Phenolic flavors and aromas in beer are most often described as clovey, spicey, smokey, band-aid-like, or medicinal flavors and aromas. Except in a few beer styles where some of these flavors are considered appropriate, these compounds are for the most part considered a flaw. The most common source of the clove-like and spicey aromas in beer are the classic German hefeweizen-style yeasts. These yeasts convert ferrulic acid from malt into a compound which smells just like cloves, 4-vinyl guaiacol. The reason most hefeweizen and other German wheat based beers display so much clovey phenolics is because wheat contains more ferrulic acid than barley.
Many other yeasts produce detectable phenolic flavors as well. Many Belgian yeast strains and some British yeast strains will produce the same compound, although on a much smaller scale. One of the primary culprits of off flavors attributed to phenolic flavors and aromas are wild yeasts. These wild yeasts will produce many types of phenol compounds, but most of these are not the commonly accepted clove or spicy aromas and flavors, but a strong medicinal band-aid-like flavor which is not pleasant at all.
Before attributing the medicinal flavors to wild yeast, you should be aware of the effect that chlorine has on beer. When you brew with highly chlorinated water, the chlorine reacts with naturally occurring phenols in the beer to form chlorophenols. This is a common flaw in home brewed beer and is easy to remedy by filtering with activated carbon or using either bottled water or Reverse Osmosis (RO) water.
The yeast strains which produce the common clove-like phenolic flavors and aromas in beer do so as a part of their normal metabolism. These yeast strains have enzymes which most of the other brewers' yeasts lack. If you are having problems with phenolic aromas and flavors in your beer, a change in yeast will probably solve the problem.
Many of the homebrewer's problems stem from their mash and sparge routine. Phenols extracted from malts during the mash and sparge are called polyphenols, or tannins. These polyphenols react with proteins in the wort to form chill haze or permanent haze. Sometimes if hot-side aeration occurs, oxidized fusel alcohols flavors in beer are created. If you are having problems with polyphenols, you can reduce these compounds by controlling your sparge water temperature (below 170°F or 77°C), reducing the sparge time, and by using brewing water low in alkalinity. Another source of polyphenols in your wort is from malt and hop additions. When hops are added in the boil, polyphenols are extracted and end up in the wort. High wort pH will increase this extraction but the polyphenols will not dissolve readily until the pH drops below 5.5. Tannins in the husks of malt are there to act as an astringent inhibitor against fungal and bacterial attack. The majority of the polyphenols in wort come from the malt. If you are having astringency problems from polyphenol extraction in your malts, try stopping the sparge when the SG reaches 1.010, keep the sparge water pH below 5.5, and keep the sparge water temp below 167°F (75°C). Sparge water acidification is not normally done by the homebrewer, but stopping your sparge early and controlling sparge water temp are easy fixes.
One problem in trying to detect phenolic problems in your beer is that many people are insensitive to these aromas and flavors which many find objectionable. These people are genetically incapable of detecting phenolic flavors or aromas. So, what do you do if you just can't detect the problems others find in your beer? About the only thing you can do is have someone else taste your beer for you. Learn to rely on their senses. I'm sure they won't mind, after all, they will be getting a constant source of free beer.
There are several other sources of phenolic flvors which make their way into your beer. One is spoilage bacteria in your brewing water. Algae produce many phenolic compounds and are found in any water supply which is exposed to sunlight. Water sources exposed to industrial waste will be high in phenols. Be wary of using water taken from a surface source. Streams and lakes used for municipal water supplies are a big source of phenols. Bacteria is another source of phenols. When you encounter a problem which you can't explain readily, start looking for bacterial infestations in hoses or valves. Make sure you rinse well if you are using a chlorine based cleanser in your brewery.
As you can see, there are many sources for phenolic notes in homebrewed beer. Pay attention to the items listed above and maintain proper sanitation and you shouldn't have any problems.
Information for this article was adapted in part from Beer Flavors from brewiki.org, How To Brew, by John J. Palmer, and New Brewing Lager Beer, by Gregory J. Noonan.