After the boil, we send yeast into a clean and sanitary fermenter with sugar, nutrients, and a whisper. "Come out bright and clean, baby, will you?" From that point on, everything is done sight unseen. Our fermenting beer is much like Schrodinger's cat, existing in multiple states at one time given our inability to observe it. There's no sight glass on our fermenters, no glass manway. It could be going well inside, or off the rails. Rolling bubbles in the blowoff bucket give us a sign. Temperature control gives us some options. Our preparation for this moment gives us confidence.
The only way for us to know what's going on inside is to sample the beer from the Zwickel, a simple screw valve made of steel, brass, and silicon. It's the only sanitary access point we have to check the beer's progress. To do so, we spray the valve inside and out with an acid-based sanitizer solution that we keep handy for times like this. Then we collect an ounce of beer or so and dump it down the drain. That first rush is usually not a representative sample of what's inside the tank.
With that clear, we draw off a quart of beer and take its gravity using a hydrometer, a tool that measures how much sugar is left in the beer by how high it floats above the surface of the sample. We chart each beer's gravity over time. Usually, we see a steep drop in gravity over the first few days before the curve levels out. When we measure the same gravity two days in a row, we know the beer is fully attenuated, or that the yeast has metabolized all of the available fermentable sugar. That final reading is the terminal gravity of the beer, and that alone tells you how sweet or dry (or full- or thin-bodied) the beer is. It also helps us determine when we should crash the temperature in the fermenter. Too soon and the results could taste ugly; too late and we're tying up a fermenter that we could be brewing into and ultimately putting less beer out in the world. We err on the side of quality and scarcity.
We also use our Zwickel samples to taste the beer's progress. Every yeast strain is different, but during primary fermentation, we can taste all the off-flavors the yeast puts out during reproduction and attenuation. Sulfur, butter, green apple, and vomit are all to be expected. In fact, we have to know when to expect those foul flavors in order to know how fermentation is going. What's that old saying about sausage and legislation? Well, add beer to that list. The good news: as yeast finishes fermenting the beer, it responds to the depleted store of nutrients and increasing alcohol concentration by absorbing all the stuff you don't want to taste in your beer and falling to the bottom of the fermenter, where we can easily bleed it off.
This is the second post in my "What's That Thing Over There?" series in which I reveal the extraordinary and mundane of brewery miscellania. Last time, we got the skinny on the troll that lives under the brewdeck. Next time, who knows? The Thing you're wondering about may be coming up next. Just please, stop asking our poor bartenders what that damn thing over there is.