I'm doing a monthly series here at Food Matters that I’m calling “Friday Happy Hour,” in which I’ll delve into the science of alcohol production.

I don't know about you guys, but after the events of the past two weeks, I could use a drink. Though the official celebration in Germany has been over for several weeks, the style of beer served every year* since 1810 at Oktoberfest can probably still be found at your local beer seller. All kinds of breweries make octoberfest-style beers, but the right to call your product "Oktoberfestbier" is a strictly guarded trademark of the Club of Munich Brewers. Only six breweries are allowed to serve their beer at the celebration, Augustiner, Hacker-Pschorr, Hofbräu, Löwenbräu, Paulaner, and Spaten.

The style that all the imitators make is "Märzenbier," a lager brewed with a bottom-fermenting yeast (I'll talk about this in another post). Märzen means March in German, so why is a March beer being used for Octoberfest? The beer is stored (lager = to store in German) in casks to age over the course of the summer. Most lagers are brewed in the fall and stored over winter, and the cold weather would keep the beers from spoiling.

In the heat of the summer, before refrigeration, there was nowhere to keep the beer cool, so brewers turned to an alternative method of preservation: more alcohol! Oktoberfestbier must be at least 6% alcohol in oder to get the title, but more practically, this is to prevent the growth of unwanted microbes. But how could a 19th century brewer hope to measure the alcohol content of his beer?

The Science: Specific Gravity

Beer starts its life as a substance called "wort," a sweet liquid obtained when malted barley is bathed in heated water to extract the sugars and other flavorful compounds. This liquid has a very high density (high weight compared to volume), but once the yeast get started breaking down the sugars and converting them to CO2, the density begins to decrease. Even when the brewer switches the yeast to anaerobic conditions, and the molecules in sugar can't be converted into a gas and released, the alcohol produced has a lower density than water, further reducing the overall weight/volume.

The principal of measuring density has been around a long time, ever since Archimedes stepped into a bathtub (or so the story goes). When a solid object is put in liquid, it displaces a volume of that liquid equal to its own volume. So if I have a container filled to the brim with water and drop in a bowling ball measuring 1 cubic foot, the amount of water that would splash onto the floor would also be 1 cubic foot.

But an object of the same size made of styrofoam would float. Why? Because the volume of water that would be displaced from the container weighs much more than the styrofoam ball. Instead of sinking, the styrofoam ball will displace a volume of water equal to its own mass, and come to float with only that volume submerged.

This is the principal behind a relatively simple tool used to measure the density of a fermenting batch of beer, a hyrdometer, an object (usually cylindrical) with a known weight, but a lower overall density than water. When placed in a very dense liquid (high weight per volume), the hydrometer won't sink very far, since it doesn't take much of the liquid to equal the weight of the instrument. As the yeast break down sugars converting them to CO2 or alcohol, the density decreases, meaning the hydrometer must push up more liquid to match its weight, causing it to sink. Markings on the side of the hydrometer indicate exactly how low it has gone.


Ancient brewers didn't have a good way to measure absolute density, so instead they relied on relative measurements. The difference depth of the hydrometer at the beginning of brewing (the "original gravity" or OG) and the density at the end ("final gravity or FG) was the specific gravity of the beer. It's not really possible to accurately convert specific gravity of a beer to it's alcohol content - the exact ratio will depend on the original sugar content of the wort, the amount of time spent in aerobic (Co2-producing) vs anaerobic (alcohol producing) conditions, and a host of other factors, but this measurement allows a reasonable approximation.

The Review: Harpoon Octoberfest

When I can, I like to stick to locally brewed beers, and it doesn't get much more local for me than Harpoon Brewery. Founded in the early 1980's, it was the first brewery to obtain a license to produce beer in Boston in over 25 years (they like to show off their permit on the tour - it's numbered 0001). Their "Octoberfest" is obviously just Octoberfest-style (they don't brew in Munich), but it's pretty good.

It pours nicely with a rich head of orangish/white foam. It's very clear, dark brown with some deep amber tinge. There's a faint, very pleasant hops aroma, slightly sweet. Tastes pretty standard for an Octoberfest style, but it's crisp. A little malty, a touch of hops, and a very clean mouthfeel. Perfect for a brisk autumn day.

Up next month: Pumpkin beers! (I'm a fan so I'll be trying a panel)


* Actually, the celebration has been canceled 24 times, mostly due to the great wars (and a couple of cholera epidemics).