Bottle Priming

This topic confused me many years ago for multiple reasons, but mainly because of all of the assumptions and loosey-goosey measurements tossed about in brewing procedures and recipes. The two fundamental questions that must be answered to go about calculating how much priming sugar to add to beer are: What is the target carbon dioxide content in the finished beer, and what is the current carbon dioxide content? Most homebrewers and commercial brewers in the United States express carbon dioxide content in terms of volumes. Another way to express carbon dioxide is by weight, typically in grams of carbon dioxide per liter. The weight method works easier for doing calculations about carbonation.

So let’s create a scenario here to review the fundamentals. Assume we have a carboy of beer containing 20 liters (5.3 gallons) of beer with 3.2 grams of carbon dioxide per liter of beer (1.6 volumes; 2 g/l = 1 volume), and assume the target carbonation level in the finished beer is 5.2 g/l (2.6 volumes). In order for this to happen, 2 grams (the final 5.2 grams minus the initial 3.2 grams) of carbon dioxide need to dissolve into every liter of our 20 liters of beer during bottle conditioning, meaning we need 40 grams of carbon dioxide. This is really an easy problem to solve if we use dry glucose (aka corn sugar or dextrose) or dry sucrose (aka table or cane sugar) as the priming sugar. Liquid sugars muddy things a bit because they don’t all contain the same amount of moisture and wort, liquid malt extract (LME) and dried malt extract (DME) all contain a mixture of fermentable sugars, unfermentable sugars, and some protein; making for less surety about what is really being added.

Glucose (C₆H₁₂O₆) has a molecular weight of 180 grams per mole and when fermented into 2 molecules of carbon dioxide and 2 molecules of ethanol, yields 88 grams of carbon dioxide. In other words, 2.05 grams of glucose yield 1 gram of carbon dioxide (180 ÷ 88 = 2.05). Hold onto that number for just a second, please!

Sucrose (C₁₂H₂₂O₁₁) has a molecular weight of 342 grams per mole and when fermented into 4 molecules of carbon dioxide and 4 molecules of ethanol, yields 176 grams of carbon dioxide. In other words, 1.94 grams of sucrose yield 1 gram of carbon dioxide (342 ÷ 176 = 1.94).

If we go back to the problem at hand, we are looking to produce 40 grams of carbon dioxide during bottle conditioning. Using glucose as the priming sugar we need 82 grams of glucose (40 grams CO₂ x 2.05 grams glucose/ gram CO₂) or 77.6 grams of sucrose (40 grams CO₂ x 1.94 grams sucrose/gram CO₂). These numbers are right in line with the online calculators I found. For this calculation to be completely correct, the moisture content of the sugar (typically 5-8%) needs to be factored in. Assuming 5% moisture bumps these weights to 86 grams and 82 grams.

There are a few difficulties with all of this. The first thing is being able to estimate the initial carbon dioxide content of your beer. All of the online calculators require you to input your beer temperature. This is used to determine the initial carbon dioxide content. Since carbon dioxide solubility increases as beer is cooled, decreasing your beer temperature will increase the carbon dioxide content. This is true if the headspace above the beer contains carbon dioxide. If you take beer at 68 °F (20 °C), for example, and cool it down after fermentation is complete, the carbon dioxide content cannot and will not increase if the headspace above the beer is not carbon dioxide. Most homebrewers who ferment in carboys or plastic buckets do not flow carbon dioxide gas into the headspaces of their fermenters upon crash cooling and what happens is that the fermenter sucks in air from the atmosphere as the beer chills and very little carbon dioxide pick-up is possible because there is simply not much (by weight) in the headspace to begin with.

If you use Corny kegs or a stainless steel fermenter for cold crashing and allow carbon dioxide to flow into the vessel during crash cooling things are different. But to assume that beer temperature is a good indicator of carbon dioxide content is a faulty assumption because it takes time for the beer below the carbon dioxide headspace to equilibrate with the headspace. This is why it takes so much longer to force carbonate beer using headspace pressure versus bubbling carbon dioxide through a stone or shaking the keg during carbonation. And keep in mind the online calculators are basing the initial carbon dioxide content on no head pressure above atmospheric (0 psig carbon dioxide pressure). I am not criticizing the assumptions used by these calculators because there must be a method to estimate initial carbon dioxide content and temperature absolutely affects this value, and all of the calculators that explain the methodology discuss this conundrum.

Without actually measuring the carbon dioxide content of your beer before bottle conditioning, all you can do is make an educated guess. It seems to me that using the fermentation temperature as the baseline for this makes the most sense as this is when the beer’s initial carbon dioxide concentration was established. If the beer is warmed or cooled for a few days you know the content is going to change, but you just don’t know how much. One online calculator suggests “tweaking” the temperature to be some sort of reflection of a weighted average. I can see doing this if you ferment a lager at 50 °F (10 °C) and warm it to 68 °F (20 °C) for a diacetyl rest for a week because this condition results in a reduction in carbon dioxide. Assuming that an ale picks up carbon dioxide when cold crashed is another story, as explained above, and you are probably best off not assuming much carbon dioxide gain when cold crashing unless you are adding carbon dioxide to the headspace and holding the beer cold for several days.

We do reader surveys here at BYO and I know that there are some scientists and engineers gritting their teeth about some of my rounded numbers earlier and my typical aversion to using decimals, although I have used a few in the above examples! My results are so, so close to the results from the online calculators that I am satisfied that my explanation of the problem you present is solid. And I know some people are thinking “what about the headspace volume!” Well, just don’t grossly under-fill your bottles or kegs and this can be blissfully ignored. And this brings us to that loosey-goosey stuff I referenced earlier.

What drives me bonkers is when a recipe makes the assumption that a known volume of beer is being carbonated. And add to this the arcane volumetric measurements often used to express the amount of priming sugar required! In order to have any hope of consistently bottle conditioning beer you absolutely must have two tools in your battery of brewing implements; a calibrated vessel and a decent scale capable of displaying grams to a tenth of an ounce.

The easiest, and perhaps the cheapest, way to calibrate a vessel is to slap a piece of tape on the side of a carboy or bottling bucket and add water one liter a time so that you can place volume marks on your tape. This sort of calibration tape will last a very long time if you are careful with how you handle your vessel. Knowing your beer volume and the amount of sugar required, coupled with an accurate means of weighing your priming sugar, is going to get you far closer to your end result.

And the very last bit of advice about bottle conditioning is to consider the age of your beer/yeast at the time of bottling. If you have used extending aging in the secondary your yeast viability may be low. A little pinch of happy yeast added to the bottling bucket is something to consider, and is certainly something that nearly all commercially brewed, bottle-conditioned beers have had added before bottling.

 

Response by Ashton Lewis.