Dr. Charlie Bamforth, Professor of Malting and Brewing Sciences at UC Davis, recently presented an engaging ACS webinar on “Getting A Head Through Chemistry: Great Beer and A Frothy Foam.” This is a brief summary of the presentation, with tips on getting that beer “head of excellence.”
Beer foam is mainly composed of the same glycoproteins and organic acids of the liquid, but at higher concentrations. The exposed glycoproteins arise from denatured proteins (such as from barley) and autolyzed yeast cells. Combined with bitter acids, the hydrophobic protein regions form spaces around volumes of carbon dioxide. Dr. Bamforth describes foam quality as a combination of stability, lacing, whiteness, texture, and robustness. He cites four physical processes at play in beer heads.
- A vigorous pour decreases beer surface tension, aiding in bubble formation. Scratches at the bottom of the drinking glass can serve as nucleation sites for bubbles, but our speaker advised against purposefully etching glasses for this purpose as it may pose a health risk.
- Creaming (bubble rise or beading) will increase with increased temperature, but too high a temperature will create a less stable head. It will also create a less refreshing beer!
- Drainage of the liquid between bubbles destabilizes foam; Dr. Bamforth hypothesizes that tall beer glasses may increase drainage.
- Disproportionation has to do with small bubbles combining to form bigger bubbles. Smaller bubbles form a whiter, “prettier” head.
Guinness pioneered the addition of nitrogen gas to increase beer foam stability, but the gas can also change the texture and decrease the aroma of hoppy beers. Some companies use propylene glycol alginate as an additive for stabilization, but this can drop out of solution, which will in turn affect beer clarity.
Dr. Bamforth blames ethanol, detergents, and lipids for inhibiting beer foam stability. Beers with alcohol content higher than 6-7% have less stable heads, and detergent disrupts the interaction between polypeptides and bitter acids. It is best to hand wash beer glasses, then rinse well and air dry. Foam clinging to the side of the gas, termed “lacing,” is actually a sign of a clean glass. Changing your look can also improve foam quality: mustaches and lipstick carry lipids that disrupt bubbles. Always pour with vigor and sip from one side only.
Dr. Bamforth is the author of “Foam,” which he plans to be the first book of a six-volume series. You can find more information about the book’s release through UC Davis’ News and Information site and view presentation slides from the webinar as of March 21st.