado

A
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.

Q
I am a first timer trying to force carbonate my keg of Oatmeal Stout. I usually use priming sugar that comes with the kits but this time I wanted to try force to carbonate the keg, and am getting 99% foam. Here are the steps I took:

1. Cold crashed both beer and keg.
2. Set CO2 pressure at 25 psi. 
3. Laid the keg on its side and rocked for 200 seconds, disconnected and placed in kegerator for 2hours, and tested. Reduced the keg pressure and set the CO2 pressure to 8 psi.
4. Had about a 2” (5 cm) head, however seemed to have very small bubbles and tasted a little flat.
5. Re-carbonated, this time at 25 psi for 2 hrs. whilst rocking every 20 minutes. Disconnected and let it set for a few hours.
6. Reduced the keg pressure and tried to pour. This time nothing but foam.

I went as far as connecting the gas on the beer in and degassing until foam sprayed out. Pressure is set at 10 psi and my beer still appears to be over-carbonated. Do I disconnect the gas and purge every hour? Seems my 5-gallon (19-L) batch is down to 3-4 gallons (11.4-15.1 L), and I am still trying to get the pour right. Any suggestions will be greatly appreciated. I have the standard gas hookup purchased from a homebrew store.
Tony Polk
Boca Raton, Florida

A
In my brewing opinion, the only redeeming quality of the old crank and shake method of beer carbonation is that it may properly “carbonate” beer to a desired level when correctly executed. That’s on a good day. On a bad day? I really cannot imagine a day much worse than the one you describe. You over-carbonated your beer by rushing the carbonation process. Then you attempted to quickly reduce the carbonation, and in the process lost about a third of your beer, likely destroyed any semblance of decent beer foam, risked getting beer in your gas lines, and generally made yourself sick with the failed effort. No, I am not going to sugarcoat this bitter pill. This was a total bomb! You know all of this, and hopefully see some humor in a painful experience. I am just hoping that others can learn from your question.

It is clear that homebrewers will continue to crank and shake, despite all of the warnings and admonitions to the contrary. So, against my better judgement, I will explain how I would go about quickly carbonating a keg using a brute-force method.

So here is the set-up:
1. 5-gallon (19 L) Cornelius “Corny” keg containing cold beer at some known temperature. It is important that the keg have some gas space above the liquid. For the sake of discussion, I am assuming a beer temperature of 38 °F (3 °C).
2.Target carbonation level of 2.5 volumes of carbon dioxide. This is “normal” carbonation for flavorful beers.
3. Carbon dioxide tank, regulator, check valve between the regulator and hose, a 6-foot gas hose equipped with a flare end, one “beer-out” fitting, and one “gas-in” fitting.
4. Gas pressure gauge with 0-25 psi range (such as a spunding valve) connected to “gas-in” fitting, sothat the internal Corny keg gas pres-sure can be measured.

I prefer ball-lock Corny keg fittings, and like those that can be attached to swivel, flare fittings. This allows for easy disassembly of hoses after use so that everything can be cleaned, sanitized, and allowed to dry before storing. Since the crank and shake method usually involves a bit of foaming and spraying, it is important to use brewing tools that are designed for this. That’s why the check valve on the gas regulator is important; beer may move into your gas line and you do not want beer being pushed into your gas regulator.

OK, now that my kit is defined, the only thing required to begin is a target pressure. Referring to the table on above, I am choosing a target head pressure after carbonation is deemed complete of 11.5 psi. My approach is tiered; the first tier is rapid and I want to jam as much gas into my beer without over-carbonation. The second tier requires more finesse because the crank and shake method is, as you know, prone to overdoing things.

So here is the technique:
1. Attach “gas-in” fitting to the gas line.
2. Adjust the gas regulator to 11.5 psi, and pressurize the keg headspace.
3. Turn off the gas from the regulator, remove the “gas-in” fitting from the keg, remove the “gas-in” fitting from the gas hose, attach the “beer-out” fitting, and turn the gas on (a valve after the regulator is handy to have).
4. Attach the pressure gauge assembly to the “gas-in” fitting of the keg; this will be handy to observe during the carbonation process.
5. Tier 1 Carbonation:
a. Connect the “beer-out” fitting to the keg so that gas can flow into the keg through the dip tube. Shake the keg for about a minute, making sure to listen to your regulator during this shaking period. As gas is dissolved in the beer, the headspace pressure is reduced, and gas flow into the keg can be heard as carbon dioxide rushes through the pressure regulator. After a minute you probably need to take a short break. With rested arms, give your keg another minute shake, again listening to the flow of gas through the regulator. Repeat until no gas is heard flowing through the regulator during the shake cycle.
b. Disconnect the gas line, move the keg back to your keg refrigerator, and close the door on the fridge. Allow the keg to rest for at least 30 minutes. This gives time for beer and gas to equilibrate, foam to settle, and a bit of cooling to occur.
c. Repeat steps a-b until very little gas flow into the beer is heard. Note that during these steps it is impossible to dissolve more carbon dioxide into the beer than what is permitted by the pressure/temperature solubility.
6. Tier 2 Carbonation:
a. Increase the carbon dioxide pressure to 3.5 psi higher than the pressure used in Tier 1 Carbonation. In this example, this means increasing the pressure to 15 psi. This is where there is a risk of potentially over-carbonating the beer.
b. Connect the “beer out” fitting to the keg so that gas can flow into the keg through the dip tube. Shake thekeg for a minute, making sure to listen to your regulator during this shaking period. As gas is dissolved in the beer, the headspace pressure is reduced, and gas flow into the keg can be heard as carbon dioxide rushes through the pressure regulator.
c. Disconnect the gas line, move the keg back to your keg refrigerator, and close the door on the fridge. Allow the keg to rest for 30 minutes.
d. If the pressure reading from the pressure gauge on the keg is less than 11.5 psi, repeat steps a-d until thekeg pressure is pretty close to 11.5 psi.
e. Allow the beer to rest at least 1 hour before serving.

I can hear the jeers already about how slow and conservative this method is, but it is controlled and will not result in grossly over-carbonated beer. One important tip about force carbonation by any method is to clean, sanitize, and purge gas hoses prior to use. A hose that is simply connected to a carbon dioxide source and equipped with a ball lock fitting is full of air, and will very effectively inject oxygen into beer if not purged prior to use. This is extremely important because an air-filled line will quickly oxidize your beer.

On a related, but entirely separate topic, foamy beer can come from a keg even when the beer carbonation is perfect. Based on your description of things, I think it is reasonable to conclude that your beer carbonation was wonky, at best. Wonky or not, draft system balance is critical to achieve a well poured beer. A simple balancing rule when using 3⁄16” beer line is to add 0.5 to your carbonation pressure, 11.5 psi in this example, and divide this number by 2.2. This gives you the length of draft line required to balance the keg pressure with line restriction. So in this example we have serving line length = (11.5 + 0.5)/2.2 = 5 1⁄2 feet.

The best way to prevent having to resort to this method is proper carbonation from the onset and it is best to exercise patience. Forced de-carbonation is a recipe for trouble. To reduce carbonation, simply vent the head space pressure of your keg and allow the beer to rest for a few hours (this time depends on degree of over-carbonation). After this venting period, boost the keg pressure back to your equilibrium pressure (11.5 psi for the above) and taste. This may take a couple of cycles to dial in.