By hopskullbeerguide
February 05, 2020

So here is the deal! Lately I have been seeing a lot of talk on different craft beer groups on Facebook about the subject “off-flavors in beer” – One specific beer from the Mikkeller Beer Club box was what sparked the discussion of “what’s wrong this beer?” Mostly people mention the off-flavor description “Oxidized” when beer tastes bad/wrong, which in fact almost all said about the before mentioned beer – I don’t really think that oxidation was the problem with the debated beer (More about the specific beer in the conclusion of this article)

I rarely see people use other off-flavor descriptors; therefore I really wanted to make an article about the subject, and clarify the most common off-flavor in beer, and how to identify what’s wrong? And perhaps give a few inputs on how you could become a better beer taster!

There are many aromas and flavors that contribute to the overall character of a beer. Some of these aromas and flavors are described as malty, fruity, bitter, roasted, juicy and so on. When it comes time to figure out why a beer tastes bad though, we need to get more specific.

In this section I’ll discuss several different flavors that are thought as off-flavors and what could cause them to occur.

Lets start with the perhaps most used off-flavor descriptor! – Make sure to bookmark this blog post, as you can use it as a little off-flavor dictionary!



Tastes like: Paper, Cardboard, Old Books, wet cardboard BUT also dried fruit and sherry like characteristics can develop.

First of oxidation is a word which is associated with the aging process of beer! What quality the beer gets depends on the beer style, how it was stored and what temperature the beer is stored at.

Some flavors associated with oxidation are important aroma and flavor characteristics of an aged beer’s profile, others, are always considered off flavors.

A really great example of good oxidization is in barrel aged

As a matter of fact, all barrel aged beers are undergoing oxygen exposure – but in a good way, as the wood acts as a semi-permeable barrier between beer and the environment, allowing for the exchange of gases such as oxygen. This way the oxidation is in fact helping to mellow out the alcohol, mature and help develop aroma and flavor profiles like e.g. Sherry.

Okay now the bad oxidation! Oxidation is probably the most common problem with beer. Aging directly causes oxidation. How fast and to what extent this process occurs is a result of oxygen levels, storage temperatures, and a beer’s ingredients.

The more oxygen a beer is in contact with the faster and more damaging the oxidation can be to the beer. Cooler storage temperatures slow the process. If you don’t treat your beer correctly, you’ll notice increase in oxidation if for example your storage temperature is high. But also Ingredients used in the beer can both inhibit or aid oxidation – One time I listened to an interview with a brewer from an American brewery (Can remember the name) he talked about brewing with coffee. He claimed that even when grinding coffee, you let in increased levels of oxygen into the coffee – which you then trans fore into you beer. There he always use whole beans – after disinfecting the beans, he steeps them in the beer after fermentation.

I also have to point out that if the wort is exposed to oxygen by aeration/splashing/spraying, or vigorously stirring at temperatures above 80°F/26,66°C. the beer will sooner or later develop wet cardboard or sherry-like flavors, depending on which compounds were oxidized.

Breweries and even home brewers today have learned to handle oxygen when packaging beer – they simply purge the bottles and cans with CO2 before packaging! – Home brewers often use soda stream CO2 tanks, which are easy to come by.



Tastes like: Buttery, buttermilk, milky, oily mouth feel and butterscotch.

Diacetyl is usually considered an off flavor, but is considered great at lower levels in some beer styles, including English Bitters, Scotch Ales, Dry Stouts, and Czech Pilsner to name a few.

You’ll notice the Diacetyl right away if your beer is affected by it! Diacetyl is most often described as a buttery taste with oily mouth feel.

Little disclaimer! Diacetyl is in fact desired to a degree in many ales and lagers.

Diacetyl is produced by all yeast during fermentation, but is usually re-absorbed by the yeast cells as a normal process of fermentation. Non re-absorbed Diacetyl or over production of it, is caused by bad or short boiling, low temperatures during fermentation, bad yeast, under pitched yeast and bottling before end of fermentation. The key to avoid is most common to simply sit back, and let the fermentation finalize.

Quick side note! It can also be formed by bacteria contamination.


Dimethyl Sulfides (DMS):

Tastes like: Sweet corn, Creamed Corn, Cabbage, and Cooked Vegetables.

Like Diacetyl in ales, DMS is common in many light lagers and is considered to be part of the character.

The precursor compound to DMS, called S-methylmethionine (SMM) which is a compounds that’s created during the malting process of barley and grains which are later converted to DMS when heated either in the mash, bad boil and slow cooling. DMS is continuously produced in the wort while it is hot and is usually removed by vaporization during the boil. If the wort is cooled slowly these compounds will not be removed from the wort and will dissolve back in. A really important thing to remember, is not to completely cover the brew kettle during the boil or allow condensate to drip back into the pot from the lid – that’s also why the wort should also be cooled quickly after the boil, either by immersing in an ice bath or using a wort chiller.

DMS can also be produced from bacteria that have managed to contaminate the beer. It can be driven off through evaporation when boiling wort.

When caused by bacterial infection, DMS has a highly unpleasant character - more liked cooked veggies than corn. It is usually the result of poor sanitation.



Tastes like: green apples, rotten apples, cider and some times like paint

Acetaldehyde is a naturally occurring chemical produced by yeast during fermentation. It is usually converted into Ethanol (alcohol). It is a characteristic flavor of some beer styles like Lambic – But generally high concentrations are considered an off flavor in most beers.

Acetaldehyde is caused when the yeast does not have enough time to convert the Acetaldehyde into Ethanol (alcohol) due to either a lack of fermentation time, not enough yeast for the wort or low/bad quality yeast. But can also occur because of infections of certain strains of bacteria.

Astringent / Tannic:

Tasting like: If you can imagine sucking on a wet tea bag or a grape skin – then you’ll get that dry, grainy, mouth-puckering, tannic feeling which is called ” Astringent”

Astringency differs from bitterness by having a almost powdery feeling in the mouth. It can also be tart, vinegary, tannin-like and dry.

Tannins are the number one cause of astringency in beer. Tannins are found in the skins or husks of the grain as well as in the skin of fruit. Steeping grain for too long or grain that has been excessively milled or crushed can release tannins. Also hopping to much can lead to the creation of astringent qualities.


Tastes like: Banana, pear, strawberry, raspberry and grapefruit.

Ales are supposed to be slightly fruity, and Belgian and German wheat beers are expected to have banana flavor components, but sometimes a beer comes with way to much esters. Esters are a naturally occurring by-product of fermentation. Strong fruity flavors or fruity flavors that are inappropriate for the style of beer are sometimes a result of under pitching or high fermentation temperatures. As a general rule, the higher the fermentation temperature, the more esters the yeast will produce. Low oxygen levels can also help increase the production of esters.



Tastes like: I don’t think I have to point it out!

A bacterial or wild yeast infection is the main cause. Can occur during the last part of wort production post boil, or after packaging.


Tastes like: Tart, Sour Milk, Acidic, Citrus fruits.

Sourness at different levels can be appropriate to some styles like Dry Stouts, Belgian sours, Berliner weisse, GosesWitbier, and Wild Ales / Lambic.

Sourness cane become an off flavor if it is at too high a level overall or too high for a given style. Any perceived sourness is due to added acids in the form of raw materials like e.g. fruits, or fermentation, and/or bacterial contamination or inoculation by wild yeast and bacteria.



Tastes like: spice, herbal, Tea-like, clove, smoke, Band-Aid, medicinal.

This is nearly always considered an off-flavor, but in fact phenols are a Key flavor component in some ales and stouts - but generally regarded as off-flavors in beers that are bottom fermented – best know as a major contributor in German wheat beers.

The main cause of phenolic character is done by various phenols, which are initially produced by the yeast. Phenolic flavorings are normally due to the presence of wild yeasts during fermentation or because usage of special yeast varieties. Other sources of phenolic flavor occur due to bacteria from bad hygiene.


Tastes like: iron, coins, ink or blood.

Metal flavors are an off-flavor in beer! It’s caused when beer or raw materials come into contact with poor quality metal - pipework or machinery, particularly by wort being boiled in steel kettles.

It can also be occurring under the packaging process – such as metal cans, bottle caps or kegs. Improperly stored grains can also cause metallic off flavors. 

Moldy / Musty:

Tastes like: Mold, corked wine, damp cellar. Imagine sniffing the cork from a beer or wine bottle.

The main cause of those moldy/musty aromas and flavors is a chemical created by mold ”TCA” which consumes chlorinated phenols and metabolizes them and it can be detected by humans at extremely low levels.

Generally TCA is caused when beer is fermenting somewhere that is damp or where the grain has developed mold in storage. Some yeast and bacteria can also produce the characteristics of that moldy/musty character, which is quite sought after in aged Lambic.


Tastes like: well.. Cheese! Or sweaty socks and must.

The main cause of this off-flavor is called ”Isovaleric Acid”. This flavor is in fact the characteristic of some beer styles, e.g. Lambic.

Even thought the typical ‘cheesy’ characters are often associated with beers that have very high bitterness, such as IPA’s. In lager beers, isovaleric character is regarded as an off-flavor.

Isovaleric acid is generally introduced by old hops. The compound arises as hops age and lose their bitter alpha acids, producing a flavor typified by flavors of cheese, sweat or must.


Tastes like: well…. Soap!

ALWAYS make sure to rinse your glass with hot water after cleaning (And always rinse after your glass which has been in the dishwasher).

Soapy flavors can also be produced by the fermentation conditions. If the beer is left in the primary fermenter for a prolonged period of time after primary fermentation is over, soapy flavors can result from the breakdown of fatty acids in the trub (The layer of sediment, left at the bottom of the fermenter).



Tastes like: Blend of these things: rubbery, animal musk, burned rubber, and hints of oxidized / cardboard and wet paper.

Skunked, Skunky or light-struck? Well it’s an odd term that’s fore sure, but it’s one of beer’s most notorious off-flavors.

Skunk” is the result UV radiation penetrating beer, which then causes a breakdown of hop molecules, called isohumulones, which then bond with sulfur, giving you a “skunked” smell. The reaction can happen quickly! Leave a glass of beer in strong sunlight for just a minute and you’ll already be able to taste it. I’ve heard stories about people protecting their newly posted beers from sunlight, as skunking/light-struck can happen after 5-10 minutes in direct sunlight. After Reading some articles about the term “skunked” I asked my self, how fast does it really happen? And what would happen to the beer, if i simulated how beer often is stored in bottle shops? This past summer I conducted a little “skunking” experiment

HopSkull’s “skunking” experiment

I’ve seen IPA’s and hop forward beers being displayed very differently in bottle shops, some put beers on display in Windows (in direct sunlight), on shelves (in direct lighting) and in fridges. — Light hurts the product, and in The end. You’ll not get the full value for your money!

Usually I never buy hop forward beers that hasn't been stored in a fridge at the bottle shop, and I tell people to NOT buy beer, which has been sitting in light for a period of time. A lot of folks do not believe me when I tell them, that sunlight/light Can ruin their beer in a matter of 10-15 minutes (sometimes quicker) therefore I’ve conducted a little experiment!

Firstly I purchased 3 bottles of Amager bryghus / Gamma brewing — Talents & Legends: Gamma.

Back home I placed a beer in direct sunlight on a ledge. I also placed a beer on a shelf, under direct light from a led spotlight, and lastly I placed one in the fridge. (I made labels marked with a W (Window), S (shelves) and F (Fridge), which were then placed under each beer bottle, so I could blind test.

The beers stood there for a period of 8 hours. After that I placed all the beers in the fridge for 2 hours, making sure all beers had the same temperature.

Here are my results!


The one from the fridge was still really hoppy with vibrant aromas and taste.

The one that had been sitting on my shelf in direct led spotlight, was also fine — it was more the less on pair with the fridge sample.

The one, which has been sitting in direct sunlight — Well where do I start? It had become really dull, and uninteresting! The aromas from the hops were so subtle, and had been overtaken by a buttery, fat, wet cardboard and sweat. The taste was also a complete letdown!

My conclusion! Never ever buy hop forward beer that has been exposed to any short of lighting, especially sunlight! Always buy hop forward beers from a fridge! Make sure to store your beer properly! — Dark places are preferred, but a fridge is The Way to go with hop forward beers! — Ohh and lastly, make sure to protect your beer from sunlight while drinking it!


Conclusion to most used off-flavor descriptor! And the case of ”Magma Tipa from The Flying in brewing”

The flavor characteristics of beer are really wide – mostly you’ll encounter rich impactful notes, which just mesmerize you! But sometime, you just happen to be the odd man out, and get a bad beer for some reason. Mostly you’ll just pour the beer down the drain and grab another, but if you like me is somewhat nerdy – well, then you want to get into details of what went wrong? Some time ago the Mikkeller Beer Club members received the beer Magma Tipa from The Flying in brewing as part of their monthly box – some where lucky to get a god can, others got a bad can, and most people wrote that their can tasted really bad and was oxidized  - I was so damn lucky, that I got 2 cans of Magma because of a packaging mistake. The first one was great the other was terrible! Here are my thoughts on, what went wrong with the Magma Tipa from The Flying in brewing.


My first thought is that the cans came from two different batch’s – next, the bad tasting can tasted very reminiscent of cooked veggies, con and had a sour hint, with oily mouth feel.

I suspect that the beer was the victim of poor sanitation, which leads to a profile of DMS, Sour and a somewhat Diacetyl like mouth-feel.

Sad, but these things sometime happen!

Lastly I want to share a little guide, on how you can experiment with off-flavors and catapult yourself to become a beer-tasting master – judge level standards!


How to practice tasting off-flavors with the general Off-Flavor Practice Guidelines:


  1. Use a bland light lager as your base – Heineken is really great!
  2. Chill the beer to the style’s appropriate serving temperature
  3. Always have a control beer  - The same beer, at the same temperature – for compression.
  4. Add small amounts of flavoring until you can perceive the off-flavor in both the aroma and taste.
  • Oxidized: Store 3 beers at 3 different temperatures, as heat accelerate oxidization a month is appropriate to create oxidized/ aged characteristics.
  • Diacetyl: Buy butter Aroma in liquid form, and taint the beer.
  • Acetaldehyde: Get some Green Apple aroma/extract and taint the beer.
  • Dimethyl Sulfides (DMS): Take the liquid form canned corn, and taint the beer with it.
  • Astringent / Tannic: make a tea of over-crushed almost powdery malt, and put it straigt in boiling water. Taint the beer with the tea once it has cooled.
  • Skunked/light-struck: Do like I did in the HopSkull’s “skunking” experiment.
  • Sour: Lemon extract for citric acid. White vinegar for acetic acid. Ascorbic acid to get a citrusy (solvent-like at higher levels) taste and Malic acid to create cider sourness.
  • Phenols: Add small amounts of clove extract for that spicy flavor – use throat spray for a medicinal character
  • Metal: Simply stick a CLEAN iron or copper pipe in the beer, and let is sit for 5-10 minutes.
  • Esters: Hard to replicate, but try tainting with fruit aromas/extracts – Banana is preferable using
  • Cheese: Make a hop-tea with old stale hops, and taint the beer with it.
  1. Take notes as you progress – and have fun! Next time you stumble upon a off-favored beer, you’ll know what’s wrong!


Hope you learned a lot to day, Till next time!


Yours truly,

- HopSkull!