Bottled Bubbles

People love to drink bubbles. Why else would Americans drink an average of 56 gallons of soft drinks per person each year?

Getting Bubbles into the Bottle

Soda is a pressurized mixture of flavored syrup and water to which carbon dioxide gas has been added. In modern bottling plants, very cold liquid and carbon dioxide (CO2) are mixed together in big tanks called carbo-coolers, or carbonators. The colder the temperature, the more gas can dissolve in the liquid.

The bottling plant transfers the carbonated soda under pressure to bottling or canning machines. Machines fill containers and cap them immediately so that contents stay under pressure. Water sprays bring the sealed containers to room temperature so that condensation won't form when they're packed in boxes. Then the soda goes to stores and restaurants.

More Bottled Bubbles

Sparkling water is generally bottled or canned like soda. Even with naturally carbonated mineral water, bottlers usually filter the water and add carbon dioxide the way soda bottlers do. That way, potential contaminants don't give the water an “off” taste. Beer starts with a porridge of malted (soaked) barley, to which brewers add hops and special yeast. Soaking releases some of the sugar in the grain. “When the yeast gobbles up the sugar, it spits out alcohol and it spits out CO2,” explains Jonathan Satayathum at Cleveland's Great Lakes Brewing Company. Some breweries release the carbon dioxide and then carbonate their product as soda companies do. Others, like Great Lakes, maintain a certain pressure in the tank. Then they transfer the naturally carbonated beer under pressure to machines that fill kegs or bottles.

Why do beer and nonalcoholic root beer form foamy “heads” when poured? As their bubbles rise, they absorb chemicals that act as surfactants. These “surface-active” chemicals like to be on the surface, says Ira Leifer at the University of California, Santa Barbara. When they are, they weaken surface tension and delay bubble bursting.

Wineries that produce champagne and fine sparkling wine use a first step of fermentation to produce a low-alcohol still wine. They bottle that, add yeast and sugar, and let it ferment again. Afterward, wineries concentrate the lees (dead yeast) by placing bottles at an angle on racks and rotating them periodically. The bottlenecks are frozen in a chemical solution, the bottles go on a conveyor belt, and the tops are opened. Gas pressure shoots the lees out, machines top off fluid levels, and within seconds the bottles get corked and caged. The pressure inside the corked bottle is six atmospheres, or three times the pressure in a car's tires. No wonder the bottle goes “Pop!” when it's finally uncorked!

Be a Fizz Whiz

Look at an unopened soda bottle, and you won't see bubbles. That's because carbon dioxide is dissolved in the soda. Some is also in the pressurized space at the container's top. Gas pressure inside the container is higher than the air outside the bottle.

When you open the container, you hear the whoosh of escaping carbon dioxide. As Ranjan Patro at Memorial University of Newfoundland explains, “The difference in pressure between inside the soda bottle and its outside surroundings causes the gas to flow from the soda bottle.” Basically, the soda obeys a principle called Henry's Law: The amount of gas dissolved in a solvent is proportional to the partial pressure of that gas over the solvent. So, reduce pressure over the liquid by opening the bottle, and the amount of dissolved gas is reduced.

Pour soda into a glass, and tiny bubbles form. “As bubbles rise, they grow bigger and bigger until they reach the surface,” observes Leifer. That's because additional CO2 flows into the bubble from the soda.

“It's really hard for a bubble to form in the middle of nowhere, like in the middle of water,” adds Leifer. But tiny defects or irregularities in the glass provide places for gas to collect into a bubble. “That's why the bubbles seem to form in the same place and all go up in the same line,” notes Leifer.

Put your nose close to the top of the soda. As soda bubbles reach the surface, they burst and release tiny aerosols, or droplets. Thus, your nose feels wet. Sip the soda and feel your tongue tingle. Carbon dioxide is converted to carbonic acid inside the mouth. As that breaks down, its byproducts bind to receptors on the tongue, and you taste fizz.

Eventually, all soda goes flat. Dissolved gas moves out of solution and bubbles up. Finally, the gas concentration outside the soda equals that of the surrounding air. “For most opened beverages, equilibrium is not a tasty situation,” observes Patro.

Vocabulary

fermentation:
A chemical process by which the sugar in a liquid turns into alcohol and a gas. Yeast or certain bacteria can cause fermentation in fruit juices.

hops:
The dried, ripe flowers of a twining vine that give the characteristic bitter taste to beer.

surface tension:
A property of liquids arising from unbalanced forces at or near the surface of the liquid. This causes the surface to contract and have properties that resemble a stretched elastic membrane.

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Activity

  1. What causes carbon dioxide to separate from flavored syrup and water in a can of soda? Why?
  2. What is Henry's Law?
  3. You have probably seen champagne served in a special glass, called a champagne flute. A champagne flute is usually a tall glass with a narrow mouth. Why do you think these glasses are designed this way? Think about what you learned about surfaces and pressure from Henry's Law.