When you bottle condition a beer, you add priming sugar to the bottle and seal it. The brewers yeast in the beer ferment the sugar and release ethanol and carbon dioxide (CO2). The carbon dioxide that is released inside the bottle is trapped, since the bottle is sealed. As such, the level of carbon dioxide pressure in the bottle increases and, in turn, the level of dissolved CO2 in the beer increases.
Different beers are carbonated to different levels. English ales may have carbonation levels as low as 1.5 volumes of CO2 whereas German weizens may top 5.0 volumes of CO2. Most American craft beers are in the range of 2.5–2.6 volumes of CO2.
Residual Carbon Dioxide
Before you add priming sugar, your beer already has some CO2 dissolved in it. During fermentation, carbon dioxide was continually bubbling through your beer. And, given that carbon dioxide is a part of our atmosphere (it reached 400 ppm in May 2013), there was some carbon dioxide pressure exerted on your beer due to atmospheric pressure. As such, when you prime your beer for bottle conditioning, the amount of residual CO2 in your beer should influence how much priming sugar you add.
The first table in this article shows the amount of carbon dioxide retained in fermented beer before it is bottle conditioned or forced carbonated. The amount varies with temperature as colder liquids hold more dissolved CO2 than warmer liquids. (This assumes the beer is at standard pressure, the average atmospheric pressure at sea level — which can be stated as 101.3 kPa, 760 mmHg or 29.92 inHg.)
Carbon Dioxide From Priming Sugar
To reach the desired level of carbon dioxide in your beer, you need to know the existing level of CO2, then add the appropriate amount of priming sugar to reach your target.
The second and third tables show how much CO2 will be added to 5.0 gallons (19 L) of beer when you add a certain amount of priming sugar. The priming sugar is assumed to be corn sugar (glucose monohydrate); priming with different sugars will yield slightly different results. (In the final installment of the German hefe-weizen series, I’ll discuss priming with Speise.) The first table covers levels of priming sugar that are reasonable for most beers; the second table is for generating highly-carbonated beers.
So, if you wanted to reach, say, a carbonation level of 2.6 volumes of CO2 in a pale ale you fermented at 68 °F (20 °C), you would first see how much residual CO2 was in the beer (0.85 volumes), then calculate how much added CO2 is required (2.6 v – 0.85 v = 1.75 v). Then, look up how much sugar it would take to produce that amount (from the table, roughly 5.25 oz. or 149 g).
A few things may complicate these estimations. For example, what if your fermentation and/or conditioning temperatures varied? What if your fermentation temperature spiked around high kräusen, but dropped down later? What if you increased the temperature during late fermentation for a diacetyl rest? What if you fermented at one temperature and cold conditioned at another? The key to untangling these questions is to realize that during active fermentation, your beer has CO2 bubbling through it. After fermentation, you can lose CO2, but you can’t gain it. (This assumes that the beer is in a sealed vessel — a fermenter with an airlock, for example.)
If you experienced an increase in temperature in your early fermentation, but later the temperature dropped, use the late fermentation temperature as the temperature to determine your residual carbon dioxide. If you fermented your beer at one temperature, then cooled it for conditioning after fermentation had ceased, you didn’t add any CO2 simply by cooling it. So, use your late fermentation temperature as an indicator of residual CO2. On the other hand, if your beer warms up after fermentation, it will lose CO2. In this case, pick the highest temperature the beer reached after it was done fermenting as an indicator of your residual CO2.
Two other potential complications are atmospheric pressure and rising levels of CO2 in our atmosphere. As atmospheric pressure changes, so does the partial pressure of CO2 on our fermenting beer. Could this affect the residual levels of CO2? Likewise, the levels of CO2 are rising in our atmosphere. Before the industrial revolution, they hovered around 280 ppm. Recently, they have topped 400 ppm. With more CO2 in the atmosphere, won’t the partial pressure of CO2 on your beer be increased?
These are great questions. On the other hand, it’s Friday and I’d rather be drinking beer than figuring out the impact these may have. Some very quick calculations lead me to believe that the effect of either of these is going to be small enough to ignore in most cases. (I’ll do the full calculations later.) If you fermented your beer in the eye of a tornado atop a high mountain (i.e. at very low pressures), you might need to add more priming sugar. If you bottled your beer on the shores of Lake Nyos, you might need to add a little less. But for now, knowing your residual CO2 (estimated from temperature) and how much sugar to let you take control of your carbonation levels.