Ask Mr. Wizard

Bottling from a Keg


Aaron Ritchie - Williston, Vermont asks,

We all (hopefully) know not to bottle-carbonate beers beyond a certain volume, such as 2.5–3 volumes. Say that you have a highly carbonated brew (3.5–4 volumes) that you force carbonated in a keg and now want to bottle from the keg. If you bottle into a regular 12 oz. bottle will there be any issues with the CO2 coming out of solution if the bottle is at room temperature? Or can you pull this off safely since the beer is already carbonated vs. what happens as yeast eats sugar when you bottle-carbonate?


Most bottled beers do fall into the 2.5 to 3.0 volume carbonation range, but there are styles that are typically carbonated to a much higher level. Many Belgian styles and German hefeweizens are normally carbonated to a higher level, sometimes pushing the 6 volume mark, and Champagne often times contains more than 8 volumes of carbon dioxide. The most important thing when bottling beers with higher carbon dioxide levels is selecting glass bottles that have a pressure rating aligned with the pressure developed in the bottle. Violating this basic rule results in exploding bottles; a safety problem and a product loss problem all in one.

So you want to have bottled beer in the 3.5 to 4.0 volume range and your plan is to carbonate in a keg and then bottle. I will address this question shortly, but will start with the easiest method first. And that method is bottle conditioning because you do not have to transfer the beer from keg to bottle. Whether you keg condition or bottle condition, the pressure rating of the bottle must be high enough to cover the possibility that the bottle is exposed to high ambient temperatures. Ordinary beer bottles are designed for use with beers with up to about 3.0 volumes of carbon dioxide, but since ordinary beer is pasteurized in a tunnel pasteurizer an ordinary beer bottle must be able to withstand well over 100 psi of pressure. Compare this to the 80 psi of pressure required to keep 5 volumes of carbon dioxide in solution at 80 °F (27 °C) and you can see that most beer bottles can be used for a very wide range of beers, as long as the beer is stored in a temperature-controlled home environment. Even so, it’s the norm to use heavier bottles — think sparkling wine heft — for beers carbonated in the 3.5 volume range because consumers buy beer in stores and then transport the beer in cars that can become very hot, resulting in increased bottle pressures during the warmer months of the year. This scenario can result in growler bombs and big messes since most growlers have low pressure ratings, but that is the subject for
another day!

In your case you have a keg of beer in the 3.5 to 4.0 volume range and you want to move it from keg to bottle. The tool for this job is a counter-pressure bottle filler that will allow you to: 1) Open a gas valve to pressurize the bottle with carbon dioxide to the same pressure as your keg, 2) Open a fill valve connecting the keg and bottle, and 3) Open a vent valve allowing carbon dioxide to escape as beer fills the bottle. When the bottle is full, the beer supply valve is closed, the headspace pressure is slowly relieved, the fill tube is removed and the bottle is capped. The trick is to do all of this with minimal oxygen pick-up and minimal foaming. Excessive oxygen pick-up during filling results in accelerated beer oxidation and foaming during filling results in beer loss and low-fills, bottle-to-bottle variation. Sometimes foaming is severe enough to make capping a nearly impossible task and this situation quickly becomes very maddening.

The thing to remember about carbonated beer is that when the pressure in the container is released, for example when opening a bottle or can of beer, the beer is supersaturated with carbon dioxide. Supersaturation means that the beer contains more carbon dioxide than allowed by the pressure and temperature of the system and the system will adjust to the saturation level associated with this new pressure and temperature condition by losing gas. When you gently pour beer from a pressurized container into a glass there is minimal foam formation and when the beer is left alone it slowly adjusts to this new condition over time. In plain terms, the beer goes flat. Compare this mental image of tranquility to what happens when roughly pouring a bottle of beer into a warm glass with a healthy dusting of salt granules in its bottom . . . you should be seeing a beer volcano that ends with half of the beer sitting on the bar top around the glass and the other half of the beer streaming bubbles of carbon dioxide into the environment. Beer gushes, or quickly releases carbon dioxide, when it is supersaturated with carbon dioxide and is treated in such a way that causes the beer to foam. Once foaming begins it builds upon itself since a gas bubble is itself a nucleation site for more gas bubbles to form.

There are numerous things that can make bottling difficult, but they all relate to the beer volcano scenario described above. The most basic rule of bottling is to start off with very cold beer. If you don’t have cold beer in your keg you are setting yourself up for trouble. Turbulent flow between the keg and bottom of the bottle is the equivalent of the rough pour, causes localized changes in liquid pressure and can result in foaming. This frustrating situation is often seen at bars where a rough spot in a draft line, often associated with a dirty line, or a rough transition causes beer to froth and sputter from the tap as the bartender struggles to fill the glass. This means your filling rig needs to be well made to minimize turbulence and kept very clean as dried beer leaves rough surfaces.

Another major cause of gushing during bottling is low headspace pressure in the bottle. This can happen if your regulator is set too low, or if you release the gas too quickly during filling. The thing about bottling that is quite different from filling a glass is that a very small amount of foam presents a very real problem because bottle necks are narrow; once foam rises into the neck and spritzes out of the gas relief valve more foam is often formed in the neck, resulting in slow filling and beer loss. The goal is to quietly fill the bottle all the way to the fill level without any foaming. Controlling gas pressure goes a very long way to accomplishing this goal. If you control the filling process correctly, you can induce a slow and predictable release of carbon dioxide after the bottle is full and before the cap is sealed. This is known as crowning on foam and is an effective means of reducing oxygen pick-up from the air that enters the bottle headspace when the filling tube is removed. One way to do this is to gently knock a full bottle of beer that has been quietly filled right before capping; that gentle knock causes gas to escape and foam the bottle. You can also do this as a rude party trick to set off bottle volcanos at will.

And the last group of factors relate to the condition of the glass. Rinse your bottles before filling. Wet glass is much, much smoother than dry glass and commercial bottle filling operations always fill beer into bottles that are rinsed immediately prior to filling. Not only does rinsing make the surface smoother it also rinses off dust that may be present on the glass surface; dust is like a salt crystal and will act as a nucleation site if present on the bottle surface.

Warm or hot bottles can also cause problems, especially when bottling beer with higher levels of carbon dioxide. Although commercial bottling lines work quite well with room temperature bottles, rinsed with ambient water, chilling glass at home is something that can be done if the need arises. This is not practical on a larger scale.

And the last major problem associated with glass is surface roughness associated with reused bottles that have either been etched by too many washes with aggressive detergents or reused bottles that have not properly been cleaned. Happy bottling!


Response by Ashton Lewis.