It is helpful to understand carbonation to know little bit about what’s going on inside your keg of beer as you dispense it. This will help you serve better draft beer.
Carbonation = Carbon dioxide dissolved in a liquid. Whether it is water (seltzer), soda, champagne or beer, dissolved carbon dioxide is the component of the drink that gives it fizz and some added taste (bittering). It is a natural by-product of fermentation that is produced by the yeast involved. This makes it the natural compound for use in beer.
Carbon Dioxide or CO2 is a clear, colorless gas that is a key componnent of our atmosphere. It is also found dissolved in oceans, lakes, glaciers and all other bodies of water - liquid or frozen. CO2 dissolves very easily in water. It is also a by product of your metabolism and you exhale large quantities of it every day.
When you are dealing with beer in a keg you have three constituent parts that interact and affect one another: the liquid beer; carbon dioxide dissolved in the beer; and the carbon dioxide in the atmosphere of the headspace above the beer. How you interact with these three parts is the key outside influence on this otherwise stable system.
Dissolution, is the process of attraction and association of molecules of a solvent (beer) with molecules of a solute (CO2). As molecules of CO2 dissolve in beer, they spread out and become surrounded by the solvent (beer) molecules. Technically, the solvent here is water, but for this discussion, we will consider the amalgamation of suspended particles, dissolved sugars, alcohols and proteins as a singular unit – Beer.
So if some one ever asks if you think beer is "the solution", you can respond that "beer is a solution."
The rate of dissolution of CO2 in beer depends on:
Temperature and Pressure
Degree of under or over-saturation
Presence of mixing
Interfacial surface area between the CO2 and the beer
Conversely, the rate of precipitation (the opposite of dissolution) also depends on these factors. Precipitation or CO2 "break out" is of particular concern when serving draft beer.
All four of these issues will come into play in maintaining carbonation when you dispense draft beer.
Here’s another key definition: Equilibrium is the state achieved when the rate of dissolution equals the rate of precipitation (the opposite of dissolution). So when CO2 is in equilibrium with your liquid beer it is going into solution at the same rate that it is falling out of solution.
Equilibrium does not mean that activity has stopped within your keg. There is a balance of activity and you are going to be affecting that balance each time you pour a beer.
If you took chemistry or physics in high school or college you probably remember the term partial pressure.
In the simpliest terms, each phase of CO2 within the keg - gas dissolved in the beer and gas undissolved in the headspace of the keg has a pressure all its own or its partial pressure. So you have in play Henry's Law:
At a constant temperature, the amount of CO2 dissolved in a given volume of beer is directly proportional to the Partial Pressure of CO2 in equilibrium with that liquid. Furthermore, the concentration of CO2 in solution is directly proportional to the partial pressure of CO2 above the solution.
If you are still with me, try these on for size:
The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. In other words the CO2 in your beer and in a keg’s atmosphere is in motion – falling in and out of solution.
(1) CO2, will always flow from a region of higher partial pressure to one of lower pressure; the larger this difference, the faster the flow. CO2 dissolves according to its partial pressures, and not necessarily according to its concentration. This is why you can have over-carbonated beer - the beer will continue to absorb CO2 as long as the partial pressure of the un-dissolved gas is higher than the partial pressure of the dissolved gas. The reverse is also true: A higher partial pressure of dissolved CO2 will force the molecules out of solution and into the headspace until equilibrium is reached. This is what happens when the beer is too warm.
(2) At a constant temperature, the amount of gas CO2 dissolved in a given volume of beer is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
(3) Because solubility of CO2 decreases with increasing temperature, the partial pressure a given CO2 concentration has in liquid increases. So, CO2 from a beer escapes much faster when the beer is warm because of the increased partial pressure of CO2 due to the higher temperature. Inversely, with decreasing temperature, the partial pressure a given CO2 concentration has will decrease – allowing for increased gas concentrations or increased solubility.
You will be applying these principles to your beer in the keg.
In effect, you’ll start off with CO2 in two phases – one dissolved in the beer and one un-dissolved in the headspace or atmosphere of the keg. Each will have its own, different partial pressure until equilibrium is reached between those phases.
Therefore: The beer's carbonation level is determined by its temperature and the pressure applied to the liquid within the keg. CO2 is more soluble in cold beer than in warm beer and the equilibrium of the partial pressures keeps that gas from escaping from the beer into the atmosphere above at any rate higher or lower then the equilibrium rate.
What Does All This Mean To You?
In a draft beer system, you control the pressure of the headspace of the keg (the applied pressure) using a regulated flow of CO2. You also control the temperature of the beer. These two variables are what matters most in a draft beer.
Always keep your beer at the right temperature and at the right pressure to avoid these:
1. Beer that is too cold can seem to be flat because the CO2 stays in solution even after pouring the beer. The CO2 will come out of solution as the beer warms up - like when you drink it and the beer warms in your stomach - causing belching and feeling overly full.
2. Under-pressurizing a keg (this includes using the wrong gas blend) will allow the keg to go flat over time as CO2 slowly bubbles out of solution until it reaches equilibrium with the low pressure of the keg's headspace. It can also bubble out through your draft system.
3. The beer in the keg can quickly become over-carbonated when the applied gas pressure is too high.
All this means is keep your beer cold and pressurize your draft system properly with the right gas blend (if appropriate).
All the above points are made regarding carbon dioxide and do not apply to nitrogen which is insoluble (doesn’t dissolve) in beer. In mixed gas situations the gas pressure applied is the total of the gases in the mix. The CO2 component of the mix has a partial pressure that is a fraction of the total. That has a significant impact on your draft service if you are using mixed gas. Learn more.