Foamy Beer: The Physics of the Perfect Pint

What makes a perfect glass of beer? Should it be cold? It depends. A lager is best chilled, but a beer is best warmer. Lager may appeal when pale, but beer strives for copper, hazy gold, or brown. Some beers aim for a clean, crisp taste, and others for a funky, thick bitterness. But one thing unites them all: frothy beer is the best. A simple upgrade from the typical pint glass can harness physics to bring out the foam that enhances any beer.

Carbonation puts gas in beer

Beer bubbles are carbon dioxide, which can enter beer in several ways. The natural fermentation process of yeast digesting the grains of beer generates CO2. But the gases produced by the fermentation at the brewery mostly escape. Thus, the beer can be bottled before fermentation is complete, trapping the last gas in the bottle. Some beers are deliberately bottled with extra sugar and yeast to allow for extra fermentation. These beverages will be referred to as “bottled” (or “barrel-fermented” if the beer is stored and served in barrels).

Most commercial beers do not carbonate during bottling. After the brewing process is complete, the beverage is placed in a pressure vessel. Carbon dioxide is then pumped into the container until a certain pressure value is reached. The mixture is kept under pressure until the carbon dioxide is completely dissolved in the liquid, and this process works faster when the beer is cold. (The solubility of CO2 in water decreases as the temperature increases.)

The details of choosing pressure, temperature and time to achieve the right level of carbonation for a particular style of beer are complex. After natural or forced carbonation, gas is trapped in the beer via the cap seal until you break it.

Nucleation releases bubbles

Nucleation is the process of a tiny microscopic event triggering a much larger transformation in a material. When material first ventures into the right conditions to melt, freeze, boil, or tear apart, it usually cannot do so until the very first atoms or molecules somewhere inside trigger the ‘action.

Nucleation is probably easier to understand by example than by definition. You have probably witnessed this in boiling water. A perfectly still container of water can be heated to boiling point without bubbling. The moment the pot or cup is moved, bubbles burst furiously. The water is ready to turn into steam. It just needs the first little pocket of gas to form and jostle the rest of the molecules to form more bubbles.

Likewise, you may have seen freezing rain. Droplets of liquid fall from the sky and instantly turn into tiny spheres of ice upon contact with the ground. A still water droplet inside can be cooled below its freezing temperature and remain as a “supercooled liquid” until the first molecules are shaken off by contact and take the form of the very first crystal of ice. Once the first crystal has formed, the crystallization spreads throughout the volume and freezes it in a flash.

In the same way that sudden movement creates a trigger event for bubble formation, a small surface imperfection can also nucleate bubble growth.

Make better foamy beer at home

Most pint glasses are pretty much the same. Some found in Taverns will have noticeably more weight; the walls and base are made with extra glass to survive clumsy drops. But a really nice beer glass will come with a built-in bubble nucleation system.

When the glass is empty, look carefully inside. Nucleation sites are tiny pits etched into the bottom of the glass. You can see them in the glass below, above the stem and just below the index finger, arranged in a small outline of the CHIMAY beer company shield crest logo.

You can also watch for the telltale sign of bubbles continually flowing through the beer from somewhere at the bottom of the glass.

frothy beer

But beware ! If the beer is bubbling like crazy but the glass isn’t deliberately etched to create that effect, then the glass is probably just dirty. The same nucleation process can occur at other localized sites where bits of dirt are stuck to the sides or bottom of the glass. Dirt particles or floating debris cause the same effect.

In a properly carbonated beer, continued nucleation at these etched sites will continually release bubbles. This creates a light foam on the beer that will last for the duration of the drink. You can buy these nucleated beer glasses for a moderate premium over standard glasses. On many frothy pints, the extra cost is well worth it.

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