How many bubbles are in a glass of beer?

 　　Modern beer is mainly made by fermenting water, grains, yeast and hops. During the fermentation process, the yeast will decompose the starch in the grain to produce delicious sugar, intoxicating alcohol and carbon dioxide-and this is the source of bubbles. When beer is being canned, additional carbon dioxide is injected by the manufacturer to increase the pressure in the container. This results in a lot of bubbles in the beer when it is opened or poured into the glass.

　　So, when you carefully pour beer into a glass, how many bubbles will pop up? In order to clarify this problem, Gerard Lieg Bellaire and Clara Silind of the University of Reims-Champaign-Ardenne in France did a thorough theoretical analysis and verified their predictions with experiments.

theoretical analysis

　　The total number of bubbles that can be produced in a glass of beer is limited by three factors: the total amount of carbon dioxide in the beer; the content of carbon dioxide in each bubble; the concentration of carbon dioxide in the beer when the beer no longer forms bubbles.

　　Carbon dioxide is soluble in water, but most of the data on the solubility of carbon dioxide are data in pure water. For beer, a mixture with an alcohol content of about 5%, the data is very rare. Fortunately, in 2013, some people have studied the dissolution of carbon dioxide in beer. However, there is one thing that is similar to water and beer: how much carbon dioxide can be dissolved in a liquid is closely related to air pressure.

　　This is called Henry's Law, which means that the higher the air pressure, the more carbon dioxide can be dissolved in the beer. Generally speaking, beer bottles will not be completely filled, leaving a little space at the mouth of the bottle. It is the air pressure here that determines how much carbon dioxide can be dissolved in the beer. Generally speaking, the solubility of carbon dioxide is not very high. However, due to the higher air pressure in the beer bottle, the solubility increases, and the volume of beer in the bottle is much larger than the volume of gas, so most of the carbon dioxide in the bottle is concentrated in the beer.

　　For beer at 0 to 5 degrees Celsius, the internal pressure is generally 2 atmospheres. When you open the wine bottle, the cap of the bottle flies with a "pop", the air pressure also drops, and the air pressure in the bottle returns to one atmosphere. At this time, the dissolved carbon dioxide in the beer becomes supersaturated, and the carbon dioxide can overflow.

The trick to bubbling

　　Obviously, whether the bottle is "gentleman" or not will affect the amount of bubbles produced when the bottle is opened. This is because even if the dissolved carbon dioxide has reached a supersaturated state, the formation of bubbles from the liquid will increase the energy of the entire system, so it is difficult for the gas to spontaneously emerge from the beer. Only when there is a ready-made gas cavity in the beer, the gas can spontaneously form bubbles from the beer.

　　According to the surface tension strength of the beer-carbon dioxide interface, atmospheric pressure, and the solubility and concentration of carbon dioxide in beer, the critical radius of curvature of the air cavity that stimulates bubble nucleation can be calculated. For the beer with 6 degrees Celsius and 5% alcohol content used in the experiment, as long as the radius of curvature is greater than 0.7 microns, or the diameter of the original air cavity is greater than 1.4 microns, it can grow from the air cavity into a bubble.

　　Even if you are not so "wild" when opening the bottle, try to avoid too much air cavity in the beer bottle due to shaking, and the glass will not let you go. Although the surface of most glasses looks very smooth, if you put them under a microscope, you will find that they are covered with tiny gaps left in the manufacturing process. Each gap is an indelible air cavity, allowing the carbon dioxide dissolved in the beer to flow out. Generally speaking, there are more uneven places at the bottom of the glass, which is why most of the bubbles will gush out from the bottom of the glass.

　　Carbon dioxide is constantly being precipitated, and the concentration of carbon dioxide in beer is getting lower and lower, so that the radius of the air cavity required is getting larger and larger. Until there is no more large enough air cavity in the beer to allow carbon dioxide to precipitate, the bubbles will no longer precipitate. Calculating the critical point, you can get the carbon dioxide concentration when the beer no longer bubbling.

A bubble

　　The evolution of gas is a dynamic process. As the bubbles are rising, there is continuous gas from the beer into the bubbles, and the bubbles are constantly growing. In addition to theoretical derivation, Bel Air and the others also directly took pictures with a high-speed camera. When the bubbles rise to the surface of the beer, the air pressure is basically equal to the atmospheric pressure, and then the carbon dioxide content in each bubble can be estimated. However, the shape of the glass and the amount of beer poured into the glass will affect the carbon dioxide content in each bubble.

　　Now, we have obtained the concentration of carbon dioxide in the beer at the beginning and the end of the bubbling, as well as the carbon dioxide contained in each bubble. With a little calculation, you can get the total number of bubbles in each glass of beer. Under Bel Air's experimental conditions, the bubbles in a glass of beer are about 200,000 to 2 million.

Rigorous physical experiment

　　Bel Air's research has summarized many factors that can affect the number of bubbles in beer. The type of beer, the alcohol concentration, the air pressure during the canning, the shape, material, and production process of the beer glass, the amount of beer poured into the beer glass, even the atmospheric pressure and the acceleration of gravity can affect the number of bubbles, as a rigorous physical experiment , Control variables are essential.

　　They chose a French brand of beer with an alcohol content of 5%. Beer is packaged in glass bottles, each bottle contains 250 ml of beer. Before the start of the experiment, each bottle of beer was kept in a refrigerator at 6 degrees Celsius for at least 48 hours. The cups should not be sloppy either. They chose four machine-made 500 ml glasses to ensure that they are the same as much as possible. First wash the glass with dilute acetic acid, then wash it with distilled water. After washing, dry it in a drying oven at 60 degrees Celsius. After the drying is completed, store the glass in an environment at 6 degrees Celsius. After that, gently pour the entire 250 ml bottle of beer into the glass to minimize the bumps in the beer pouring process.

　　Only under these conditions, the total number of bubbles in a glass of beer is 200,000 to 2 million. When the experimental conditions change, the number of bubbles produced in a glass of beer will change drastically. 2 million and very few are possible, because there are so many factors that can affect the total number of bubbles in a glass of beer.