Joe Sixpack: Beer primer: 16 things to know about foam

W E 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 West Chester University chemistry professor 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 are 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 the glass 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 kill 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: The widget is a hollow plastic sphere or cylinder with a small hole in one end. As the can is filled under high pressure, compressed nitrogen gas and a tiny amount of beer is forced into the widget. When the can is opened, the can's pressure drops suddenly, and the compressed gas and beer rushes back through the widget opening. This surge kicks off a chain reaction of tiny nitrogen bubbles that produces a creamy foam when the can is poured.

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.