I’m guessing at some point in their homebrewing experience, most brewers have made a bad beer. Either through poor process, poor fermentation, and/or poor sanitization, the beer has developed aromas and flavors not acceptable for the style being created. A great way to figure out what went wrong is by identifying the offending flavors and aromas in order to link them to the source of the problem.
The BJCP (Beer Judge Certification Program) occasionally offers a sensory evaluation class to help brewers and judges learn about such flavors and their sources. At the class, a sensory evaluation kit with 24 vials from the Siebel Institute in Chicago is used to spike a light, neutral tasting base beer (a control), allowing the aroma and flavor of each sample to come through clearly.
If you’d like to get in on one of these classes, join up with a local homebrew club and find out when a BJCP judge in their ranks can get a subsidized kit. You can also order one straight from siebelinstitute.com; it’s pricey at $250 with shipping, but if you consider that the kit serves about 20 people, that’s roughly $12 per person.
The first step in the class I attended was to get familiar with the base beer, noting the aroma and flavors present, so you know what to look for in the spiked samples. A spreadsheet was passed out to everyone listing the flavor, flavor descriptor, sources of the flavor, related terms, and the concentration of the flavor in the one-liter pitcher of beer. The class was broken up into five categories – process-related, hops, aging/storage, fermentation, and contamination/infection.
Process-related off flavors included DMS (Dimethyl Sulfide), metallic, and grainy. DMS will produce a cooked vegetable character that is similar to creamed corn and/or cooked tomatoes, making the beer less sweet. DMS can occur when you cover your boil kettle while boiling, not cooling your wort fast enough, or by contamination. A metallic flavor has a tinny, blood-like character that can come from bad water sources or non-passivated brewing vessels. A grainy flavor has a stale, tannic malt character that can come from excessive lautering/sparging, lautering at too high of a pH level, or from an insufficient boil.
Hops provide many flavors to beer but the kit highlights three of them. Geraniol is a floral, rose-like flavor that adds a big perfume character to the beer. Hops can also provide a lot of bittering, and in this example, an intense spice and herbal bitterness similar to quinine. Certainly the least favorable hop character is isovaleric, a flavor that is similar to cheesy, sweaty socks. This can occur with aged hops stored past their prime.
In the case of clear or green bottles, storing a beer is important. If left in sun for several minutes, a beer will become lightstruck and produce a skunky aroma and flavor. This is due to a chemical reaction between photons from the light and the hops in the beer. Aging a beer for a length of time can produce two types of oxidation characteristics: papery and sherry. Papery oxidation can make the beer taste like cardboard while a sherry-like (fruity, cherry-like) oxidation character can be quite pleasant in certain styles of beer.
Fermentation can produce quite a few flavors, both good and bad, depending on the yeast used. Pleasant characteristics include spicy clove and banana (found in Hefeweizen and Belgian style ales), almond (marzipan, bitter almond character), banana and pear (isoamyl acetate), flora, anise and apple fruitiness (ethyl hexanoate), and custard, cream soda-like character (vanilla). Unpleasant characteristics include(green bitter bruised apple character from staling or contamination (acetaldehyde), (oily, buttered popcorn character from contamination or improper maturation (diacetyl), rotten, dirty diaper, sewage character from poor yeast health and autolysis, the destruction of the yeast cells from its own enzymes (mercaptan), and nail polish remover character from poor yeast health or extreme fermentation temperatures (ethyl acetate).
Contamination can occur with improper sanitization procedures. Most contaminations occur when undesirable wild yeast and/or bacteria get into the wort or beer and produce undesirable flavors ranging from vinegar-like (acetic acid), sour milk (lactic acid), bleu cheese and vomit (butyric acid), damp top soil with beets (earthy), rancid dirty diapers and body odor (indole), waxy goat cheese (caprylic acid), clove spiciness, and buttery tartness. The exception to the above will be in wild/sour ales where pleasant levels of lactic and/or acetic acid are present.
Despite some very unpleasant flavors, it was a great learning experience going through the kit. Doing the class with a large group of people highlighted how different everyone’s palate can be. Certain flavors were more intense for some and harder to detect for others. It was also enjoyable seeing everyone’s reaction to some of the horrible flavors mentioned above.