There is, it is said, a time and place for everything. While I suppose this is generally true, I will suffer no grief when TV executives finally relegate reality shows to their rightful spot on Civilization’s Trophy Case of Shame, alongside Jar Jar Binks, and the Eighteenth Amendment.
Sourness in beer, however, is definitely a right place, right time phenomenon. Rodenbach Grand Cru would be a mere shell of itself without its puckering dryness, but the same acidity in your house pale ale probably points to a sanitation problem.
The Beer Judge Certification Program (BJCP) score sheet has this to say about sourness:
Tartness in aroma and flavor. Can be sharp and clean (lactic acid) or vinegar-like (acetic acid).
By far the most common cause of sour flavors in your homebrew is contamination (flaw) or inoculation (intentional) with souring bacteria. Here are the major roles.
Lactobacillus bacteria produce lactic acid and are responsible for a relatively smooth sourness. It’s the same tang you find in yogurt and sourdough breads. Lactobacillus bacteria are ubiquitous in our environment: If you don’t believe me, go ahead and leave that spent grain in the mash tun for a couple of days and then sniff. See?
Pediococcus bacteria also produce lactic acid, and with time, they can also create diacetyl, that movie theatre buttered popcorn flavor. Wyeast Laboratories also notes that Pedio can cause “ropiness,” which is every bit as pleasant as it sounds.
Acetobacter is the bacterium that makes acetic acid, which is responsible for vinegar’s puckering qualities. Unlike Lactobacillus and Pediococcus, Acetobacter requires oxygen, which is one reason why many intentionally sour beers are aged in wood, the porosity of which permits some oxygen.
Certain wild yeasts such as the various strains of Brettanomyces can also lend a sour note, though usually in combination with fruitiness (think over-ripe pineapple) or horse blanket funk. Clean sourness is more likely from bacteria than from Brett.
For the purposes of this article, let’s assume you aren’t intentionally making a sour beer and that these flavors are showing up in, say, a brown porter or Dortmunder. How do you prevent them? The answer, you might guess, is simple: sanitation, sanitation, sanitation!
A one-time issue probably means you weren’t quite as meticulous with your sanitation regimen as you normally are. In that case, chalk it up to experience and be more careful next time. But if you’re finding that these issues crop up repeatedly, then you probably have an infection somewhere in your brewery.
Stainless steel and glass are fairly easy to sanitize, but plastic easily scratches and can host bacteria from one batch to another. Next time you brew, whip up a big bucket of sanitizer, and let all of the plastic pieces soak for an hour or more while you make beer. If that still doesn’t fix your problem, then it may be time to replace those plastic hoses, maybe even your auto-siphon or fermentor. It’s a good idea to replace siphon tubing periodically.
Contrary to popular belief, Brett and bacteria aren’t really harder to kill than regular Saccharomyces, and good sanitation should be sufficient for most issues. You’re simply more likely to notice the effects of a Lacto infection than you might cross-contamination from, say, an English ale strain.