Common Off Flavours
Below you will find a brief description of some of the most common off flavours that are found in beer. Some of these flavours in low quantities are an important part of “beer” flavour but become an off flavour when they are found in larger quantities.
Acetaldehyde
Acetaldehyde is one of the steps along the way from wort to ethanol and is also the substance that your body oxidizes alcohol into, which is then metabolised into acetate. It is mainly created in early fermentation and is later reduced into ethanol. The most common reason for Acetaldehyde is not letting the beer ferment completely; this can be remedied by conditioning/lagering for a longer period. (Only if there is still sufficient yeast to convert it into ethanol) Fermenting at too high a temperature or pitching too much yeast can result in the creation of excessive levels of acetaldehyde however if you have pitched too much yeast and let it ferment fully this should clean out the beer. Oxygenating the wort before fermentation reduces the production of acetaldehyde. The use of excessive amounts of non-malt sugars without adding yeast nutrients will result in increased levels of acetaldehyde. Bacteria can also cause the production of acetaldehyde so as always sanitation is a key ingredient to good beer.
Flavour Descriptor: Green Apples/ Green Beer
Common Sources: Fermenatation stalling or contamination
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
It can also be perceived as a harshness in the mouth.
Acetic
Acetic acid is the acid contained in vinegar. It can be created by some yeast strains but is usually below the flavour threshold. When easily identified in a beer it is usually due to acetobacteria, which can survive in ethanol, is acid tolerant and has no problem with hop compounds. The bacteria only requires oxygen so over oxygenation is one of the main causes of this off flavour, along with poor sanitisation. An alternative to dumping the beer if undrinkable is to allow the acetobacteria to do its thing and the result will be a batch of malt vinegar. In some sour beer (like Iambic), acetic acid is a desirable compound that adds to the complexity of the beer. Acetobacteria can be found everywhere; on fruit, plants and in the air. Acetic Acid can become a problem when dispensing from a cask as the beer is exposed to increased amounts of oxygen. (Note that I have used acetobacteria to describe all acetic acid bacteria. There are other bacteria that produce acetic acid)
Flavour Descriptor: Vinegar-like
Common Sources: Contamination (mash, bacteria, or wild yeast)
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Almond
Almond flavours can occur due to storage at high temperatures or oxidation of beer.
Flavour Descriptor: Marzipan
Common Sources: Specific styles, yeast growth, or raw materials
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Butyric
Butyric acid (also known as butanoic acid) is a compound found in all beers. The flavour is often described as cheesy or rubbery in high concentrations but can taste putrid, rancid or like baby vomit. The main reason for a high amount of butyric acid in beer is due to too much unconverted starch in the wort that has been fermented, by some yeasts or more commonly by bacteria from the genus clostridium. (Unconverted starch in wort is normally a result of steeping instead of mashing malts/grains that should be mashed or the use of adjunct syrups that contain starch)
Flavour Descriptor: Rancid butter/putrid
Common Sources: Bacterial Contamination
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Hold your nose to prevent any air intake. Now take about 20-25ml of beer into your mouth for about 10 seconds before swallowing. As you swallow, let go of your nose and breathe out.
Diacetyl
Diacetyl is a compound commonly found in beer but is considered an off flavour in higher concentrations. Butter, butterscotch, buttermilk and toffee are amongst the flavour descriptions given to diacetyl. The yeast excretes a compound that is then chemically converted into diacetyl which is later converted by the yeast into a flavourless compound. The main reason for diacetyl in significant quantities in beer is not letting the yeast ferment the beer fully or in the case of lager not increasing the temperature at the end of fermentation for a diacetyl rest (15.5 C). There are also some bacteria that produce diacetyl, most notably post fermentation during storage or beer lines not being cleaned properly. Certain yeast strains are known to result in higher levels of diacetyl in beer. High fermentation temperatures and increased pitching rates produce higher levels of diacetyl but later clean up the diacetyl. Over/under aeration can lead to increased diacetyl production depending on the yeast strain being used. Non malt fermentables without sufficient yeast nutrient additions lead to increased levels of diacetyl. Fermenting cold and conditioning warm will reduce diacetyl. It is quite common to mistake hallertauer hop aroma (in large quantities) for diacetyl. It is considered a positive aroma in some ales or stouts.
Flavour Descriptor: Butter/butterscotch
Common Sources: Microbial contamination or improper maturation
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Take short sniffs while holding the beer near your nose.
D.M.S. (Dimethyl sulphide)
Dimethyl sulphide is found in all beer. At low concentrations it has a malty/grainy aroma and is quite common in some lagers and kölsch. In higher concentrations it results in a cooked corn or vegetable aroma and sometimes in dark beers a tomato aroma. DMS is normally due to not cooling wort below 70ᴼC within 30 mins after the boil has complete. DMS is produced in the wort when it is above 70ᴼC and boils off at 98ᴼC. If the kettle is left partially open during a vigorous boil (for at least 60 minutes) and the wort is then cooled immediately afterwards there is little chance of high concentrations remaining in the beer. Pilsner malt contains up to 8 times as much S-methyl-methionine as pale malt, this is converted to DMS at temperatures greater than 70ᴼC. Some yeast produce DMS during fermentation but this is rare especially when fermenting at higher temperatures. Very high levels of DMS are most likely due to a bacterial infection in the beer.
Flavour Descriptor: Cooked corn/cooked vegetable
Common Sources: Wort boil, wort cooling or contamination
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Take short sniffs while holding the beer near your nose.
Earthy
Earthy, soil-like aroma which is mainly due to contamination, particularly water-based contamination. Other common descriptions are freshly dug soil or damp soil.
Flavour Descriptor: Geosmin/soil-like
Common Sources: Packaging contamination or water derived contamination
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Take short sniffs while holding the beer near your nose.
Mercaptan
Mercaptan at low levels produces a sulfur-life aroma. At higher levels it gives a rotten vegetable or drain-like aroma. It is primarily formed by yeast during fermentation but can also be caused by yeast autolysis during beer production (Yeast autolysis is where the cell bursts open and dumps its content into the beer).
Flavour Descriptor: Sewer-like
Common Sources: Poor yeast health, autolysis
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Take short sniffs while holding the beer near your nose.
Ethyl acetate
Ethyl acetate is commonly described as a nail polish remover or turpentine aroma. It is the most common (quantitatively) ester found in beer. In low quantities ethyl acetate gives a desirable fruity aroma. Larger quantities are generally due to wild yeast and are common when using brettanomyces. The main causes of high concentrations are high fermentation temperatures (>21ᴼC) and wild yeast infection. Under-pitching yeast is another reason for increased ester production, along with continuous fermentation, low oxygen levels or higher gravity wort. Other solvent-like aromas include non-food grade plastic fermentors or transfer tubes leaching.
Flavour Descriptor: Solvent-like esters
Common Sources: Wort composition and yeast growth
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Ethyl hexanoate
Ethyl hexanoate is a common ester contained in all beer. In high concentrations it produces an apple or aniseed aroma. Concentrations of ethyl hexanoate can be reduced by fermenting at a lower temperature or changing to a different yeast strain. Sometimes acetaldehyde is mistaken for ethyl hexanoate.
Flavour Descriptor: Apple-like esters
Common Sources: Ferment product, wort composition or yeast health
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Spicy
Certain yeast strains have a spicy aroma, for example the yeast strain used by Chimay. Higher fermentation temperatures are a common reason for spicy flavours. It can be a desirable aroma in certain big beer styles (>7%) or wheat beers (e.g. Kellerweiss from Sierra Nevada) however is generally accepted to be an off flavour in light lager style beer. Some hop varieties lead to a spicy aroma, e.g. crystal, saaz and tettenager. And obviously the use of spices in a beer will lead to a spicy aroma. The spicy aroma also is sometimes referred to as allspice or eugenol.
Flavour Descriptor: Cloves
Common Sources: Microbial contamination, wild yeast or ageing
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Metallic
Metallic taste can be due to a high iron (or other metal) content in your water. Filtering your water and ensuring all metal that comes into contact with your beer is food grade (like stainless steel, aluminium or copper) and ensure the protective oxide layer is left after cleaning. i.e. don’t over scrub, leave the grey oxide layer. Grain stored in a damp or humid environment can lead to oxidised unsaturated fatty acids leading to a metallic flavour. High levels of metal can also effect head retention.
Flavour Descriptor: Metal/tin-like
Common Sources: Water sources, non-passivated vessels
How to taste:
Take about 20 – 25 ml of beer into your mouth. Move it around your mouth with your tongue then swallow.
Dip your finger into the beer, rub some onto your arm and take a short sniff.
Geraniol
Geraniol comes from hops, it is an important character of hop oil and is the primary component in rose oil. It is generally described as rose-like, floral or citrus-like. About 67% of the population have a threshold of 350µg/l where the other 33% can perceive it at 18µg/l (450µg/l in Siebel sample). It is sometimes used as an insect repellent, however it attracts bees. It is found in the following hops amongst others; Cascade, Citra, Centennial, Chinook, Sothern Cross and Styrian Goldings.
Flavour Descriptor: Floral/rose-like
Common Sources: Hop addition and variety
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Take short sniffs while holding the beer near your nose.
Indole
Indole is found in small quantities in all beer, however it can be produced in larger, less desirable quantities by the coliform bacteria. Even if you can ignore the smell it is best to dump the batch as it could lead to stomach ache or diarrhoea. Around half of the population are very sensitive to Indole. The use of adjunct sugars or poor sanatisation are the most common causes for increased levels of Indole. Other aroma descriptions include faecal or baby diaper. This is often found in association with DMS.
Flavour Descriptor: Farm/barnyard
Common Sources: Bacterial infection during fermentation
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Isoamyl acetate
Isoamyl acetate is an ester found in all beer at varying levels, particularly in wheat beers (e.g. Hoegaarden). This compound is most commonly caused by the yeast during fermentation, especially at higher temperatures. High glucose content in the wort also produces a more pronounced banana aroma. It is generally accepted as a positive aroma in Bavarian weissbier but is not expected to be present in an American hefeweizen. Isoamyl acetate is widely used as artificial banana flavouring.
Flavour Descriptor: Banana esters/peardrop
Common Sources: Ferment product, wort composition, or yeast health
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Grainy
Grainy aroma is generally due to the use of malt that is too young. The aroma should be boiled off. It can be a positive aroma in some lagers, but is generally perceived as an off-flavour except at the lowest concentrations. Another cause of husky aroma is the over-milling of the grain. Long mashes, over sparging or alkaline water are also common causes of husky/grainy aroma in beer. Cold conditioning the beer can help to drop the grainy/husky aroma out of suspension. The aroma is sometimes described as spent grains.
Flavour Descriptor: Husk-like/nut-like
Common Sources: Excessive run-off or insufficient wort boil
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Take short sniffs while holding the beer near your nose.
Isovaleric
The isovaleric flavour note can be imparted to beer through the use of old or degraded hops or use of high hopping rates. Flavour impact character in some beer styles. Putrid, Stale Cheese, Old hops or Sweaty.
Flavour Descriptor: Husk-like/nut-like
Common Sources: Excessive run-off or insufficient wort boil
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Lactic
Produced by mashing by bacteria. Also formed by contaminating bacteria during fermentation and storage, and in packaged beer. Lactic makes an important contribution to the acidity of beer and to flavour balance. Off flavour or taste at high concentrations. Sour milk or Yoghurt.
Flavour Descriptor: Sour/sour milk
Common Sources: Beer spoilage bacteria
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Take about 20 – 25 ml of beer into your mouth. Move it around with your tongue then swallow.
Caprylic
Caprylic flavour is produced by yeast during conditioning (cellaring) of beer. Caprylic is a desirable flavour character of some pale lager beers. Off flavour at high concentrations. Goaty, Waxy or Tallowy
Flavour Descriptor: Sour/sour milk
Common Sources: Beer spoilage bacteria
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Take about 20 – 25 ml of beer into your mouth. Move it around with your tongue then swallow.
Papery
The papery flavour note is formed during beer storage. Development of character depends of time and temperature of storage and oxygen of packaged beer. Papery is an off flavour in beer associated with ageing. Cardboard or Oxidized
Flavour Descriptor: Cardboard/oxidized
Common Sources: Product of oxidation/staling
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Take short sniffs while holding the beer near the nose.
Take about 20 – 25 ml of beer into your mouth. Move it around with your tongue then swallow.
Vanilla
Formed in some beers dureing ageing. Derives from breakdown of the barley cell wall materials. Can also be formed from the breakdown of certain phenolic compounds produced by wild yeasts. Vanilla imparts a positive aroma to some beer styles. Custard like, Cream soda or Ice cream
Flavour Descriptor: Custard powder
Common Sources: Specific styles, phenol flavour compound, or wild yeast
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
Bitter
Bitterness is derived from addition of hops during wort boiling, or addition of hop extracts to wort or beer. Bitter is one of the most important flavours in beer and one of the basic tastes. Quinine or Bitter hops
Flavour Descriptor: Hoppy/bitter
Common Sources: Hopping/hop addition
How to taste:
Take about 20 – 25 ml of beer into your mouth. Move it around with your tongue then swallow.
Infection
Infection is a combination of acetic and diacetyl.
Flavour Descriptor: Sour + Buttery
Common Sources: Microbial infection/contamination
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff (acetic).
Take short sniffs while holding the beer near your nose (diacetyl).
Hefeweizen
Hefeweizen is a combination of isoamyl acetate and spicy.
Flavour Descriptor: Spicy + banana esters
Common Sources: Specific beer styles
How to taste:
Cover the beer with your hand and swirl the glass to release the aroma.
Remove your hand and take a single long sniff.
