I was fascinated by the discussion in your book about why beer bottles are made of dark glass. A friend pointed out that he has seen some beer, such as Miller High Life, in clear bottles. Does this clear-bottle brew somehow lack the light-sensitive properties of other beers? It's not completely immune, but it is much less sensitive to light than most other beers are.

Light sensitivity is caused by hops, the dried female flowers of the hop plant that have been used for about 1,000 years to impart bitterness and a mellow aroma to beer and other brews. Hops boiled during the brewing process contain chemicals called isohumulones, also known as isomerized alpha acids, which when struck by visible or ultraviolet light produce some of the same chemicals that skunks spray at their antagonists. Beer drinkers understandably are not fond of the accompanying scent and spurn light-struck beer as being "skunky" or "skunked."

Skunkiness, incidentally, is quite different from the unsavory flavor that develops in beer that is stale or that has been subjected to warm or fluctuating temperatures. Stale beer can taste like cardboard, while skunked beer tastes like -- well, like a polecat smells.

The deleterious effects of light on beer have been known for more than a century. Historically, beer has been bottled in green or brown glass because those colors are easily produced by natural iron oxide in the sand used to make the glass. Brown glass is better than green, because unless it is very thick, green glass permits the passage of the primarily skunkifying green and blue light, while brown or amber glass blocks it. The widespread appearance of canned beer in 1935 sidestepped the problem, of course, but bottles still hold the allegiance of craft brewers and finicky drinkers.

Only recently (in 2001) did we figure out how light causes skunkiness. Chemists at the University of North Carolina and Ghent University in Belgium found that when exposed to light, the alpha acids in hops break down into free radicals that then react with sulfur-containing proteins to make a chemical called 3-methyl-2-butene-1-thiol, which is virtually identical to the principal constituent of skunk juice. Any time you see "thiol" in the name of a chemical, you can bet it's going to stink. Humans can detect this particular thiol at concentrations as low as one-billionth of a gram per 12-ounce bottle of beer. Apparently, skunks really know their chemistry.

So if skunkiness is caused by light acting on chemicals in hops, and if virtually all beers contain hops, how does Miller get away with using colorless bottles for its Genuine Draft, which has replaced its High Life? (It's the same recipe, but Genuine Draft is cold filtered instead of being pasteurized, as High Life was.)

Chemical trickery, that's how. Instead of using actual hops for bittering the beer, Miller uses a chemically modified form of hops' alpha acids known by several brand names, among them Tetrahop Gold. It does not produce 3-methyl-2-butene-1-thiol when struck by light, but according to the Ghent chemists, it can still produce rotten-egg odors. Uncolored bottles are cheaper than colored ones, however, so Miller's fiscal folks apparently prevailed over their flavor mavens.

Mexico's Corona Extra beer, also sold in a colorless bottle, is customarily served -- at least to us gringos, but not so much in Mexico -- with a wedge of lime stuck in its neck. Nobody really knows how the garnish originated, but cynical armchair historians have two theories: Either it was needed to wipe the rims of hygienically challenged bottles before putting them to the lips, or it served to cover up any skunkiness that developed during shipping and storage.

Me? I never know what I'm supposed to do with the lime, so I discard it and take my chances. Who needs the sourness of lime with his beer, anyway? It's not tequila.

Robert L. Wolke can be reached at wolke@pitt.edu.