Carbonation: Homebrew Science
They say that first impressions count. Your initial judgment of a beer is highly correlated with its carbonation and clarity. When you open a bottle of beer, your first impression (aside from the packaging and possible brand association) is the satisfying “phhhht” sound of carbon dioxide gas escaping. If you are served a beer at a pub, or from the Corny keg of a fellow homebrewer, the first thing you can assess is the color, clarity and head of the beer. Many different variables — including ingredient choice, fining and filtration options — influence a beer’s appearance. In this article, I’ll discuss two of them — carbonation and clarification.
Traditionally, brewers matured lager beers for many months at low temperatures. This was designed to achieve three key goals. First of all, the flavor would develop with time as off-flavors — such as diacetyl, hydrogen sulfide, dimethyl sulfide and acetaldehyde — were removed and mature beer flavors, such as some higher alcohols and esters, were formed. (The majority of these flavors were formed during the primary fermentation and just “topped up” during maturation. This was discussed in the previous article in this series, “Flavor Matura-tion,” in January-February BYO.)
During this time, beer would also naturally carbonate as residual fermentable extract left behind in the “green” beer fermented to produce carbon dioxide gas. This gas became trapped in solution in the beer as the maturation vessel was closed to prevent gas escaping. The degree to which carbon dioxide (CO2) dissolved in the beer is determined by the temperature and the pressure of the tank.
Another issue in maturation is the clarity of the beer. It was discovered that beer that was kept for long periods at low temperatures developed a brilliant clarity as both yeast and non-biological hazes settled out of the beer to the bottom of the vessel.
For traditional ale brewers, these goals are the same. However, since the yeast is not as active at lower temperatures, the beers tended to be less carbonated and were more prone to hazes. In a modern brewery — including the one you have in your own home — the goals of maturation can be shortened. Beer can be clarified by fining or filtering, and CO2 can be added after the fact using bottled gas.
The level of CO2 dissolved in beer after normal fermentation is between 1.2 and 1.7 volumes, depending on the temperature. One volume refers to the amount of gas forced into solution. For example, one volume of CO2 means that one pint of gas has dissolved in one pint of beer. This is the amount of gas, produced from normal fermentation of sugar to alcohol and carbon dioxide, that beer at fermentation temperature will hold in solution without a top pressure being applied. It is usual to raise this level to between 2.4 and 2.8 volumes for packaged beer. This can be achieved two ways, by trapping the natural carbonation produced by finishing the fermentation under pressure or by forcing CO2 into the beer later, during the processing phase.
The law governing the amount of gas that will go into solution is Henry’s Law, which states that the concentration of a slightly soluble gas in a liquid is directly proportional to the partial pressure of the gas. (If a container holds more than one type of gas, each exerts its own pressure — its partial pressure — on the container.)
In our case, the gas is CO2, the liquid is beer and the partial pressure is equal to the total pressure, either in the headspace of the bottle or the maturation container. The higher the pressure under which the beer is kept, the more CO2 will stay in solution.
Carbon dioxide solubility is also influenced by the temperature. A colder beer will allow more CO2 to stay in solution, hence a cold lager will hold more natural carbonation than a cask-conditioned British ale at cellar temperature. Thus, carbonation in beer is a balancing act between temperature and pressure. In general, the lower the temperature and the higher the pressure, the more CO2 gas will stay in solution.
Professional brewers may transfer their beer from the fermenter to the storage tank (keg, Cornelius, can or bottle) with 1.0–1.5 °Plato of fermentable extract remaining. The fermentation that occurs produces sufficient CO2 to carbonate the beer to 2.8 volumes without raising the tank pressure above 15 psi (780 Torr). This will only work at around 40 °F (4.4 °C) and only lager yeast will ferment at that temperature. If this technique is used on ales, ale fermentation temperatures of 50–60 °F (10–16 °C) will require that the pressure in the tank will need to be around 30 psi (1600 Torr). The beer will not pour well at that pressure, so the beer will need to be refrigerated for serving. This technique is much easier to control in a keg where excess pressure, if developed, can be vented. Bottling is trickier because either the CO2 level is correct or it isn’t (and you won’t know until you pop the top).
Carbonation can be forced into the beer by manipulating the temperature and pressure. There are two main techniques for achieving this, in-line and in-tank carbonation.
In-line carbonation: This method does not apply to homebrewers and is used in all large breweries, either as a primary carbonation or as a final fine-tuning to reach the standard specification for carbonation just prior to packaging. CO2 can be injected into beer as it passes through a pipe on the way from one container to another. The gas is injected through a sintered stainless steel diffuser which creates very fine (10–100 µm diameter) bubbles that readily dissolve in unsaturated beer. The carbonation is rarely done before filtration due to the risk of CO2 bubbles disturbing the filtration.
In-tank carbonation: A top pressure of CO2 is applied to the tank, which is picked up and gently rocked back and forth. Any beer exposed to the high pressure of CO2 in the tank headspace will carbonate to the required level immediately. Once all of the beer in the container has passed by the headspace, even carbonation is achieved throughout the can. With a Cornelius keg this is even more rapid if the keg is placed on its side to increase the surface area. This method will disturb any settled yeast.
With time, the suspended solid particles in beer will sediment to the bottom of the vessel. The rate at which they settle is determined by several parameters, including the particle’s size and density, the viscosity and density of the liquid, and gravity. The law governing the rate of sedimentation is Stokes Law and it is stated as:
In the equation, V is the terminal settling velocity of the particle (in cm sec-1), D is the diameter of the particle (in cm), dp is the density of the particle (in g cm-3), dm the density of the liquid (in g cm-3), g is the acceleration of the particle due to gravity (in cm sec-2) and µ is the viscosity of the liquid (in dyne sec cm-2).
It can be seen that the best way to speed up the rate of settling of particles is to increase their diameter. Yeast with strong flocculation characteristics tend to stick together more easily and increase their particle size dramatically. The best way to improve the time taken for the beer to clarify is to reduce the distance they have to travel. Large brewers do this with horizontal aging tanks. In some cases, larger brewers use centrifuges that decrease the settling distance and dramatically increase the gravity component (the “g” in the equation) of Stokes Law with centrifugal force.
Isinglass is extracted from the swim bladder of the sturgeon, and some other tropical or subtropical fish. It is prepared by soaking in a mixture of acids for many weeks. The colorless, viscous liquid produced by this treatment is rich in collagen, with a net positive electrical charge. The structure is like that of a large net that falls through the beer, attracting and binding the negatively charged yeast cells, along with some proteins, lipids and antifoaming agents. The particles form large flocs that rapidly sink to the bottom of the tank or vessel. It is most often used in the United Kingdom, but many microbreweries and brewpubs in the USA use it rather than filtering the beer. Used in conjunction with a silica-based auxiliary fining agent, isinglass can significantly reduce the yeast in suspension in beer. Isinglass does not work too well with high yeast counts (i.e. over 4 million cells/mL) and it works best if the temperature is allowed to rise a little. Since it is a protein extracted from fish, it is denatured at relatively cool temperatures and should never be allowed to rise above 68 °F (20 °C), even in storage, although different species of fish provide collagen with different temperature stabilities. This is related to their particular protein structure. Preparations of isinglass are available that are already activated by acid and are then dessicated for storage. These preparations still require careful rehydration, however, so the manufacturer’s instructions should be followed accurately.
Many brewers choose to accelerate the maturation process by kraeusening. This involves adding back 10–20% of actively fermenting wort to finished beer in the aging tank. The kraeusen beer is 12–24 hours old and still contains fermentable sugar and a large number of actively growing, healthy yeast. This fermentation will produce the required carbonation in the beer and the active yeast will more quickly mop up the diacetyl and acetaldehyde, and purge off the volatiles. Using this process does require that the same beer is being produced repeatedly, and the beer still has to be chill-proofed and clarified. Homebrewers reproduce this process in a way every time they add back priming sugar and leave behind a small amount of yeast for bottle conditioning. Bottle conditioning differs from this technique in that the yeast count is much lower and the yeast is at the end of a fermentation. So, it has a much lesser ability to “mop up” the mess than the freshly aerated “active” yeast added with kraeusening.
You may have noticed that when you take your brilliantly clear beer from the cupboard under the stairs where it was maturing and put it in the fridge, it turns cloudy. This is due to certain compounds present in beer that form a solid precipitate when chilled. This haze is known as “chill haze” and it is formed when compounds extracted from malt and hops, known as polyphenols or tannins, combine chemically with proteins. Traditional lagering methods involved forming them and then allowing them to settle out. Modern brewers treat beer with adsorbant chemicals, such as silica gel or PVPP, which remove either the protein or the polyphenol or both. Many homebrewers simply live with the haze.
Since the term “lager” means “to store,” there is a tendency among some brewers to think of lager beers as the only style that requires maturation. Ales too require a period of maturation while the yeast work their magic. Since the yeast is more active at higher temperatures, this means that the biochemical reactions leading to flavor improvement occur more quickly. A British-style cask-conditioned beer is a classic example of a beer matured and then served in the same container.
The beer is transferred from the primary fermenter after a period of reduced temperature maturation at 50 °F (10 °C) to a stainless-steel barrel with some residual extract (or added sugar in the form of primings), some yeast, and perhaps some additional hops. The ale then carbonates to a slightly higher level, and the flavor develops. Finings are then added to clarify the beer of yeast and it is served at warmer temperatures so chill haze shouldn’t be seen. This all takes place in about a week. For homebrewing ale brewers, when primary fermentation is carried out in carboys, the beer is usually transferred to closed Cornelius kegs, or bottles for aging. The beer will then be chilled to a more appropriate temperature for consumption. Beer fresh out of primary fermentation would contain far too much yeast for appropriate maturation, so the beer should be reduced in temperature in the primary for a couple of days to slow yeast metabolism and drop the bulk of the yeast to the bottom of the vessel. Leaving the beer sitting for too long on the bulk of the yeast at high temperatures will undoubtedly lead to off-flavors due to yeast autolysis.
Steve Parkes writes the “Homebrew Science” column in every issue of BYO.