16 things to know about foam
We don't pay much attention to bubbles in our beer, unless they're foaming over the coffee table.
When it comes to beer pleasure, though, bubbles are right up there with color, aroma, flavor, body and buzz. So, with the help of Roger Barth, Ph.D., author of "The Chemistry of Beer" (Owl's Nest Publishing), Marty Nachel, whose second edition of "Beer for Dummies" (Wiley) was just released, and the draft beer gurus at Anheuser-Busch, here's 16 things to know about foam.
1. Foam is the expansion of pressurized carbon dioxide that is created during yeast fermentation. Upon opening a bottle, the pressure drops, bubbles of expanding gas form then burst, allowing dissolved gas molecules to escape the liquid.
2. Zzzzzz ...
3. Wake up, or it's going to gush all over your coffee table! This phenomenon is not uncommon in homebrews, where bacterial contamination creates excessive bubble nuclei that expand uncontrollably.
4. Foam is good in moderation. It allows aroma to escape the glass and rise to your nose.
5. The best way to pour a beer: Tilt the glass at a 45-degree angle, commence pouring, then slowly straighten and aim for the center.
6. If it foams all over the place, that's your fault. Always use a larger glass so there's room for the foam. A half-liter bottle of hefeweizen won't fit into a pint glass.
7. Wheat beer foams more than barley because it has a higher protein content.
8. Foam is profit. A properly poured 16-ounce glass with a 1-inch head contains only 12.25 ounces of actual beer, which gives bars more servings per keg — about 35 more per half-barrel. At five bucks a glass, well, you do the math.
9. Foam is not a waste of money. When you ask a bartender to eliminate foam by pouring down the side of a glass, the dissolved CO-2 gas is not properly released — until it (belch) fills your stomach.
10. Some beer glasses are marked with a tiny "plimsoll line" (named after Samuel Plimsoll, who devised the marks for ship hulls). The actual liquid should reach that line, with foam rising above.
11. Foam stability is known in brewing circles as "head retention." Go ahead, you can laugh now.
12. Detergent and grease kills foam. Bars use special cleaning compounds and sanitizers instead of soap. You can tell if your glass was properly cleaned if foam adheres to the glass in parallel rings after each sip, a pattern known as "lacing."
13. Yes, the bubbles in Guinness go down. They also go up. It's actually a complex circular pattern created by the way the beer is poured and the shape of the glass. It happens in all beers — it's just easier to see in a dark, bubbly stout.
14. How a beer widget works: It's a hollow plastic container with a small hole. Air inside the widget is replaced with nitrogen, then the can is filled under high pressure, driving beer into the widget and further compressing the gas. When the can is opened, the compressed gas drives the beer out through the opening, producing a creamy head.
15. Bubbles need a place to grow, called a nucleation site. It can be either another bubble or an imperfection in a smooth glass. Some brewery glassware (Duvel, Sam Adams, Chimay) is etched at the base to create a neat-looking stream of bubbles.
16. Why beer explodes when you tap a bottle hard: Instead of gradually releasing and rising to the surface, the bubbles growing on nucleation sites are shaken loose all at once. This allows them to attract even more bubbles, which rise quickly through the neck and onto — you got it — the coffee table.