All About Carbon Dioxide for Homebrewers
Mount Vernon Brew Club
Tuesday, March 19, 2024
Carbon dioxide (CO2) is normally a gas at atmospheric pressures and room temperature, and it's the gas responsible for making our fizzy drinks fizzy. However, where you get your carbon dioxide and how you treat it can affect the outcome of your beers and other carbonated beverages.
Producing Carbon Dioxide
Most industrial gases, like nitrogen, oxygen, acetylene, argon, or helium are produced either by picking it out of the air we breathe or through chemical synthesis. Carbon dioxide, however, is rarely produced by either method, instead being produced as a byproduct from other industrial processes. Shipping cylinders of gas is expensive, so exactly what industrial process produced your carbon dioxide depends more on where you live and what industries are popular in your local area.
Most industries that purchase carbon dioxide don't necessarily care about where the carbon dioxide came from or what other gaseous impurities might be in their carbon dioxide. Industries like fire control and suppression, greenhouse plant growing, and metal fabrication don't really care if a very small percentage of their carbon dioxide is actually argon or nitrogen. If your carbon dioxide is produced from the petroleum/oil/gas industry, these impurities may be hydrocarbons, such as benzene or acetaldehyde, or other compounds like carbon monoxide.
The food and beverage industry does care about these small percentages of impurities, though. You may have observed how just a little bit of nitrogen used to push a carbon-dioxide-carbonated beverage affects its flavor, like in dark milk stouts. As a result, several grades of carbon dioxide were defined. In the United States, for most industrial applications, carbon dioxide must be at least 99.5% pure, which means that 99.5% of the gas in the cylinder must be carbon dioxide. The other 0.5% can be any variety of other gas. Food and beverage grade carbon dioxide must be at least 99.9% pure, meaning that up to 0.1% can be any variety of other gas.
The difference in cost between both varieties is just a few cents, but it can have a massive impact on your beer. While the concentration of impurities in even industrial grade carbon dioxide is unlikely to cause illness (even if the impurity is ammonia or benzene), it can affect the flavors of the beer, especially its aromas and textures. Beyond 99.9%, there are diminishing returns to improving the purity of the gas, and the cost of a higher purity generally outweigh the benefits.
Carbonic Acid Formation and Decomposition
Carbonic acid (H2CO3) is formed when carbon dioxide and water meet and are held under pressure. Carbonic acid is a gas (not a liquid), and it is formed when one water molecule (H2O) meets one carbon dioxide molecule (CO2). In the carbon dioxide, one of the oxygen atoms and the carbon atom are tightly held in a double bond, while the other double bond is broken to form two single bonds. One of the single bonds remains with the other oxygen molecule in the carbon dioxide, while the other single bond is formed with the single oxygen in the water molecule. Freed hydrogen atoms from the water molecule then form single bonds with the oxygen molecule.
However, under atmospheric carbonic acid dehydrates (sheds the water part from itself) into carbon dioxide and water again. This is what causes the "fizz" in carbonated beverages. The carbonic acid is splitting back apart, letting the water fall back into the beverage and the carbon dioxide escape into the atmosphere, bubbling up.
In carbonated beverages, the amount of carbonic acid formed is very, very small, and it falls apart quickly. If it did not, carbonated beverages would have no effervescence, as the carbonic acid would not break down into water and carbon dioxide very easily. While it is acidic, having a pKa of 3.49 ± 0.05, it also breaks down quickly, so it does not contribute a sour taste to even pure water. Given these properties, we believe you should focus your efforts on the texture effects of flavor, rather than the aroma or taste effects of flavor.
Carbon Dioxide Safety and Handling
Carbon dioxide can safely be handled at home, but some precautions should be taken to avoid injury. Smaller, lower-cost carbon dioxide cylinders are sometimes constructed out of aluminum, but many times, cylinders are constructed out of steel. Cylinders can be heavy, and you should be careful to avoid injuring yourself by using good lifting practices while moving cylinders. If a cylinder is too heavy for you to move alone, you should get help moving them. The gas inside of a cylinder also has mass. It's easy to think of a gas as not weighing very much, but a 20 pound cylinder can hold, well, 20 pounds of gas in addition to the weight of the cylinder!
To cram that much gas into a cylinder, it must be under high pressure. Most cylinders are pressurized to approximately 2,200 to 2,600 PSI, though the exact pressure is generally not recorded, since gas is dispensed by weight. Naturally, there is a risk associated with anything under pressure. Make sure that pressure relief valves are equipped on your regulator, and never open a cylinder without a suitable regulator attached. When valves get knocked off of cylinders, the cylinder itself can turn into a rocket, punching holes in walls and causing severe bodily harm. To avoid breaking off valves, never store a cylinder with a regulator attached. Cylinders larger than 10 pounds should be kept with some retention mechanism, such as a chain attached to a wall, which should prevent the cylinder from being knocked over.
Carbon dioxide is stored under pressure. When pressure is released, so is the potential energy. Quickly releasing lots of gas from an area of high pressure (inside the cylinder) to an area of low pressure (the atmosphere) also converts that potential energy to thermal energy. Specifically, the potential energy in the cylinder is used to sap thermal energy from the environment, meaning that the cylinder will get colder. If you accidentally release lots of gas, be careful of frostbite from cold surfaces, such as your regulator or valves.
Your gas cylinder must be inspected regularly. The inspection is to ensure that the cylinder won't burst or rupture when it is next filled, which can be very dangerous. Technicians at your local gas supply store can test or have your cylinder sent off to be tested using a hydrostatic pressure test. In a hydrostatic test, the cylinder's valve is removed and inspected. The cylinder itself is attached to a hydrostatic testing machine, which pumps the cylinder full of water and holds it over its pressure rating for several weeks. After the cylinder passes the test, it is drained, dried, and refilled with gas.
Cylinders must be tested every five years. If you have a cylinder that is older than five years old, you can continue to use it until it is empty, as long as the cylinder is in good condition. If there is corrosion or the cylinder is damaged, you should have the cylinder tested. When it comes time for your cylinder to be tested, you'll probably want to exchange the cylinder. If your cylinder is on lease from the gas supply company, they'll just take the old cylinder back and give you one of theirs. If you own the cylinder, you'll be without a cylinder for the time it takes to have it tested. Make sure the gas supply store knows that you own the cylinder and want it back, otherwise they may just take it!
Carbon dioxide can also be toxic, especially in the larger amounts that we store it at. While it is true that we breathe out carbon dioxide, the concentration of carbon dioxide in the air is relatively low. However, if you have a system leak and a large cylinder, you could suffocate yourself and those around you. Sometimes, leaks can be so small that they are not audible, especially if they are on the low-pressure or secondary side of your regulator. Therefore, you should keep an eye on the system pressure. If you notice the system pressure dropping quickly, you may have a system leak. When your carbon dioxide system is not in use, you should close valves to prevent the accidental loss of gas. Aside from causing a risk to safety, it's also super inconvenient to lose a bunch of carbon dioxide!
Carbon dioxide is heavier than air, so it settles on the floor and will go into your basement. Make sure that if you've recently encountered a system leak, you clear people and pets from the lowest levels of your home. Depending on the size of your cylinder, you may also want or need to ventilate the lower levels to return the atmosphere to a breathable condition. If you are lucky enough to have a gas system with a capacity larger than 100 pounds, you should also invest in a carbon dioxide alarm, which will alert you to a gas system leak.
Citation
Mount Vernon Brew Club. (2024). All About Carbon Dioxide for Homebrewers. Mount Vernon Brew Club. https://mountvernonbrewclub.org/site/index.php/ontap/all-about-carbon-dioxide/.
This work is licensed under Creative Commons Attribution 4.0 International.