By Greig McGill
Carbonation - the amount of “fizz” in our beer caused by dissolved carbon dioxide (CO2) - can make all the difference between a good beer and a truly great one. While getting the correct level of carbonation can’t save a faulty beer, it can really transform and elevate an already well brewed beer. Therefore, it makes sense to understand this facet of our brewing craft, and really make the most of those magical bubbles! In this article, I hope to cover the science of carbonation in an easily digestible way, and leave you with another tool in your kit for making great beer. Let’s start with the real basics.
What is carbonation and why do we want it?
Carbonation is the process of adding CO2 to a liquid, beer in this case, to make it fizzy. It is important to beer because it provides texture in the form of bubbles, some acidity helping to cut through sweetness (more on this later), and also elevates aromatic compounds adding depth and complexity of flavour.
The main cause of carbonation in home brewed beer is the presence of yeast during the fermentation and conditioning processes. CO2 is produced by the yeast in addition to ethanol and other compounds during fermentation, and a proportion of it dissolves into the beer. In some commercial breweries, this CO2 is stripped out during filtration and re-added via pressure in a bright tank, injection inline during transfer, or injection inline just before packaging. Home brewers will usually allow a secondary fermentation in the bottle to carbonate their beer, but many will also force carbonate via adding CO2 pressure in a keg.
Once packaged in a keg, bottle, or can, the CO2 remains trapped inside the liquid. When you open the container (or pour from a keg), the pressure is released, and the carbon dioxide begins to come out of solution, creating a pleasing foamy head that not only looks great, but helps with the release of aroma compounds to your nose as you drink the beer.
What is the “right” amount of carbonation?
With our drinker hats on, I think we all pretty much know that different beer styles suit different levels of fizz, right? Imagine a highly fizzy English Bitter, or Dry Irish Stout, or a really flat Belgian Tripel or Double IPA? The former two would taste washed out and thin, lacking in both flavour intensity and body, while the latter two would be sweet, flabby, and difficult to drink. For each style there is a range of carbonation levels that suit a beer, but finding that unique sweet spot for your specific beer is a particularly difficult challenge. It requires logical thinking and rigorous process, as well as a whiff of luck and artisanal skill.
There are many charts and tables for the correct range of carbonation for each beer style, but here’s a PDF from asianbeernetwork.com that also includes dosing rates and pressure levels to get you there - don’t worry, I’ll explain all this soon! Note the ranges are given as volumes of CO2? A volume in this parlance means an equivalent volume of gas to liquid, at zero degrees Celsius under one atmosphere of pressure at sea level. So if you have twenty litres of beer, and want to carbonate to one volume, you’d need to dissolve twenty litres of CO2 at zero degrees - there are tables to correct for your altitude, but unless you live at a particularly high elevation, it won’t make enough difference to matter. We’ll get into how you dissolve this CO2 shortly.
In practice, it pays to start at the lower end of the carbonation range, and slowly increase carbonation, tasting as you go, to find that magical sweet spot. This can only really be done via pressurised carbonation, as those of you bottle or keg conditioning can’t easily add more sugar without relieving the existing pressure, and accounting for the reduced yeast health, but hopefully it’ll still serve as a guideline. You could jury-rig a small pressure carbonator using those cool bottle carbonator caps from brewshop.co.nz (plug plug) and do a test carbonation alongside your normal bottle conditioning, dial in your “perfect” carbonation level, then adjust sugar levels for the next batch. Remember that if you overshoot your sweet spot while tasting (and after all, how do you know “best” until you’ve gone past it?) you can always relieve pressure to de-gas the beer a little and start again to get back there.
It’s the LAW - Boyle’s, Charles’, and Henry’s!
As a home brewer, understanding the gas laws can help you better understand how your beer will carbonate and dispense, and how to make adjustments to your equipment or process to achieve the desired carbonation level.
The three gas laws that are most relevant to home brewing are Boyle's law, Charles' law, and Henry's law.
Boyle's law states that the pressure of a gas is inversely proportional to its volume at a constant temperature. This means that as the volume of gas decreases, the pressure increases, and vice versa. It’s fairly obvious when you think about it - you’re squeezing the gas into a smaller space, so the pressure in that space will increase. This is important when considering the pressure inside a keg of carbonated beer. When you reduce the volume of beer in the keg or bottle by pouring out some of the beer, the volume of gas inside will increase, reducing the overall pressure, which can cause the remaining beer to foam up. To avoid this, it's important to monitor and adjust the pressure inside the keg as needed.
Charles’ law states that the volume of a gas is directly proportional to its temperature at a constant pressure. This means that as the temperature of a gas increases, its volume increases as well. This is important when considering the temperature of the beer when it's being carbonated. If the beer is too warm, it may not absorb enough carbon dioxide, resulting in a flat beer. If the beer is too cold, it may absorb too much carbon dioxide, resulting in over-carbonation and potentially even exploding bottles or kegs.
Henry's law states that the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of the gas above the liquid. This means that the more carbon dioxide you have in the headspace of your keg or bottle, the more carbon dioxide will dissolve into the beer. This is important when considering how much carbon dioxide to add to your beer during the carbonation process. It's also important to note that the solubility of carbon dioxide in beer decreases as the temperature increases (see Charles’ law above), so you may need to adjust the pressure or temperature depending on the desired level of carbonation.
Overall, understanding these gas laws can help you better understand the carbonation and dispensing process of your beer, and make adjustments as needed to achieve the desired level of carbonation and avoid any potential issues.
Carbonation Methods and Considerations
As a home brewer, your method of carbonation will largely be dictated by your packaging requirements. If you are kegging, you have the most flexibility, as you can simply add more CO2 (or degas to remove some) at any time. Life is good! If you’re packaging into bottles, you really only have two options. The carbonation methods are:
Bulk priming with Sugar: using the chart linked above (or any equivalent - the science is the same regardless of the source) simply calculate the required dose of sugar based on your total beer volume - eg. if you have 20 litres of a porter, and you want the minimum carbonation level for the style at 1.8 volumes, you’ll need 3.5 grams per litre - 20 x 3.5 = 70 grams of sugar. Ensure the sugar is well dissolved and mixed with your beer before you package it to avoid uneven carbonation. Leave your bottles at fermentation temperature for the beer for at least two weeks, testing carbonation when done by… well… drinking a beer! It’s a tough life.
Priming using carbonation drops or individual dosage: This is basically the same method as above. With individual sugar dosage, you simply take your bulk calculation and divide by your package size. Eg. in the above example, you need 70 grams for 20 litres, and you’re packaging into 500ml bottles, you’ll need 70 / (20 / 0.5) = 1.75 grams of sugar per bottle. Measure it out, add it to each bottle, fill, cap, gently swirl and tilt to mix well, then condition as above - including testing… always! Carbonation drops are, in theory, the same thing, though it pays to check their composition. If they are 100% fermentable sugar, you can simply use them as “sugar” for the exact same calculation above. Some have magical compounds that claim to enhance mouthfeel or other things salespeople think you will believe. If so, work out the percentage that IS fermentable sugar, and calculate as above.
Force carbonation: As we talked about before, force carbonation is the application of Charles’ and Henry’s Law, utilising temperature and pressure to dissolve exactly the required amount of CO2 in our beer. The remaining variable is the time it takes for that CO2 to dissolve at the given pressure and temperature, and those are provided in the chart linked above. This method is mainly useful for home brewers who package to keg, but could be applied to bottles using plastic flagons and using the carbonator caps mentioned earlier. This would not be advisable for beers requiring higher carbonation levels though, and some brewers do not like packaging beer in plastic bottles.
Well, this article is raging wildly out of control! We haven’t even talked about carbonation as a component of beer foam, or balancing your dispense lines for perfect pouring every time, no matter the style and carbonation level of beer… perhaps that’s a job for another day! I hope that consideration of the science behind carbonation will help you make better beers, and dial in that sweet spot every time!