Malt Monday: For Your Eyes Only

4 May 2010
By Richard Pliny

by Richard Pliny, Beer Correspondent

Style: “Chocolate Stout”
Color: Black
ABV: 10.6%
Price Point: $22 for 750 mL
Technical Notes:
From the bottle, “Brooklyn Black Ops does not exist. However, if it did exist, it would be a robust stout concocted by the Brooklyn brewing team under cover of secrecy and hidden from everyone else at the brewery.  Supposedly Black Ops was aged for four months in bourbon barrels, bottled flat, and re-fermented with Champagne yeast, creating big chocolate and coffee flavors with a rich underpinning of vanilla-like oak notes.  They say there are only 1,000 cases.  We have no idea what they’re talking about.”

Hedonic Notes:

The beer pours an inky black with a coffee colored head.  Bubbles in the foam are small and persistent even as the beer warms.

A malty caramel smell with faint bourbon notes make up the aroma.  As the beer warms a very distant solvent flavor can be detected.  Vanilla notes come out as the beer reaches near room temperature along with slight coffee scents.

Sweet, thick, roasted malt flavors form the start of the taste and are followed by a vanilla and oak notes, with a faint red fruit flavor in the middle.  Cocoa and toffee form the transition to the finish along with some more sweet malty notes and hints of coffee following.  The vanilla, in particular, comes out nicely as the beer warms.  As in the aroma, an ethanol solvent character comes out slightly as it warms.

From the start, the {{mouthfeel}} has a noticeable viscosity and feels very heavy and mouth-coating.  The alcohol character comes out a bit as the beer warms, but does not feel like a 10.6% beer.

Both the flavor and aroma have great balance.  Though complex, the flavors complement each other and strike a balance between the smoky and sweet notes that is simply remarkable.  The beer is, despite its robust and formidable character, surprisingly palatable.  That said, it is certainly not a lawnmower beer, but would be great with smoked salmon and plenty of time to drink it.  Serving as a dessert beer would also work very well.  Even when warm, the beer does not taste as alcohol-solvent like as one might expect for the content, I suspect that aging will improve this characteristic.  Overall, Brooklyn’s Black Ops is a great beer that is definitely worth the investment.

Ratingcorkcorkcorkcorkhalfcork 4.5 out of 5 corks .  This is an excellent stout that has a wonderful amount of balance.  Try aging this for a few months and see how it develops.


Science!

Foam is a critical component in a beer’s appearance, texture and even aroma.  Foam color tells one about the malts used and bubble size lends hints about the level of carbonation and the protein content.  Bubble size and persistence influence how beer feels when drunk.  Because of the very high surface area, foam is able to act as a great interface for mass transfer, accounting for a great deal of the aroma one smells in beer.

Bubble size and persistence are two very important features of the foam that lend insight to the beer’s composition.  The size of bubbles is an indicator of the pressure inside the bubbles and the overall carbonation of the beer.  Bubble stability or persistence indicates a large number of properties responsible for maintaining the beer-air interface.

Beer Bubbles

A collection of bubbles in beer foam. Notice that some of the smaller bubbles appear to be growing into larger ones. Thanks to Placbo on Flickr.

Smaller bubbles require more internal pressure to maintain curvature.  The force on a bubble’s interior surface due to pressure difference is described as the area integral of the normal vector to the bubble’s surface (this simplifies to

, where ΔP is the pressure difference between the inside and outside of the bubble and r is the bubble radius), while the force of surface tension is described as

, where γ is the fluid’s surface tension.  Balancing the two equations so that the force is zero (indicating a stable bubble) yields the solution

This equation says that the pressure of a bubble is inversely proportional to the radius and directly proportional to the surface tension.  In other words, at constant internal pressure, smaller bubbles indicate higher surface tension.  We thus see that beers which tend to have smaller bubbles (like stouts and porters) tend to be a bit more viscous and have higher sugar and protein content.  Conversely, light lagers tend to have larger bubbles due to lower content of interface-stabilizing compounds.

As demonstrated by A. Leike, beer foam decay obeys an exponential decay.  In a paper for which he won an Ig Nobel prize in 2002, Leike shows empirically that the foam height obeys the equation

where h0 is the initial height and τ is a decay constant dependent on the particular beer.  One may thus compare foam stability of beer on a quantitative basis given measurements over time if observations are conducted carefully.

The exact parameters that determine the value of τ are quite detailed and varied.  Sugar and protein content play important roles, but so do polyphenol concentrations, presence of lipids or sterols and even alcohol content.  Foam stability in general is a topic of great interest currently and beer is just one of many areas of application for such research.


Further Reading:
A Leike. “Demonstration of the exponential decay law using beer froth.” European Journal of Physics 23, 21. 2001

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3 Responses to Malt Monday: For Your Eyes Only

  1. Trevor on 4 May 2010 at 6:30 pm

    I really like how you guys do your blog format, taste first than science…very nerdy. Just thought I should let you know that there was a Nature article published about the mathematics of grain growth in metals and in the abstract the authors mention that it would be similar mathematics when describing the foam on the head of beer. It was published in April 2007 by two guys, MacPherson and Srolovitz.

    As a Cornell alum, I am glad to see people are still taking their drinking seriously on the hill.

  2. Tom Mansell on 4 May 2010 at 7:49 pm

    thanks, Trevor! at first i read that second name as Slivovtiz…

  3. Rafe on 18 May 2010 at 8:27 am

    Hey there! Nice post. I actually attended at great bag-lunch lecture here at the University of Chicago concerning beer bubbles and foam. It was presented by the lead researcher in the foam department from Coors brewing. Learned some fascinating stuff.

    Just as a heads up, you’ve got the year wrong in your reference. The Leike paper came out in 2002. {Eur. J. Phys. 23 (2002) 21–26}

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