Beer is fizzy because it contains dissolved carbon dioxide (CO2) gas. That CO2 gets into beer one of two ways: yeast produce it naturally during fermentation, or brewers inject it directly under pressure. Either way, the gas stays dissolved in the liquid until you open the bottle or pour it into a glass, at which point it escapes as the bubbles you see and feel.
How Fermentation Creates CO2
The fizz in most beer starts with yeast. When yeast cells consume sugar during fermentation, they break it down into two main products: alcohol and carbon dioxide. Roughly two-thirds of the sugar becomes alcohol, while the remaining third is converted into CO2. Louis Pasteur was the first to demonstrate this experimentally, confirming that the bubbles observed during fermentation were a direct byproduct of living yeast transforming glucose.
In traditional brewing, this happens in two stages. During primary fermentation, yeast works through the bulk of the sugar in an open or vented vessel, so most of the CO2 escapes into the air. The beer at this point is essentially flat. Carbonation comes during secondary fermentation or conditioning, when the beer is sealed in a bottle, can, or closed tank. A small amount of sugar remains (or is added back), and the yeast continues to produce CO2. Because the container is sealed, the gas has nowhere to go and dissolves into the liquid under its own pressure.
Force Carbonation: The Shortcut
Not all brewers rely on yeast for their fizz. Force carbonation skips the waiting game entirely. Instead of letting yeast generate CO2 over days or weeks, brewers pump gas from a high-pressure CO2 cylinder directly into a sealed keg of beer. The pressure from the tank slowly infuses the beer with carbon dioxide, achieving in hours what natural conditioning takes weeks to accomplish.
Most large commercial breweries use some form of force carbonation because it gives them precise control over the final fizz level. The brewer sets a specific pressure and temperature, and the CO2 dissolves into the beer at a predictable rate. This consistency is harder to achieve with natural bottle conditioning, where slight variations in sugar or yeast activity can produce batch-to-batch differences.
Why Temperature and Pressure Matter
How much CO2 stays dissolved in your beer comes down to two variables: temperature and pressure. Cold liquid holds more gas than warm liquid, and higher pressure forces more gas into solution. This relationship, described by a principle in chemistry called Henry’s Law, is why breweries store and serve beer cold, and why a warm beer foams over the moment you open it. The gas that was happily dissolved at fridge temperature becomes far less soluble at room temperature and rushes out all at once.
Breweries measure this precisely. The industry standard method involves shaking a sealed sample vigorously until the gas and liquid reach equilibrium, then measuring the pressure and temperature with a manometer. A formula converts those readings into an exact CO2 concentration. For bottom-fermented beers like lagers, the target is typically 0.40 to 0.60 percent CO2 by weight. Top-fermented beers like ales and wheat beers range from 0.40 to 0.80 percent. Even factors like the beer’s sugar content and alcohol level affect how much gas the liquid can hold.
Different Styles, Different Fizz
Not all beer is equally fizzy, and that’s intentional. Brewers measure carbonation in “volumes of CO2,” meaning how many volumes of gas (at standard atmospheric pressure) are dissolved in one volume of liquid. A beer with 2.5 volumes of CO2 contains two and a half times its own volume in dissolved gas.
English-style cask ales sit at the low end, around 1.5 to 2.4 volumes, which gives them a soft, gentle carbonation. Dry stouts and brown porters are similarly restrained at 1.8 to 2.5 volumes. American pale ales and IPAs land in the middle at 2.2 to 2.8 volumes, delivering a crisper, more lively mouthfeel. Belgian tripels push toward 2.4 to 3.0 volumes, and German wheat beers (Dunkelweizen) hit 2.5 to 2.9 volumes, which explains their famously thick, billowing foam. The style of beer dictates its target carbonation level, and brewers adjust their process accordingly.
Where Bubbles Actually Come From in Your Glass
Once beer is poured, dissolved CO2 needs a way to escape, and it can’t just leap out of the liquid on its own. Bubbles form at what scientists call nucleation sites: tiny imperfections in the glass surface, microscopic scratches, or even small particles like cellulose fibers from the air or a towel. High-speed video imaging has shown that most bubble streams in a glass of beer originate from gas cavities trapped inside microcrevices in the glass wall, some as small as 20 micrometers across.
This is why some beer glasses have a laser-etched pattern on the bottom. Those deliberate imperfections create a concentrated zone of nucleation sites that produces a steady, attractive column of bubbles rising through the center of the glass. A perfectly smooth glass would actually produce fewer bubbles and look oddly still, even though the beer contains the same amount of CO2. A dirty glass, on the other hand, is covered in nucleation sites from dust, soap residue, and lint, which is why beer poured into an unwashed glass often foams wildly and goes flat faster.
What Creates the “Bite” You Feel
The fizzy sensation in beer isn’t just about bubbles popping on your tongue. Most of what you feel is a mild chemical sting. When CO2 dissolves in the moisture on your tongue, an enzyme on the surface of your taste cells converts it into carbonic acid. That acid activates pain-sensing nerve fibers in your mouth, which send signals through the trigeminal nerve to your brain. Your brain interprets this as the sharp, tingling “bite” of carbonation.
Researchers confirmed this by applying a drug that blocks the enzyme responsible for the conversion. When the enzyme was inhibited, the nerve response to carbonated water dropped significantly, and subjects reported a much weaker fizzy sensation, even though the CO2 concentration hadn’t changed. So the fizz you taste is really a chemical reaction happening on your tongue in real time, not just the physical sensation of bubbles bursting.
Why Beer Foam Sticks Around
The head on a beer is just CO2 bubbles trapped in a thin film of liquid, but not all beers hold that foam equally well. The key player is a small, heat-stable protein from barley called lipid transfer protein 1 (LTP1). This protein survives the brewing process and concentrates in the finished beer, where it coats the surface of bubbles and strengthens the film between them. Compounds from hops also contribute to foam stability by interacting with these proteins.
This is why a well-made beer can hold a creamy head for minutes while a glass of carbonated water goes flat almost immediately. The water has plenty of CO2 but lacks the proteins and hop compounds needed to stabilize the bubbles once they reach the surface. Pour technique matters too: a hard, aggressive pour releases more CO2 upfront and builds a bigger head, while a gentle pour down the side of the glass preserves more dissolved gas for a fizzier drinking experience that lasts longer.