Cell Culture: Oktoberfest

At noon on September 18 th , 2010, the mayor of Munich will kick off Oktoberfest with the traditional ''O'zapft is'' as he taps the first keg of the festival. Over the course of the next 2 weeks, more than six million people will celebrate the 200 th anniversary of Oktoberfest by savoring roasted chicken, juicy weisswurst, and giant pretzels. But, most importantly, there will be beer—enough beer to fill six Olympic-sized swimming pools. In this Cell Culture, we follow a typical beer at Oktober-fest, from its frothy pour into a 1 liter stein to its breakdown in the liver of an unseasoned tourist. Foam, fizz, froth, or bubbles—the multiple names given to the head of beer is a testimony to its upmost importance for beer lovers around the world. But why does this layer of foam form only in beer and not other carbonated beverages, such as soda and cham-pagne? The secret lies at the beer-bubble interface, where a protein in barley, the lipid-transport protein (LTP1), prevents the small bubbles from coalescing into bigger, more fragile ones. Now, according to Mills et al. (2009), a partial unfolding of LTP1 during the brewing process is key to stabilizing beer bubbles and increasing the quality of the head. LTP1 is the primary protein in beer foam (40 mg/ml). Like detergents , LTP1 possesses both hydrophobic and hydrophilic surfaces, causing it to align at the air-water interface of bubbles, where it lowers the surface tension. However, unlike detergents , LTP1 molecules form an elastic network of proteins that can stretch. Thus, when the bubble starts to expand, the protein layer is held in place, pulls the bubble back into shape, and prevents rupturing. In other words, the protein makes the bubbles ''sticky.'' Most proteins in barley denature and precipitate when the barley and hops are boiled together for 1–2 hr before fermentation. Using a combination of nuclear magnetic resonance spectroscopy and circular dicrohism, Mills and colleagues find that LTP1 refolds after 2 hr of boiling. However, the refolding isn't perfect, and a small segment of the protein at the C-terminal region stays unfolded. This structural rearrangement enhances LTP1's surface pressure and localization at the air-buffer interface , improving its ability to stabilize bubbles. Thus, remarkably, brewing conditions have evolved to match the subtle biophysical properties of barley LTP1; the proper amount of boiling before fermentation maximizes the quality of foam by inducing …