[Photograph: Flickr user Robert S. Donovan]
Tingly, effervescent, and fun—who doesn't love the tiny bubbles found in beer, Champagne, and a good ol' G&T? But what are those bubbles, exactly? Today, we look at the science of carbonation.
What is Carbonation, Anyway?
Carbonation is a solution of carbon dioxide gas in water. The carbon dioxide is generally kept in the water through pressure (either in a bottle or in a natural spring), and will slowly release once that pressure is relieved, forming bubbles of carbon dioxide gas.
How does carbonation occur?
- In nature, some famous mineral springs produce naturally-carbonated spring waters.
- Carbon dioxide is one of the byproducts of fermentation (along with alcohol) which means that many alcoholic beverages naturally become carbonated in the bottle.
- Carbon dioxide can also be forcefully dissolved into water with the aid of pressure. This is the technique most often used with mass-produced sparkling waters, sodas, and even some beers and sparkling wines.
Why do carbonated things taste good? Three reasons: first, when carbonated bubbles come out of solution, they tingle the tongue in a pleasant way. Second, carbon dioxide combines with water to form carbonic acid, which gives carbonated beverages a slight acidic bite. Third, bubbles lift aromatic molecules to the nose, which intensifies perception of flavor.
Is there an optimal carbonation level? Kind of. There are recommended carbonation levels for different styles of beer. Sodas tend to be more carbonated. Classic seltzer and mineral waters are about on par with soda*, but can vary depending on type and brand. Champagne generally boasts the strongest carbonation—roughly 1.5 times as much carbonation as a soda.
How is carbonation measured? Confusingly. Most carbonation systems dose out CO2 at a set pressure, measured in PSI or pounds per square inch. But, PSI is a measure of how hard a gas pushes against the container it's contained in, which means there's no way you could measure the PSI of, say, a glass of cola. Instead, carbonation levels in liquids are measured in "volumes," "bar," or grams per liter (g/L).
Carbon dioxide volumes refers to "volume of carbon dioxide per equal volume of liquid at atmospheric pressure and 20°C." Bar is a unit of measure that, after some fancy math, ends up being the same thing as volumes. g/L, on the other hand, measures the mass of carbon dioxide in a volume of water. Here are some rules of thumb:**
Beer = 2 to 4 volumes, or 2 to 6 g/L.
Seltzer = Around 4 volumes, or 6 g/L.
Champagne = Around 6 volumes, or 8 g/L.
*I'm pretty sure the chart in this link is supposed to be in g/L, not mg/l. **to convert from g/L to volumes, divide by 1.53, a conversion factor.
How to Make Strongly-Carbonated Drinks at Home
The most common complaint of people who use home seltzer machines is that the sodas these machines produce aren't as bubbly as store-bought versions.
How strong you'll be able to carbonate at home depends partly on what sort of carbonating rig you're using:
- All-in-one kits. The popular SodaStream series of carbonators come with proprietary gas canisters that release CO2 at a set PSI. Once that PSI is reached, a valve releases any excess gas, so it's difficult to increase carbonation beyond the manufacturer's default.
- Devices that use chargers. Carbon dioxide chargers are small capsules each containing about 8 grams of CO2. So, in theory, you could use one to add about 8 grams of carbon dioxide to a liquid. In reality, though, other dissolved gases will fight for room. Plus, some carbon dioxide stays in the headspace of the bottle. So if you want carbonation actually approaching 8 g/L, expect to use two or even three chargers per liter of liquid. Even then, you're limited to pretty imprecise levels of carbonation.
- Carbonation rig, with regulator. A home carbonation rig uses a large standard-format carbonation tank that feeds gas through a regulator. To carbonate using a standard plastic 2-Liter bottle, you'll need a carbonator cap. For classy presentation from a glass bottle, there's a custom adapter you can buy. You can then use the regulator to carefully control carbonation level. For even more precision, simply place the container on a kitchen scale and watch your grams/Liter creep up.
Now, some handy tips for any setup.
- First, start with really cold liquid because CO2 dissolves more easily into cold liquids.
- Flush the liquid at least once with carbon dioxide to help release dissolved oxygen. That is, pressurize the liquid, vent quickly to release carbon dioxide, then pressurize again.
- Finally, make sure to keep as little head space as possible in the bottle you're using, so more of the gas makes it into the liquid. If headspace can't be decreased, naturally occurring gases in the air (like nitrogen and oxygen) will be forced into solution along with your CO2 and will fight it for room. In these cases, simply flush the container first to replace the gaseous air gases with CO2, then carbonate.
Another thing to keep in mind: carbon dioxide is more soluble in alcohol than it is in water, which means it takes more grams per liter of CO2 to produce the same tingle. Most of the time, you can tell if you've gotten the carbonation levels right by simply tasting it, though the kitchen scale trick I mentioned above can come in handy for some.
Did You Know?
Carbon dioxide bubbles trigger pain. Specifically, they trigger pain cells that incorporate the TRPA1 protein, a mechanism responsible for the detection of general pain. If it seems odd that people like the feeling of this "pain," keep in mind that both alcohol and spicy foods trigger TRPV1, a similar protein that detects a different sort of "burning" pain.
The exact tingling mechanism isn't completely understood. As this Popular Science article explains, we know it's not purely mechanical because carbonated drinks stay tingly even deep underwater, when the bubbles don't pop due to pressure. And it's not just the acid, since there are plenty of sour drinks that don't taste bubbly. The latest research suggests a specific enzyme found on the tongue might be the mechanism, but apparently there's no true consensus.
Carbon dioxide can get you drunk faster. Though probably not enough for it to really matter. I remember when this news circulated a year or two back, but there only appears to be one real study on the subject. It found that the effect only lasts for about 45 minutes, and then it only affected 14 out of 21 test subjects.
Some Important Safety Notes
Some soda machines, such as the SodaStream, come with explicit instructions to not carbonate anything except for water. Honestly, I've ignored this advice on many occasions (thereby voiding my warranty), and the main problem is foaming: when foam gets into the upper housing of the machine, it can sit there and develop harmful mold. For that reason, I don't generally use my machine for anything but water anymore.
If you're using a CO2 tank and carbonator cap or similar rig, keep in mind that the tank can put out gas at hundreds of PSI, so a regulator is critical. Most plastic and glass bottles can handle somewhere between 100 and 200 PSI, but this can change depending on the atmospheric temperature and whether the bottling material has any flaws or defects.
If you're chasing high pressures, make sure that the bottle you're using can handle the pressure—in any situation. Here's an example of what I mean: let's say you carbonated a soda in a glass bottle designed for still wine. And let's say you get it up to a respectable 8 g/L CO2. At room temperature, the bottle would probably only be experiencing around 30-50 PSI—no problem for the bottle, and just fine, from your regulator's perspective.
If, however, you took that bottle and shook it, gas would rapidly come out of solution, raising pressures to 90 PSI. And if the bottle happened to be in direct sunlight when it was hot out at the same time, the pressure would increase even more. And what if then, you dropped it from a height of a few feet, right where the glass happened to be weak?
Here's what I'm trying to say: Always use a bottle that is rated for well above the maximum PSI you think you'll be exposing it to.
Ever tried to carbonate a cocktail (or other beverage) at home? What's your favorite thing to carbonate?