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Phys In Your Fizz

April 2, 2010

Have you heard about Guinness White?

Guinness put out this ad a few years ago:

Good Things Come to Those Who WHITE
A pint of the black stuff becomes a pint of the white stuff with the launch of a creamier new version: “Guinness White”.
Guinness is made from the same raw ingredients as regular Guinness Draught – hops, barley, water and yeast, but the barley is frozen instead of roasted, which provides the white colour.

 But this beer isn’t a new invention. In San Seriff they have been making Guinness White since 1977. Back then, they sowed the barley seeds upside down, to get the inverted colors.

 Some bartenders came up with their own method of making Guinness White: They use a special glass with etching on the inner surface, which stops just where the (black) head starts.
The roughness of the surface is what makes the bubbles form.
You can order a white Guinness, then order a normal Guinness, and and the only thing that changes is the glass, not the beer. 

OK, so all three are clearly an April fool’s prank, but the last one is based on actual science. A modern beer glass very often has a few concentric rings etched on the base, which functions as a nucleation site to promote the formation of bubbles, and form the ‘head’ of the beer. Nucleation sites are where something starts to form, where a bunch of molecules meet and then stick together. Once a tiny bubble forms, it continues to grow because the gas molecules diffuses from the liquid into the new bubble. What drives the inflation is the difference between the higher pressure of the gas dissolved in the liquid compared with that inside the bubble. When the bubble develops enough buoyancy, it detaches from the nucleation site and continues to grow as it rises. This growth makes the bubble increasingly buoyant, causing it to rise more and more quickly toward the surface.

A Minty Spin On An Old English Favorite

Remember the Mentos-Coke fountains? (two of my favorite videos are here and here). Mentos, or anything added to a fizzy drink (sugar, salt, sand…) acts as a nucleation site for the carbon dioxide dissolved in the liquid. Gas takes up more space than liquid, even though the number of molecules doesn’t change. Once you have a lot of gas contained in the same space the liquid was in, the pressure increases and the gas escapes violently, and takes some of the liquid along with it.

Why was there gas in the liquid to begin with? The English chemist William Henry (1775-1836) formulated one of the gas laws (Henry’s law), which says that gas can dissolve in a liquid, and the amount that dissolves depends on the temperature and pressure of the system, and the type and volume of the liquid. Our fizzy drinks are all water-based and packed at a high pressure, to insure a maximum amount of gas dissolved. When you open the bottle, the pressure drops, and the carbon dioxide leaves the liquid and reverts to the gas form. The gas will continue to leave until it reaches equilibrium with the new pressure level (Le Chatelier’s principle: If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established). This process may take some time (days), but if you add nucleation sites it will go much faster (seconds), hence the Mentos-Coke fountain.

The same thing happens with divers and the nitrogen from the air they breathe. The air we breathe is about 78% nitrogen, the pressure is 1 atmosphere, and very little of the atmospheric nitrogen dissolves in the blood. As you dive into the ocean with an air tank, the deeper you go, the higher the pressure, and more nitrogen is dissolved in your blood. When you rise back to the surface, the pressure decreases and the nitrogen in your blood reforms as tiny gas bubbles, which might clog you blood vessels and cause a heart attack (a condition known as ‘the bends’). For this reason, it is recommended not to dive too often (as to not accumulate too much nitrogen in the blood) and also not to fly on a plane too soon after a dive (the higher you go, the lower the pressure, the bigger the gas bubbles).

The carbon dioxide in beer, Champagne, hard cider and other alcoholic beverages comes from the action of yeast. The yeast eat sugar and emit carbon dioxide as waste products. This is also what makes bread rise. This is the beginning of biotechnology, and of a different post.

Dissolved carbon dioxide functions as an amphoteric acid/base duo and has some very interesting functions in our bodies. That’s another biochemical post altogether. 

Sinking Bubbles

Another interesting phenomenon related to Guinness and to inverted things is the marvel of the sinking bubbles. We are very much used to seeing gas bubbles floating up to the surface of a liquid- an event related to the difference of density between the gas and the liquid- but sinking bubbles were thought to be nothing more than an optical illusion. In 1991, Dr Andrew Alexander (Edinburgh University, Scotland) and Professor Richard Zare (Stanford University, CA, USA) managed to film the phenomenon and prove that it is real (check out their site for the video and more). Some time later, Professor Clive Fletcher (University of New South Wales, Sydney, Australia) used a computer simulation called FLUET to explain how and why the bubbles go down instead of up. The conclusion was that the “normal bubbles”, which float up in the center of the glass, create a current in the beer, so the beer rises in the center and falls near the glass circumference. The bubbles close to the side of the glass are pushed downwards by the beer, and sink to the bottom.

Not every glass of Guinness will necessarily exhibit this effect. According to the researchers, the simulations only apply to beer in the trademark, barrel-chested Guinness glass.

The Gender Bias Champagne

photo by Gérard Liger-Belair  

Professor Richard Zare also found that Champagne tends to go flat when women drink from it. Most people drink Champagne on special occasions, and on those occasions, women usually wear lipstick. The lipstick contains an anti-foaming agent, which kills the bubbles in Champagne or any other bubbly liquid.

Champagne Bubbles Vs. Guinness Bubbles

There ways in which the beverages and their bubbles are dramatically different. For example, the champagne bubbles rise faster, expanded much more rapidly and form at higher rates than their beer counterparts, as was reported by Professor Gérard Liger-Belair (University of Reims, France) in 1999. He used a high-speed camera, strobe light, photographic enlarger, and ruler to measure sizes, speeds, and rates of formation of champagne bubbles.

Beer and champagne differ from each other in three properties: the amount of dissolved gas, the percentage of alcohol, and the concentration of large molecules, such as protein and starch. Champagne has three times the dissolved carbon dioxide and more than twice the alcohol content of beer. However, beer has 30 times champagne’s concentration of protein.

The higher gas concentration in champagne may cause the bubbles to form more quickly, the faster to leave the liquid and reach equilibrium with the atmospheric carbon dioxide

The large concentration of protein in beer acts as surface-active compounds, coating the bubbles and making them stiffer. The stiffer the bubble, the more drag it encounters, the slower it rises to the surface. Champagne, by contrast, has too little protein, and its bubbles grow too quickly to be covered by what little protein there is, so they remain flexible and rise to the surface quickly.

Ucorked: the science of champagne, by Gérard Liger-Belair seems like a really interesting book about, and it’s in plain English, for everyone to enjoy.

Who cares?

Why would renowned scientists spend their time worrying about bubbles in beer and champagne? Because it’s interesting and because even though we take those bubbles for granted, they have considerable value in the food and beverage industry. Plus, many things that seem trivial in one field have important applications in another. In the case of sinking bubbles and anti-foaming agents, the subject is very relevant to the study of emulations, which is extremely important to the food and pharmaceutical industry, dyes and paints, nano-biotechnology and many, many other fields. You start with beer, and then you take over world!


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