Q. I pressure-ferment, then prime and bottle my beers. Some require cold crashing as well. I’ve read articles related to cold crashing but haven’t seen anything about the effects of pressure fermentation on dissolved carbon dioxide. How long does it take for “still” undisturbed beer to equilibrate CO₂, for example from 10 PSI to ambient at ~70 ºF (21 °C)? Can I simply ignore the increased solubility of CO₂ when half-crashing, for example at 55 ºF (13 °C) for 2–3 days?
Tom Brown
Via email

Mr. Wizard says …

Tom, these are great questions! I’ll start with the second because it has a clear answer. No, ignoring the carbon dioxide absorbed during a brief rest at cooler temperatures is not a good idea if you want to control carbonation in bottle-conditioned beers. There is, however, a useful workaround that I’ll cover after addressing your first — and much deeper — question about the effect of time.

Determining how much carbon dioxide is absorbed by beer after 2–3 days at 55 °F (13 °C) depends on several factors and is described mathematically by the rate of gas diffusion. The equation that follows describes how quickly a gas dissolves into a liquid over time:

V dC/dt = kL A (C* − C)

This relationship is a dynamic mass balance, meaning the rate of change in dissolved gas concentration depends on how far the system is from equilibrium and how effectively mass transfer occurs.

In this equation, V is the beer volume; C is the dissolved gas concentration at a given moment; C* is the equilibrium (saturation) concentration — a function of temperature and pressure; kL is the liquid-side mass transfer coefficient; A is the gas–liquid interfacial area; and dC/dt is the rate of concentration change. The driving force is (C* − C), which is initially large and diminishes as the beer approaches equilibrium carbonation.

When you cool your beer to 50 °F (10 °C) for some period, it absorbs carbon dioxide at a rate governed by this framework. Changing pressure raises C*, meaning more or less CO₂ can ultimately dissolve, while lowering/increasing temperature increases/decreases C*.

However, the rate of carbonation depends on kL A. In a still keg/pressurized fermenter, mass transfer is slow because the interfacial area is limited and boundary layers remain intact. Agitating or rocking the keg or fermenter, even minimally, increases both effective surface area and mixing, which increases kL and accelerates carbonation. Bubbling gas through a stone dramatically increases interfacial area compared to occasional rocking and has a much larger effect on the carbonation rate.

The value of kL depends on the beer itself and varies with specific gravity, the presence of other gases like nitrogen, pH, and alcohol concentration. In other words, kL is not something you can simply look up in a table. When gas diffusion rates are calculated, kL is determined experimentally, and its combined effect with interfacial area is expressed as kLA.

This is why it is difficult to know how much CO₂ is dissolved after storing a keg/pressurized fermenter for a few days at 10 PSIG and 55 °F (12 °C). While the equilibrium level is defined by those conditions, the path to equilibrium is not. The actual dissolved CO₂ depends on the system’s mass transfer history — whether the keg or fermenter was moved, how much headspace it has, and how diffusion occurred. Without knowing kL A, you cannot reliably determine how close the system is to equilibrium.

In practical terms, if you pressure-ferment at 10 PSIG, cool your beer to 50 °F (10 °C) while maintaining that pressure, and hold it for some period, the carbonation level will increase to an unknown value. This makes consistent bottle conditioning nearly impossible. Fortunately, there is a straightforward alternative if you approach it step by step. All you need to know is your fermentation temperature and the head pressure maintained during fermentation.

Step 1: Determine your current carbonation level using a gas table, such as this one from Zahm & Nagel. Because you fermented at a fixed pressure and temperature, it’s reasonable to assume the beer is saturated under those conditions. For example, beer at 10 PSIG and 70 °F (21 °C) contains less than 1.54 volumes of CO₂. Since the Zahm & Nagel table doesn’t go lower, we’ll use 1.54 volumes as an estimate.

Step 2: Determine the pressure at 55 °F (12 °C) that corresponds to 1.54 vol. From the gas chart, this is about 7 PSIG.

Step 3: Disconnect your gas supply to your keg/pressurized fermenter, cool the beer to 55 °F (12 °C) and maintain the pressure at 7 PSIG. Because this matches the equilibrium condition for the existing dissolved CO₂, the carbonation level will remain unchanged. The main reason for connecting gas to the keg/fermenter is to make sure there is no loss of gas due to a minor leak. Not always required.

Step 4: Calculate the priming sugar based on your beer volume and 1.54 vol. of CO₂, then prime and bottle as usual.

Hopefully, this combination of background, theory, and practical guidance helps answer your beast of a question. 

Fermenting Under Pressure (sidebar)

When beer is fermented under atmospheric pressure, the pressure at the bottom of the fermenter is strictly a function of liquid (hydrostatic) head — the distance between the bottom of the fermenter and the liquid level at the top. One atmosphere of pressure is equal to about 407 inches, or 33.9 feet, of water column. This value varies with liquid density; for example, 388 inches (32.3 feet) of wort with a specific gravity of 1.048 is equivalent to 407 inches of water with a specific gravity of 1.000.

As beer fermenters became taller to allow breweries to expand vertically, brewers began noticing differences in fermentation, yeast health, and beer aroma as a function of liquid head. Fermentation speed, gas evolution (fermenter foaming), and mixing tend to increase with fermenter height because carbon dioxide bubbles expand as they rise from the bottom of tall vessels, creating turbulent — sometimes violent — mixing. Increased hydrostatic head may also have a progressively negative effect on yeast health, as higher carbon dioxide solubility places stress on yeast cells. This stress often becomes apparent in subsequent fermentations. In addition, beer from tall fermenters tends to have a less pronounced ester profile. 

Over time, as vertical fermenters grew taller — and often narrower to minimize on-site tank fabrication — brewers empirically discovered there is a practical limit to liquid head. While no single value applies universally, many brewers established a maximum hydrostatic head suitable for their operations. For many, that limit was around 32 feet, or roughly one atmosphere of liquid head, based on 12 °Plato wort. Although beer aroma plays a role in setting this limit, yeast health is the primary driver. 

In 2006, I worked for a stainless-steel company that received an order from Sierra Nevada Brewing Co. to build a 10-barrel pilot brewhouse along with its accompanying cellar vessels. Because most of their production fermenters were 100- and 200-barrels, the pilot fermenters were built at 20 and 40 barrels. I was told that when they first began brewing their flagship Pale Ale, they increased the top pressure on the pilot fermenters to better match the hydrostatic head of their much taller production vessels. This adjustment made the pilot fermentations more similar to their production fermentations. For me, this was the moment when tank top pressure became part of my mental brewing toolbox.