Ask Mr. Wizard

Priming When Kegging

TroubleShooting

Murray Nunn — Happy Valley, Oregon asks,
Q

All the recipes in BYO call for corn sugar “if priming.” There are many articles about kegging and carbonating and a few of them I read mentioned priming when kegging but did not say how much or if it is a standard practice. I am currently using 5-gallon (19-L) “corny” soda kegs and use bottled CO2 to carbonate. I brew all-grain IPAs and winter ales, normally around 6.5% ABV. Is priming for kegs recommended and if so how much corn sugar for a 5-gallon (19-L) batch?

A

This is a popular topic and we have run many variations on this basic question in past issues of BYO. I believe that there are some reasons to keg condition and will give some practical considerations for you to mull over about this topic. Whether carbonating in bottles or in a keg, you must measure the volume of beer you are carbonating in order to know how much priming sugar to add for a given level of carbonation. Without going into the nitty gritty detail of past material, 110 grams/3.9 ounces of sugar will increase the carbonation level of 18 L (4.75 gallons) of beer by 3 grams of CO2 per liter of beer or 1.5 volumes. You can check out this answer for the details on carbonation calculations https://byo.com/mr-wizard/bottle-priming/.

One common rule of thumb among brewers who routinely keg condition is to use about 75% of the priming sugar suggested in a recipe for bottle-conditioned beer. I have never been able to find the original source of this advice. Most of the explanations for this are vague and simply state that keg-conditioned beer becomes over-carbonated if the priming addition suggested for bottle-conditioned beers is not reduced. Some suggest that the problem is due to the common practice of pressurizing the headspace of the keg after filling to seal the lid. By doing this, the priming sugar has less headspace to pressurize. I am not sold on this explanation because a Corny keg usually has more total headspace volume than 5-gallons of beer filled into 53 bottles. Assuming the typical beer bottle contains 15 mL of headspace, 53 bottles have a total of 795 mL of headspace, which is greater than the headspace volume when a Corny keg is filled to 5 gallons (19 L). Volumetric details and hypothetical asides, the contemporary rule is to use less sugar for keg conditioning than you do for bottle conditioning.

The easiest and, in my opinion, best way to repeatably carbonate beer in a keg is with a spunding valve.

I spend a lot of time visiting commercial brewers as part of my job as Technical Sales Manager – Central Midwest at BSG. One thing that I am seeing and hearing more of lately is spunding. For many brewers the practice of capping a fermenting tank of beer with an adjustable pressure relief valve for the sole purpose of capturing carbon dioxide and controlling the head space pressure of the tank, in other words spunding, is a new technique and there is a lot of excitement about this method. Old school lager heads, like me, have been spunding for a long time and have wondered why other brewers have not used this handy method of carbonation. Not only does spunding make use of carbon dioxide that is produced during fermentation to carbonate beer, it also saves time, can improve foam retention when compared to methods that result in beer foaming, such as carbonation stones, and reduces gas stripping of aromatics. This latter point is a big selling-point for brewers who are adding gobs of dry hops, like the crazed haze head crowd.

The easiest and, in my opinion, best way to repeatedly carbonate beer in a keg is with a spunding valve. You don’t have to make adjustments for your batch volume or initial carbon dioxide content because the spunding technique works by releasing any excess pressure from the keg. This means that a surplus of carbon dioxide is intentionally produced. A typical carbonation level for draft beer is 5 g/L or 2.5 volumes. If a keg is being conditioned at 72 °F (22 °C), the equilibrium pressure is 31 psig. I indicated earlier in this discussion that 110 grams (3.9 oz.)of sugar are needed to increase the carbonation level of 18 liters (4.75 gallons) of beer by 3 g/L or 1.5 volumes of carbon dioxide (this takes into account the carbonation of the beer after/during fermentation).

When using a spunding valve, the exact amount of sugar is not critical as long as there is a bit more than needed. Boost the 110 grams (3.9 oz.) by 20% and add 132 grams (4.7 oz.) when you rack your brew from the primary to your Corny keg, seal the keg lid by pressuring to about 10 psig, and attach your spunding valve. Most of these valves do not have calibrated markings on them. You will need to adjust the valve to relieve at 31 psig before you are ready to walk away for a few days. This is easy to do by pressurizing your keg to about 35 psig, adjusting the valve so that it stops venting gas at 31 psig, and then reducing your headspace pressure to about 10 psig by pulling the tab on the pressure relief valve that is on top of the Corny keg.

Over the next several days, the headspace pressure will increase as the priming sugar is fermented. When the pressure reaches the relief set point, excess gas will vent until the priming sugar has all been fermented. The same method can be used without priming sugar as long as the fermentation is capped with the spunding valve when the beer is about 2 °Plato (0.008 SG) above terminal gravity. When the carbonation process is complete, simply remove the spunding valve, transfer the keg to your cooler, and allow your beer to cool and settle before serving. No further adjustment is needed. The last thing to check before serving is the equilibrium headspace pressure associated with the beer carbonation level. In this case the target was 5 g/L or 2.5 volumes. If the beer is stored in a 38 °F/3 °C cooler, ~11.2 psig is needed to maintain this level of carbonation.
Easy peasy!

Response by Ashton Lewis.