The human nose is capable of detecting trillions of scents. Unfortunately, we all have a different threshold. Below I will list common “ off flavors ” and well-known alternative scents to help you with a mental frame of reference.
Acetaldehyde - Commonly associated with the taste of green apples or cut grass. This off flavor is most often caused by poor yeast health, or not allowing the yeast to fully finish fermenting. Often stated as “green beer,” you can prevent acetaldehyde by pitching a proper amount of healthy yeast and ensuring a full fermentation. The flavor can be found and is acceptable in small doses in American Lager style beers.
Alcoholic - The alcoholic scent is reminiscent of nail polish remover and can be described as solvent-like. It is a noticeable smell which can create a harsh and hot alcohol feel in the mouth. The Alcoholic off flavor is most likely caused by fermentation at a temperature above the yeasts optimal range. Overall yeast health is also very important in prevention of this taste.
Astringent - Astringency is not really a flavor, but it is a noticeable flaw. It can be identified mostly in-mouth-feel due to a an extremely drying sensation on the tongue and mouth. Tannic in nature, it can be felt if you have ever put a tea bag in your mouth. Astringency is caused in all grain brewing from having mash water that is too alkaline (can be from over sparging or improper pH adjustments) or from too much bittering hops. To correct this flaw consider boiling bittering hops for less time and looking into water mash adjustments to prevent this flaw.
Buttery - The butter (think movie theater popcorn) or butterscotch flavor is usually caused by diacetyl. It can occur in the aroma and can attribute a slick feeling in mouthfeel. Diacetyl is a natural product of fermentation and one that yeast will generally “clean-up” by themselves. Although acceptable in some styles, and more prevalent in certain yeast strains, reducing this off flavor is usually accelerated by raising the temperature of your ferment towards the end of fermentation. Diacetyl can also be the sign of a pediococcus infection, so if you are seeing this off flavor in all of you beers look towards your sanitation procedures for remedy.
Cardboard - This is an easy one to recognize, just think about that wet package that was left on your doorstep. This off flavor is caused by oxidation - the addition of oxygen in your beer after fermentation. As oxidation increases over time the flavor can change from cardboard to more like sherry. The key to preventing this flavor is to try and minimize the amount of oxygen that is available to your beer. Don’t pour beer into a bottling bucket or keg, but instead gently siphon avoiding splashing. Don’t allow too much head space in a bottle, and make sure that when adding water (for priming sugar) that it has been boiled to reduce oxygen levels.
DMS (Cooked Corn) - which stands for dimethyl sulfide is attributed to a cooked corn or vegetable (cabbage) smell. DMS is often a result of not boiling your wort for a long enough time and is most prevalent when using pilsner malt. To avoid DMS make sure to vigorously boil for at least 60 minutes on all-grain batches not using pilsner malts and 90 minutes or more with pilsener.
Fruitiness - While fruitiness covers a rather large umbrella of olfactory scents the smell of ripe bananas is most often attributed as the undesirable fruity flavor. While acceptable in some wheat ales and Belgian styles, it can be considered a major flaw in clean styles of beer. The fruit scents come from esters which can be a naturally derivative of the yeast strain used or a result of improper fermentation profile: too high a temperature, or stressed yeast. Additionally, a high percentage of alcohol can combine with other flavors to create fruity flavors.
Grassy - I bet you would never guess how this off flavor is defined, right? The grassy smell and taste is often associated with a freshly mowed lawn or hay. It is usually a result of improperly stored ingredients. While some hops can present excessive grassy notes and are deemed acceptable in IPAs, the cause is usually old malt. Make sure not to crush grain too far in advance of brewing and that all grain is stored in air-tight containers. If you determine it is hop derived, then consider using a different hop or utilizing less of that hop in future recipes.
Phenolic - There are two categories to phenols: The first is what I categorize as “spicy” and can be pleasant like cloves and smoke, or it can be unpleasant like band-aids. The second category is chlorophenols which are phenols containing chlorine and can give off plastic or medicinal smell and flavors. Spicy phenols are yeast derived and can be wanted like in some Belgian strains, or they can be unwanted and come from wild yeast. The chlorophenols are often the result of your city water (chlorine added for sanitation) or using chlorine bleach as your sanitizer. If these phenolics are unwanted then it boils down to watching your sanitation procedures to prevent unwanted yeast or introducing chlorine into your beer. Also consider filtering your water if is known to have high chlorine concentrations (you will be able to smell it).
Lightstruck - Ah mercaptan, the off flavor which is synonymous to the smell of a skunk. Skunky beer is caused by UV rays reacting with the hops in your beer (isomerized alpha acids). It is most often associated with beers like Heineken and Rolling Rock because they come in green bottles which don’t block as much light as brown bottles. Be sure to use brown glass, or stainless steel kegs for packaging your beer to avoid ‘Pepe Le Pew’ in your brew.
Sourness - As one of the four human tastes we are all familiar with sour tastes in the mouth. Sour beer is a style in and of itself and the levels of sourness can vary. Unwanted sour beer is often caused by wild yeast or a bacterium which consumes sugars while lowering the pH of the beer making it more acidic. Sour beers can range from tart to vinegar-like. If you didn’t plan on a sour beer, you are most likely looking at a sanitation issue. Make sure you are cleaning and sanitizing all of your equipment post-boil.
Sweet - Beers can range from rather sweet and dessert-like to melt-your-mouth bitter hop bombs. If your beer ends up being too sweet for the intended style (you often taste a sweet flavor which reminds you of a syrupy cloying sweetness) then the cause of this could be recipe design, but it is also often caused by under attenuation - the yeast did not convert as much sugar as expected. This could be cause by a fermentation temperature too low (yeast went into “hibernation”) or flocculated (fell to the bottom) and became inactive. It could also be due to adding too many crystal malts which contain less fermentable sugars. Make sure you pitch the proper amount of healthy yeast and ferment in the yeasts desired temperature range. Well now you know what off flavors to look for and how they are caused. If you noticed some redundancy in the prevention steps it was for good reason. Sanitation and yeast health/fermentation are the most vital aspects of creating quality beer at home. Master these things and you will be quickly on your way to brewing award winning beer!
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