While we all wish it weren’t so, you will sometimes encounter off-flavors in beer. Poor brewhouse technique, problems during fermentation or packaging, product mishandling, even dirty draft lines can all contribute. Here’s a roundup of some of the most common miscreants.
Oxidized This typically shows up as a “wet cardboard” or papery aroma, and is most often a sign of stale beer. Brewhouse errors may also contribute.
Diacetyl No mistaking the reek of artificial butter in this common contaminant. Small amounts may add complexity to English ales, but if you smell theater popcorn or butterscotch, it’s too much. Stressed yeast, poor fermentation techniques or contamination of kegs or tap lines may be responsible.
DMS This sulfur-based chemical most commonly is described as “creamed corn,” but it can be reminiscent of overcooked asparagus, green beans or cabbage, and in dark beers it may be more like tomato juice. A welcome complexity in lagers, its presence is usually a sign of poor brewhouse practice, although it can be produced in large quantities by some contaminant organisms.
Skunky Don’t blame the brewer. By refusing to put the beer into brown bottles, the marketing guys bear the shame for this. When blue light hits beer, it transforms a precursor molecule into methyl mercaptan, a powerfully stinky, sulfur-based chemical. This reaction can happen in just a few seconds exposure to the sun, or over a longer time by fluorescent lights in stores.
Solventy This nail-polish remover aroma is caused by ethyl acetate, which actually is nail polish remover. Most often encountered in strong beers, it may be detected by its smell, but also as a solventy, eye-watering sensation. This is a sign of some problem during fermentation such as stressed yeast or too-high fermentation temperature
Lactobacillus/Pediococcus While these microbes may be welcome in the context of Berliner weisse or lambic-style beers, in most other contexts, these bacteria spew a vile brew composed most often of diacetyl, plus the sweaty odor of capryllic acid (and related organic acids usually summed up as “goaty”), topped off with the yogurty tang of lactic acid. These beer spoilage bacteria commonly appear in draft beer due to poor keg cleaning, or more often dirty tap lines. Send it back!