Yeast Dosing Specifics

The basic method for this process uses a carboy and plastic bottling bucket, and begins with relatively clear beer. If you want maximal clarity, cold crashing definitely makes a big difference; this is true of gravity clarified, fined, and filtered beers. Independent of clarifying methods you may use, ale fermented at 68 °F (20 ˚C) contains about 0.8 volumes of carbon dioxide (1.6 g/L) after non-pressurized fermentation, and lager fermented at 50 °F (10 °C) contains about 1.1 volumes of carbon dioxide (2.2 g/L) after non-pressurized fermentation. You need to know the base-line carbonation level so you can calculate how much sugar to add to achieve the desired level in your beer, and you must know your beer volume. The volume is also needed to determine how much yeast to add. I will come back to some basic calculations, but want to stick to process for the moment.

Using this basic set up of carboy and bottling bucket, you start by determining the beer volume in the bottling bucket because the dosing rate of yeast and sugar ties directly to beer volume (I will get to these details in moment). Next, hydrate your dried yeast in about 100 mL (3.4 fl. oz.) of boiled and cooled water, make up your priming sugar as usual, and very gently stir these into your beer to mix. At this stage of the process, your beer may be warm or cold depending on the use of a cold-crash step; it really makes no difference at this point. If you bottle the beer within an hour, you should have no major issues with yeast settling in the bucket, but if this concerns you, gently stir the bucket a couple of times during the bottle run. The basic set up has two main limitations: The first is that the beer is flat going into the bottle, and the second is the very real risk of oxygen pick-up and subsequent oxidation when using an open bottling bucket.

Yes, I did just write that flat beer going into the bottle is a limitation to this method of bottle conditioning. While bottle conditioning does add carbonation to the beer in the bottle, hence the term bottle conditioned, it is not the only source of carbon dioxide. In fact, most commercial brewers who bottle condition their beers use the method to improve shelf life and/or achieve higher carbon dioxide levels than are possible by bottling carbonated beer (anything over about 3.0 volumes or 6 g/L is not practical due to excessive foaming).

A more advanced bottle conditioning technique is to add priming sugar and yeast to a keg of beer that is carbonated to about 2.2 volumes (4.4 g/L) of CO₂. This method presents the challenge of knowing how much beer is in the keg, but also makes the mixing process less risky from an oxygen pick-up perspective. The easiest way to know how many liters of beer are in the keg is by weight. Simply weigh the keg before and after filling (kilograms are used for simplicity), and dividing the difference in weight by the specific gravity of the beer. For example, if the empty keg weighs 1.5 kg (3.3 lbs.) and the full keg weighs 19 kg (42 lbs.), the beer weight is 17.5 kg (38.6 lbs.). Divide this by the beer specific gravity and you now know beer volume. For example, if the final gravity after fermentation is 1.008, 17.5 kg (38.6 lbs.) ÷ 1.008 kg/l (38.4 lbs.) = 17.4 liters (18.4 qts.). As long as the keg was purged with carbon dioxide before filling, you can gently rock the keg to mix your yeast and sugar solutions with your beer. After allowing the beer to rest for a few minutes, you can bottle using a counter-pressure filler and cap on foam for the best shelf life. Again, the reason for bottle conditioning is not just beer carbonation. This method gives you better beer stability in the bottle and it also allows you to produce highly carbonated beers without struggling during filling.

Both of the methods described above discuss bottling cold beer. It is important to store the filled bottles at room temperature for conditioning. Since fresh yeast is being used, the process is typically complete within 2 weeks of bottling.

Now that the process has been discussed, what about the math? I like to use grams of CO₂/liter of beer because the math is easiest (2.00 grams of carbon dioxide per liter of beer equals 1.02 volumes of carbon dioxide). Chemistry shows that 2.05 grams of glucose produces 1 gram of carbon dioxide when fermented (dextrose, corn sugar, and invert sugar are common names for glucose). So here are two examples for a “flat ale” and a lightly carbonated lager:

Flat Hefe Weizen (ale) Example

• 17.4 L of beer
• Estimated carbonation level is 1.6 g/L (20 °C at 0 psig)
• Target carbonation level of 7.0 g/L using corn sugar (sorry speise lovers!)
• Need to increase carbonation by 5.4 g/L (7.0 g/L – 1.6g/L)
• 17.4 liters beer x 5.4 g CO₂/liters beer x 2.05 g glucose/g CO₂ = 192.6 grams glucose
• 193 grams ÷ 28.4 grams/ounce = 6.8 ounces glucose
• Target 0.5 to 1.0 million yeast cells per mL of beer using dried yeast with density >6 billion cells per mL; add 0.1-0.2 grams of yeast per liter of beer
• 17.4 liters beer x 0.1-0.2 grams yeast/liter = 1.8 to 3.5 grams of yeast

Carbonated Lager Example

• 17.4 L of beer
• Estimated carbonation level of 4.4 g/L
• Target carbonation level of 5.2 g/L using corn sugar
• Need to increase carbonation by 0.8 g/L (5.2 g/L – 4.4 g/L)
• 17.4 liters beer x 0.8 g CO₂/liters beer x 2.05 g glucose/g CO₂ = 28.5 grams glucose
• 28.5 grams ÷ 28.4 grams/ounce = 1.0 ounces glucose
• Target 0.5 to 1.0 million yeast cells per ml of beer using dried yeast with density >6 billion cells per ml; add 0.1-0.2 grams of yeast per liter of beer
• 17.4 liters beer x 0.1-0.2 grams yeast/liter = 1.8 to 3.5 grams of yeast

Response by Ashton Lewis.