BY ETIENNE BENSON
People have watched bubbles rising in a glass of beer and wondered where it all began, but few have taken the question as literally as chemist Richard Zare, the Marguerite Blake Wilbur Professor in Natural Science. Zare published a seminal paper on the "fizzics" of beer 10 years ago, but spoke on this evergreen topic Oct. 18 at a meeting of the Santa Clara Valley Section of the American Chemical Society.
Zare's obsession with beer science began, he says, when a friend pointed out that beer bubbles got larger and rose faster as they floated to the top of a glass. The reason seemed obvious at first: As bubbles rose, pressure from the surrounding beer decreased, allowing carbon dioxide in the bubbles to expand and bubble buoyancy to increase.
But then Zare noticed that beer bubbles at the top of a glass were as much as twice the size of bubbles at the bottom. To generate that much of an increase, he realized, the pressure at the bottom of the glass would have to be eight times the pressure at the top. Some back-of-the-envelope calculations showed that a glass of beer would have to be 240 feet tall to generate that much pressure.
"That squashed my idea that I had a simple explanation," he says.
Another problem with the pressure theory: It couldn't explain why bubbles in beer should act any differently from bubbles in water. "If you took an air bubble and released it in water, it would rise at a constant velocity," says Zare. "That's not what's happening here."
It turned out that pressure differences make only a minor contribution to bubble expansion. Instead, like snowballs rolling down a hill, bubbles expand because they accumulate material as they go.
The process starts when you open a bottle of beer. The sudden drop in pressure encourages dissolved carbon dioxide to escape from the beer. Most escapes in bubbles that form at the sides and bottom of a glass, where microscopic cracks serve as starting points, or nucleation sites, for carbon dioxide to gather. When the carbon dioxide at a nucleation site reaches critical volume, a bubble detaches from the glass and launches itself toward the beer's head.
The reason that bubbles expand and accelerate as they rise is that bubbles themselves act as nucleation sites. Each attracts more escaping carbon dioxide -- or, as Zare puts it, "bubbles nucleate bubbles."
Nucleation also is responsible for the fizz that results from adding ice, sugar or salt -- all crystalline in microstructure -- to a carbonated beverage. Like cracks in glass, edges on crystals provide nucleation sites where molecules of carbon dioxide can gather.
Why does beer contain carbon dioxide in the first place? To understand that, you have look at how beer is made, says Zare. Brewers start with four ingredients: yeast, hops, malted grain and water. Turning those ingredients into a carbonated, alcoholic beverage is a three-step process. First, the brewer pours hot water over the malted grain to produce a sugar- and protein-laden soup called wort. He or she then adds hops for flavor and boils the mixture for several hours. Finally, after the wort has been cooled to room temperature, yeast is added to spur fermentation, which can take several weeks.
As yeast grow, they feed on complex sugars and other nutrients and quickly use up the oxygen dissolved in the wort, explains Zare. That forces them to switch to Plan B -- a method of metabolizing sugar that works without oxygen. Luckily for beer lovers, Plan B has two major byproducts: carbon dioxide and ethyl alcohol. In a sealed container, the carbon dioxide dissolves into the beer and sets the stage for nucleation.
Zare also addressed the mystery of why most beer bottles are made of green or brown glass, which is more expensive than clear glass. The answer has to do with a process called "skunking," in which sunlight causes chemical changes that can render a bottle of beer undrinkable.
Skunking occurs when rays
of ultraviolet light strike hop-derived molecules called
isohumulones, which give beer its distinctive bitter taste. By
breaking a bond that bridges two parts of the isohumulone molecule,
light creates two new compounds. The smaller one binds to sulfur
atoms from nearby proteins to create what Zare calls "essence of
skunk" -- not exactly the same foul-smelling chemical that gives
skunks their swagger, but close. The compound is so potent that
even amounts in the parts-per-trillion range can ruin a beer.
Stanford Report, November 7, 2001