Keg Shaking to Carbonate
TroubleShooting
Matt Campo • Staten Island, New York asks,
I recently kegged my beer (almond brown ale) for the first time. I tried the “crank and shake” technique that was taught to me, shaking my corny keg vigorously for five minutes almost every hour for two days at room temperature before lowering the CO2 pressure to 5 PSI and inserting the keg in my kegerator. The beer comes out fairly quickly even when I release some of the built-up pressure and has a massive head to it (it takes up about half the volume of the glass); when I drink it, I don’t get any bubbles in the beer itself. What am I doing wrong here?
I’ve been writing this column for 15 years now and often times I feel like Abby of “Dear Abby” fame. I hear about brewers making the same mistakes year after year and just sometimes wonder why advice givers don’t seem to learn from solved problems.
The “crank and shake” method to carbonate beer, which has a scrumptious name, is widely suggested and is probably the crudest method imaginable for carbonation. You have done nothing “wrong” since you were following the bad advice given to you by others who unfortunately are fairly large in number.
What you did do is to use a poorly planned method. Carbon dioxide solubility is affected by two variables you can control; beer temperature and carbon dioxide pressure. The goal of carbonation is usually to dissolve somewhere between 5 to 6 grams of carbon dioxide per liter of beer (in US terms this equates to 2.5 to 3.0 volumes). The units are not important; the important thing is that we have a tangible goal to carbonation.
When adding carbon dioxide to beer using a gas cylinder, as opposed to bottle conditioning, it is best to begin the process with cold beer since carbon dioxide solubility increases as the beer temperature decreases. If your goal is a normal level of carbonation you will be targeting about 5 g/l or 2.5 volumes of carbon dioxide. Consulting a gas solubility chart will tell you that if your beer is 38 °F (3 °C) the corresponding equilibrium carbon dioxide pressure for 5 g/l of carbon dioxide is 13 psig (the “g” indicates that this is gauge pressure instead of absolute). What this means is that if you supply 13 pounds of regulated carbon dioxide pressure to a keg of beer maintained at 38 °F (3 °C) that the beer will absorb carbon dioxide until equilibrium is reached.
The important thing about this method is the use of a properly functioning regulator and an accurate pressure gauge. That’s an article unto itself, so I will let that thought linger. If you have a properly functioning regulator gas will flow into the keg as your beer absorbs carbon dioxide. This continues until the headspace pressure ceases to drop over time and that is when the process ends.
In a small keg this takes about three to five days to complete if you simply hook the gas up and leave your beer alone. In larger batches the process takes longer since the headspace area is small compared to the beer volume. Commercial brewers’ carbonation stones and in-line gas injection systems are used to create a much larger gas surface area and to reduce the time required for carbonation.
You can do this at home by shaking your keg as you did. The important thing, however, is to crank up the regulator to a pressure based on your carbonation goal. Otherwise, the whole endeavor is absolutely aimless. That’s why this method has the lovely nickname, “crank-n-shake.”
So why do people do this? One thing drives this method: speed. If you crank the pressure above the equilibrium target the gas drives into solution at a faster rate. The same is true with all types of equilibria. Take mashing as an example. If you put your mash pot into an oven maintained with a very good thermostat at, say, 152 °F (67 °C) the mash will eventually reach 152 °F (67 °C). This takes hours so we use a higher temperature for heating and then turn the heat down as the temperature approaches the set-point. This is pretty easy to control because we can easily measure temperature with a thermometer and we can respond to this information by reducing the heat and avoiding an over-shoot. But when carbonating we cannot measure the carbon dioxide content of the beer continuously and often end up with overly gassy beer.
There is one other thing that you need to investigate and that is your draft system. The easiest to use draft systems use enough beer line to provide pressure drop nearly equal to the keg pressure. Most beers are stored at 38 °F (3 °C) and contain about 2.5 volumes, so 13 psig is a common pressure seen in bars with draft beer. If you use 51⁄2 feet of 3/16” beer line you will have restriction about equal to this pressure (3/16” beer line has 2.2 psig of pressure drop per foot of line). Why is this important? Well, if you match the pressure drop of your draft system to the keg pressure you don’t have to drop the keg pressure every time you want to draw a beer. Not only does this practice waste carbon dioxide, granted not much, it also vents aromas from your beer.
So the next time you brew a batch of Almond Brown Ale, finish off this great sounding brew by 1) chilling the beer before initiating carbonation, 2) using the proper equilibrium pressure for carbonating your brew, 3) exercising a little patience — a few shakes a day won’t hurt if you cannot resist the urge and 4) use a longer hose to make pouring more controllable.
