St. Patrick’s Day is one of my favorite holidays. Any celebration that rotates around drinking beer and whiskey is always going to be a winner in my book. The addition of corned beef and cabbage just confirms the day’s place in my heart (historical accuracy and origins of the holiday be damned). But to me the highlight of the celebration is the popularity of stout. I turn a blind eye (and keep my cringes to myself) to the green beer that colors the holiday but what truly delights my heart is the sight of so many stouts (mostly Guinness) in the hands of the people.
An Irish stout is one of the most bewildering beers to watch. In each drinker’s hand they hold a glass sparkling with tiny bubbles floating downward as if magically resisting science with what normally would go up, coming down. This enchanting effect makes the beer appear like a black landscape illuminated by fireworks dissolving into the darkness of the night. It is this effervescent cascade of bubbles in a freshly-poured Irish stout that makes the holiday seem all the more celebratory.
These tiny bubbles moving down the glass is more than just a mesmerizing display of tasty beer. The sinking bubble phenomenon is actually related to the fluid dynamics of the glass and had long puzzled physicists on its exact mechanism. In May 2012, Irish scientists (how fitting is that?) determined how this conundrum occurs.
First off, Guinness and the other stouts are poured using nitrogen gas. This element is harder to dissolve in beer and capable of producing smaller bubbles than carbon dioxide (the normal bubbly in your brew). These fine, dense bubbles are what give Irish stouts their creamy, velvet-like mouthfeel. They are also responsible for the sinking bubble effect that occurs when these stouts are first poured.
While scientists had known that this distinctive cascade effect was a property of nitrogenated beer and the fluid dynamic properties of these brews, the true breakthrough was showing that this effect was a result of the glass shape as well (and people still question whether all of the different beer glass shapes really matter). They proved this using the anti-pint model. No, the anti-pint is not the evil doppelganger of our most common glassware, the pint (although I think that would make the best Star Trek episode ever). It is a glass that gets wider near the bottom.
In a standard pint glass the walls flare outward as you reach the top. When bubbles are in beer they normally want to go up (due to their low density) in a vertical movement. In a pint glass, bubbles in the center of the glass meet less resistance in the liquid and can flow quicker. This causes a current in the beer that overpowers the nitrogen bubble’s ability to float upwards along the glass wall. This downward movement is only possible if the glass gets wider near its top. As the bubbles rise in the glass they are moving vertically but the glass wall is moving outward at an angle. This forms a low concentration of nitrogen in the liquid (technically calculated as a void fraction but this is beer article and not a fluid dynamic course) along the wall surface and allows bubbles to flow downward. If the pint glass’s shape is inverted (aka the ominous anti-pint) then the movement is reversed and the bubbles flow in the normal direction (upwards). This is called the Boycott effect and was first observed by tilting test tubes of blood to get the cells to quickly settle in the bottom.
The Stout Styles
In stouts, the nitrogen pour is only common with the Irish style of stout. But dry Irish versions are not the only type of these black brews out there. Strong versions of stout exist as well. Foreign (export) stouts were made with more malts, alcohol and hops to help them travel the seas and reach their destination in a still delicious state. Russian Imperial stouts, first brewed for Emperor Peter the Great of Russia, are similar to export stouts but are brewed even stronger.
There are also sweet stouts which entrance the glass with sensations of dessert. Their rich flavor distracts the palate from stout’s roastier components and replaces them with flavors of dense, gooey chocolate. The aptly named milk stout contains lactose, the sugar from milk. This unfermentable sugar is not sweet itself but instead adds a fuller body that compliments the sweet stout style. Mackeson Triple XXX Stout is a prime example. Oatmeal stouts have added oats that lace the mouthfeel with a silken, elegant experience (Samuel Smith Oatmeal Stout is a classic) and when they are poured on nitrogen it is like having your palate dive into a bed of feathers.
Oysters are the perfect accompaniment to stout but this marriage is accentuated in the oyster stout style. Yeah that’s right. A stout brewed with oysters. This surprising ingredient adds flavors of smoke and brine that bind with stout’s coffee and chocolate-like characteristics Actually many ingredients have found their way into stouts. From berries, chiles, to the more obvious coffee and chocolate (Young’s Double Chocolate Stout is a beautiful representation of chocolate and stout). So explore the style of stout because it goes a lot farther than just Guinness (or even St. Patty’s Day).
Originally published in the Santa Barbara Sentinel, March 13, 2013