Beer, a complete course

Class P2.1

The ten faults, and how to name them

Ten things that can be wrong with a beer, every one of them findable by smell. Learn the smell first and the chemistry second, because the smell is the part you get to use in a bar.

By the end of this class you will be able to

Name ten common faults from smell alone, in ordinary words
Say what caused each one and which beers it turns up in most
Work out whether a fault was made in the brewery, on the way to you, or in the bar

P2.1.1 Smell it before you taste it

The nose finds a fault faster than the tongue does. The drill first, then the first five faults.

A beer that has gone wrong nearly always says so in the smell. Fault compounds are volatile, which means they leave the liquid and travel through the air. Your nose reads them at levels your tongue would never find in the glass.

The gap is not small. You need roughly three grams of sugar in a litre of water before you can taste it. The compound behind the skunk smell in a light damaged lager is detectable at a few billionths of a gram in the same litre. Smell is the more sensitive instrument by a very long way.

There is a second reason to lead with the nose. It stops noticing a smell that stays constant, usually inside a minute. The first two sniffs are the honest ones, so do not waste them on a beer you have already been holding under your chin.

The drill

Pour it and look. Note the colour, and whether the head lasts more than a few seconds.
Swirl the glass once. That drives aroma out of the liquid and into the space above it.
Two short sniffs, not one long one. Short sniffs read better and you can repeat them.
Sip, and hold the beer in your mouth for a second before you swallow.
Come back to the glass five minutes later. A warmer beer smells louder, and several faults only appear once the glass has warmed up.

The fifth step is the one people skip

A cold beer hides almost everything. Bars serve lager cold enough that a fault can pass unnoticed for the first two mouthfuls. If something feels slightly wrong and you cannot say what, wait five minutes and smell it again.

Covered in Module F3, How to taste

The tasting drill in full. Look, smell, sip, and what happens after you swallow. The steps and the reason for each one are taught once, in Module F3. What is above is that drill pointed at faults.

The first five

Read the first column and stop there if you like. The name is only useful once the smell is familiar.
What you smell	The name	What caused it	Where it turns up
Wet cardboard, wet newspaper, old sherry, stale honey	Oxidised	Oxygen got into the beer after fermentation. Time and warmth then did the rest.	Imported bottles, anything old, anything stored warm. Dark beers hide it longest.
Almost nothing at all	Stale	The aroma compounds broke down or escaped. Nothing arrived. Something left.	Old hoppy beer above all, plus old wheat beer and a keg that has been on for months.
Butter, butterscotch, cinema popcorn, and a slick feel on the tongue	Buttery	Yeast made it and was taken off the beer before it could clear it up again. A dirty draught line makes more of it.	Draught beer in bars that clean badly. Low levels are allowed in Czech lagers and some English ales.
Skunk, burnt rubber, cat	Skunky	Light reached the beer through clear or green glass and broke a hop compound apart.	Green and clear bottles. A great many imported lagers arrive this way.
Cooked sweetcorn and canned vegetables, or struck matches and eggs	Sulphury	Sweetcorn is a compound from pale malt that a short or gentle boil failed to drive off. Struck match is sulphur from young lager yeast.	Pale lagers above all, because a pale lager has nothing to hide behind.

Two of those five deserve a sentence more, because they are the two beginners get wrong most often.

Buttery beer is the work of a compound called diacetyl, which is the same thing that gets sprayed on cinema popcorn. Yeast makes it during fermentation and healthy yeast takes it back out again afterwards, given a few warm days. Rush the beer away from its yeast and the butter stays in.

The skunk smell is not the beer going off. It is a chemical reaction driven by light, and it can happen to a perfectly fresh beer in an afternoon. That is why it turns up so reliably in the same handful of green bottled imports.

Faults have addresses

Every fault was made somewhere, and knowing where changes what you do about it.

Made in the brewery. Sweetcorn, butter, green apple, hot solvent, plastic, drying harshness. These are in every bottle of that batch.
Made on the way to you. Oxidised, stale, skunky. These are about time, warmth and light, and the brewery may have done nothing wrong at all.
Made in the bar. Butter again, and sourness or vinegar from draught lines that are not cleaned. These are in every beer coming off that tap.

The practical version: if a fault was made in the bar, the beer on the next tap will usually taste of the same thing. If it was made in the brewery, it will not. That one check costs you a sip of whatever your friend ordered, and it settles the question.

Try this before the next class

Buy two bottles of the same pale lager. Leave one on a sunny windowsill for a day and keep the other in a dark cupboard. Smell both. If the bottle is clear or green you will have made yourself a skunked beer, and you will never need the fault described to you again.

Check yourself

A beer smells faintly of wet cardboard and old paper. Which fault is that?
Answer: Oxidised. Oxygen got in, and time did the rest
Wet cardboard is the standard description of oxidation and it is remarkably consistent between tasters. Skunk is tempting because it is the fault everyone has heard of, but skunk smells of an animal rather than a material. Cardboard, paper and old sherry all belong to the same fault, and they are stages of it rather than different problems.
A very cold beer shows its faults more clearly than a warm one.
Answer: False
False, and it is the reason so much faulty lager gets drunk without complaint. Cold suppresses aroma, so a beer served near freezing gives your nose very little to work with. Let the glass sit for five minutes and smell it again. Anything that was hiding will come out as the beer warms.
Pair each smell with what caused it.
Answer: Wet cardboard with Oxygen got into the beer after fermentation · Butter or butterscotch with The beer was taken off its yeast before the yeast had cleared up · Skunk with Light got through the glass and broke a hop compound apart · Cooked sweetcorn with A compound from pale malt that the boil did not drive off
Each fault has one dominant cause, which is what makes them learnable. The pairing people get wrong is skunk, because the word sounds like spoilage and spoilage sounds like age. Light is the whole story there, and a skunked beer can be two days old.
Every beer on every tap in the bar has the same faint buttery note. What does that tell you?
Answer: It is the bar. Something shared is making it, and the draught lines are the usual answer
Anything shared by beers from different breweries has to come from something they share, and what they share is the plumbing and the glassware. Blaming your own palate is the tempting answer because it is the modest one, and it is why bars with dirty lines keep getting away with it.
Sort each fault by where it was made.
Answer: Cooked sweetcorn goes in In the brewery · Hot solvent goes in In the brewery · Wet cardboard goes in On the way to you · A beer that smells of nothing goes in On the way to you · Skunk goes in On the way to you · Sourness on every tap goes in In the bar
Brewery faults are decisions and mistakes made during brewing, so they are in the whole batch. Transit faults are about time, warmth and light, so a good brewery can still hand you a bad bottle. Bar faults come from the lines and the glass, which is why they show up on drinks that have nothing else in common. Skunk sits in the middle bucket because light usually reaches the bottle in a shop or a delivery van rather than in the brewery.
One word for the smell of a beer that has been damaged by light.
Answer: skunky
Skunky, or lightstruck if you want the technical name. It is worth having both, because a bar will understand skunky and a brewery will use lightstruck. The smell really is the animal, which is why the word stuck rather than something more polite.
Which of these genuinely speed up oxidation in a bottle of beer?
Answer: Time on the shelf · Being stored somewhere warm · Oxygen picked up when the beer was packaged
Time, warmth and the oxygen that got in at packaging. Distance is the tempting fourth answer and it is not one, except that a beer which travelled a long way usually also spent a long time in a hot container. It is the weeks and the temperature doing the damage, not the miles.
A beer that smells of almost nothing can still be counted as faulty.
Answer: True
True, and it is the fault people are least likely to report. Nine of the ten faults are a smell that arrived. This one is a smell that left. A pale ale that should smell of grapefruit and smells of wet bread instead has lost the thing you paid for, even though nothing unpleasant has appeared.
Before you read on. Your nose stops noticing a smell that stays constant. Roughly how long does that take?
Answer: 60seconds. Think about walking into a house where bread is baking. You notice it at the front door. Are you still noticing it while you take your coat off?
Under a minute for most smells, and close to complete within two. That is why the drill asks for two short sniffs rather than a long one, and why you should smell a beer before you start talking about it. Most people guess several minutes, because the fade is gradual and you do not notice yourself stopping.
Which statement about the skunk smell is correct?
Answer: Light causes it, and it can happen to a beer packaged last week
Light. It is a photochemical reaction, so a fresh beer in a clear bottle on a sunny shelf can be skunked while a year old beer in a can is untouched. Heat is the popular answer and it is wrong. Heat does real damage to beer, but the damage it does smells of cardboard rather than skunk.

P2.1.2 The other five, and the table to keep

Five more smells, then all ten in one place, and the two part rule for saying what you found.

What you smell	The name	What caused it	Where it turns up
Green apple, cider, or raw grain and honey	Unfinished	The yeast came off the beer before it was done, or was too stressed to finish. Green apple is the step just before alcohol. Raw grain is sugar the yeast never got to.	Any beer made in a hurry. Raw grain flavour is the standing problem in non-alcoholic beer.
Vinegar, a sharp sour bite, or cheese and baby sick	Infected	Bacteria are eating the beer. Vinegar means oxygen got in as well. Cheese means the wrong bacteria arrived first.	Draught beer from lines that are not cleaned. Sour styles where the souring was not controlled.
Nail varnish, solvent, and a burn at the back of the throat	Hot	Fermentation ran too warm or too fast, so the yeast made heavier alcohols alongside the ordinary one.	Strong beers, home brew, and anything fermented in a hurry or in a hot room.
Plastic, sticking plaster, mouthwash, a swimming pool	Medicinal	Chlorine from tap water or from a sanitiser met compounds from the yeast and made chlorophenols.	Home brew above all. Occasionally a small brewery working on untreated municipal water.
Drying and chalky, like stewed tea or a tea bag left in too long	Astringent	Tannins came out of the grain husks or the dark malt, usually because the grain was rinsed with water that was too hot.	Dark beers and hard roasted stouts. Also anything with cacao, coffee or fruit stems in it.

Three of those need a word of explanation

Green apple is acetaldehyde. Yeast makes it on the way to making alcohol, then turns it into alcohol if you leave it alone long enough. Finding it in the glass means somebody did not leave it alone long enough. The raw grain and honey version is the same story taken further back: sugary unfermented liquid, which brewers call wort, that never became beer.

Medicinal beer is described as phenolic, which is simply the adjective for clove, pepper, smoke and that plastic or sticking plaster note. The same word covers a wanted flavour and an unwanted one. Clove in a German wheat beer is phenolic and correct. Sticking plaster in a pale ale is phenolic and wrong.

Astringency is a feeling rather than a flavour. It is the drying, grippy sensation you get from strong tea or an unripe banana skin, and it happens at the sides of your tongue rather than in your nose. Do not confuse it with bitterness. Bitter is a taste, and it can be pleasant. Astringent is a texture, and it almost never is.

All ten in one place

Ten smells, ten names. This is the table the rest of the course points back at.
Smell	Name	One line
Wet cardboard, old sherry	Oxidised	Oxygen plus time plus warmth
Nothing at all	Stale	The aroma has gone, and nothing replaced it
Butter, butterscotch	Buttery	Yeast rushed, or a dirty draught line
Skunk, burnt rubber	Skunky	Light through clear or green glass
Sweetcorn, or struck match	Sulphury	A gentle boil, or young lager yeast
Green apple, or raw grain	Unfinished	The fermentation was stopped early
Vinegar, or cheese	Infected	Bacteria, usually from a dirty line or a dirty tank
Nail varnish, throat burn	Hot	Fermentation ran too warm
Plastic, sticking plaster	Medicinal	Chlorine met yeast
Stewed tea, chalky drying	Astringent	Tannins from husk or dark malt

Worth remembering

Cardboard, butter, skunk, corn, nothing. Then apple, vinegar, solvent, plastic, tea.

The first five in that order are most of what you will ever meet, and roughly in order of how often. The second five are real but rarer, and four of the five belong more to home brewing than to bars. Ten words, ten faults, and every word is a smell rather than a chemical, which is the way round you need it while you are standing at a bar.

Name the smell, then the fault

Two parts, in that order. Say what you smell in ordinary words. Then offer the name, if you have one.

This smells of wet cardboard to me. I think it might be oxidised.
There is a buttery note on this one. Could the line need cleaning?
This has gone skunky. Has it been sitting somewhere bright?

Leading with the smell keeps you honest and keeps the conversation friendly. If you are wrong about the name you are still right about the smell, and nobody can argue with what your nose is telling you.

You will not meet all ten

Medicinal and unfinished beers are mostly a home brewing problem, and you could drink commercial beer for years without meeting either. Oxidised, stale and skunky account for the large majority of disappointing bottles. Buttery and sour account for most disappointing pints.

Check yourself

Sort each description into the fault it belongs to.
Answer: Wet cardboard goes in Oxidised · Old sherry in a pale beer goes in Oxidised · Vinegar goes in Infected · Cheese, or baby sick goes in Infected · Green apple goes in Unfinished · Raw grain and honey goes in Unfinished
Oxidation is a slow chemical change, so it produces flat, stale, papery smells. Infection is something alive eating the beer, so it produces sharp acids. An unfinished beer is one that stopped partway, so it smells of the ingredients rather than the product. Sherry catches people out because it sounds pleasant, and in a very strong dark beer it can be. In a pale beer it is the same fault wearing a nicer coat.
What is the difference between bitterness and astringency?
Answer: Bitter is a taste and can be pleasant. Astringent is a drying, grippy feel and almost never is
One is a taste, the other is a texture. You can love a bitter beer. Nobody enjoys the feeling of a tea bag left in the cup. The confusion is understandable, because both arrive at the end of a mouthful and both come from plants. The tell is that a well made bitter beer feels smooth at the same time as it tastes bitter.
A beer smells of nail varnish and burns slightly at the back of your throat. One word for the fault.
Answer: hot
Hot, or hot alcohol. The smell of solvent is the giveaway, because a strong beer can be warming without smelling of anything from a hardware shop. This is one of the few faults that is unambiguous. There is no style anywhere that wants nail varnish.
Pair each smell with its name.
Answer: Sticking plaster and swimming pool with Medicinal · Stewed tea and chalky drying with Astringent · Green apple and cider with Unfinished · Cooked sweetcorn with Sulphury
These four are the ones that need the vocabulary most, because none of them has an obvious everyday name. Sweetcorn is the one worth burning in. It is extremely specific, it is common in cheap pale lager, and once you have noticed it you cannot stop noticing it.
A medicinal, sticking plaster smell in beer usually means the brewery used bad ingredients.
Answer: False
False. It almost always means chlorine, either from untreated tap water or from a sanitiser that was not rinsed off. The chlorine meets compounds the yeast produced and makes something new. The ingredients were probably fine, which is why this fault is a cleaning problem rather than a shopping problem, and why home brewers meet it far more often than drinkers do.
Which five faults will you meet most often as an ordinary drinker?
Answer: Cardboard, butter, skunk, sweetcorn and a beer that smells of nothing
The first five in the mnemonic, and they are first because they are common rather than because they are interesting. The second option is close to a list of home brewing faults, which is why it looks plausible if you have read a brewing book. Buying beer in shops and bars exposes you to time, light and plumbing, and those are the three that produce this list.
Put the two part naming rule in the right order for saying it out loud.
Answer: 1. Describe the smell in ordinary words 2. Offer the name of the fault, if you have one 3. Ask a question that gives the bar somewhere to go
Smell first, name second, question third. Starting with the name sounds like an accusation and invites an argument about whether you are right. Starting with the smell is simply a report of your own experience, which nobody can contradict. The question at the end matters because it lets the bar fix the problem instead of defending itself.
You are buying a bottle of pale ale from a shop. Which of these should make you expect a problem?
Answer: It is in a clear bottle on a shelf under a bright light · It is dusty and the date code is fourteen months old · The shop keeps it in a warm room near the window
Light, age and warmth are the three things that damage beer on a shelf, and all three are visible before you pay. Cloudiness is the tempting fourth. Plenty of excellent beers are cloudy on purpose, including wheat beers, unfiltered lagers and most modern hazy pale ales. Haze is a look, not a fault.
Which of these beers is most likely to arrive stale, meaning it has lost its aroma rather than gained a bad one?
Answer: A hoppy pale ale that was canned eight months ago
The hoppy one. Hop aroma is the most fragile thing in beer and it starts leaving the day the beer is packaged. A big dark beer at eight months has barely started, and many Belgian ales are designed to change slowly in the bottle. The trap here is thinking that all beer ages at the same rate. Style decides shelf life far more than strength does.
The word phenolic always describes a fault.
Answer: False
False. Phenolic covers clove, pepper, smoke and sticking plaster, and the first three are the point of several styles. A German wheat beer without a clove note is the one that has gone wrong. The word tells you which family of compounds is present, not whether anybody wanted them, and that distinction is most of the next class.

Class P2.1 done

You can name ten faults from smell alone, say what caused each one, and work out whether the problem started in the brewery, on the road or behind the bar.

The three lines worth keeping

Cardboard, butter, skunk, corn, nothing. Then apple, vinegar, solvent, plastic, tea.
Smell before you taste, and smell again after five minutes.
If the beer on the next tap tastes of it too, the problem is the bar.

Class P2.2

The faults one by one

Four of the ten do most of the damage. Oxidation and staleness explain nearly every disappointing bottle. Hot alcohol and butter explain most of the rest, and skunk explains the imports.

By the end of this class you will be able to

Explain what oxygen does to beer, and why time and warmth speed it up
Tell the pleasant warmth of a strong beer apart from the fault called hot alcohol
Say why an old beer smells of nothing before it smells of cardboard

P2.2.1 Oxidation, and why imported beer suffers from it

The slowest fault, the most common one, and the only one that is happening to every bottle you own right now.

Some oxygen gets into every beer when it is packaged. A little is dissolved in the liquid and a little sits in the gap under the cap. Breweries measure that amount in parts per billion and spend real money reducing it, because once it is in the bottle there is no getting it out.

What follows is slow chemistry. Oxygen reacts with compounds that came from the malt and produces a family of new ones called aldehydes. The best known of them smells of wet cardboard and wet paper, and your nose finds it at levels far below anything you could taste.

Oxidation has stages, and they smell different

Stage	What you smell	What it means
Early	A dull edge, a slight lack of sparkle	Almost nobody notices this one, including experts
Middle	Wet cardboard, wet paper, damp books	Unmistakable once you know it. This is the classic
Late	Sherry, toffee, dried fruit, old honey	Wrong in a pale beer. Sometimes welcome in a very strong dark one

That last row is worth sitting with. The same reaction that ruins a pale lager can improve a barleywine, because a big sweet dark beer has flavours that sit comfortably next to sherry and dried fruit. It is the same fault. The difference is the beer it happened to, and how far it went.

Covered in Module 9, Strong and specialty beers

English Barleywine, where sherry is welcome. A strong dark English beer built to be kept. Some oxidation over years turns into sherry and dried fruit, which is the point of ageing one. Too much still gives cardboard.

Time and warmth, and nothing else

Two dials control how fast this happens. Time is the obvious one. Temperature is the one people underestimate, because chemical reactions speed up sharply as things get warmer. A useful rule of thumb is that the rate roughly doubles for every rise of 10C, which is 18F.

Run that forward. A case of beer kept at 30C, which is 86F, is ageing something like four times as fast as the same case at 10C, which is 50F. A month in a hot warehouse can cost a beer as much as four months in a cold one. Cold storage, at 40F to 45F which is 4C to 7C, is the single cheapest thing anybody can do for beer.

Covered in Module P3, Buying, keeping and serving

Storing beer at home. Cold, dark and upright, and which beers are worth keeping at all. Taught once, in Module P3.

Why imported beer suffers

Imported beer is not damaged by distance. It is damaged by everything that happens during the distance. A shipping container crossing the tropics gets hot. The beer then waits in a warehouse, then in a distributor, then on a shelf. By the time you buy it the beer may be six months old and may have spent weeks above 30C, which is 86F.

None of that requires anybody to have done anything wrong. A German lager that is superb in Munich can be tired by the time it reaches a shop in Chicago, and the brewery will never know. Importers who use refrigerated shipping exist, and it makes a difference you can taste.

Covered in Module P3, Buying, keeping and serving

Reading a date code. How to find the date on a can or bottle, and what the different formats mean. Taught once, in Module P3. Everything on this screen is the reason it matters.

Aroma goes before flavour does

An old beer does not smell bad first. It smells of nothing first. Aroma compounds are small, light and volatile, which is exactly what makes them reach your nose in the first place. Those same properties mean they break down early and slip out through the cap liner over months.

Bitterness and malt sweetness are made of tougher stuff and stay put. So an ageing beer follows a predictable pattern. The smell fades. The taste holds on for a while. Then cardboard arrives and joins what is left.

This is why an old hoppy beer is such a disappointment. You bought it for the grapefruit and the pine, and those are the first things to leave. What remains is bitterness with nothing on top of it, and a beer that looks completely correct in the glass.

Covered in Module 9, Strong and specialty beers

Fruit beers, where this bites hardest. A stale fruit beer looks right and smells of nothing, because the fruit aroma is the most fragile thing in it. Buy the newest one on the shelf.

Why a weak beer goes stale faster

Alcohol is a preservative, but it is worth being precise about what it preserves against. Alcohol makes life difficult for bacteria. It does very little to slow down the reaction with oxygen. A low alcohol beer is therefore much more vulnerable to spoiling organisms, which is why nearly all non-alcoholic beer is pasteurised or made under close to sterile conditions.

There is a second reason, and it is about proportion. A beer at 0.5% has less flavour in it to begin with than a beer at 6%. The same small amount of staling is a much bigger share of what was there. So it shows up sooner and reads as more serious.

The practical rule is the same either way. For a non-alcoholic or low strength beer, buy the newest date you can find and keep it cold. It has less margin than anything else on the shelf.

Covered in Module 11, Non-alcoholic and low alcohol

Non-alcoholic beer, and its own particular fault. Raw grain and honey character, which comes from a fermentation that was stopped or never started. Common in this category and rare everywhere else.

Worked example: Two bottles on the same shelf

Both are pale ales from the same brewery, at the same price, sitting side by side in a shop. One is in a brown bottle at the back of a chiller. One is in a clear bottle in a display near the window. Which do you buy, and why?

Light: the clear bottle is exposed and the brown one is not. Clear glass gives almost no protection, so the display bottle may already be skunked.
Temperature: the chiller runs at roughly 40F, which is 4C. The display is at room temperature, so it is ageing several times faster.
Time: check both date codes. If the display bottle is also older, the two effects stack.
Style: this is a pale ale, so the aroma is the thing you are paying for, and aroma is the first thing to go.

Buy the cold brown one, every time. Nothing here required you to taste anything or to know the brewery. Light, temperature, date and style are all visible from where you are standing.

Rough shelf lives, and they are rough

Hoppy pale beer: at its best inside about three months of packaging, and noticeably faded by six.
Ordinary lager and most everyday ales: fine for six to nine months if kept cool.
Strong dark beer: a year and often much longer, and some improve.
Non-alcoholic and low strength beer: the newest you can find, always.

These are habits rather than rules. Storage temperature moves every one of them.

Check yourself

What actually causes the wet cardboard smell in an old beer?
Answer: Oxygen reacting with compounds from the malt, over time
Oxygen plus time. The reaction produces a family of compounds called aldehydes, and the best known of them is the cardboard one. Bacteria is a reasonable guess because we associate spoilage with things being alive, but a sealed oxidised beer is usually perfectly sterile. Nothing is living in it. It is just chemistry going slowly in the dark.
Before you read on. Chemical reactions speed up as things get warmer. Roughly how many times faster does beer age at 86F, which is 30C, compared with 50F, which is 10C?
Answer: 4times faster. A common rule of thumb in chemistry is that a reaction roughly doubles in rate for every rise of 10C, which is 18F. There are two of those steps between the two temperatures here.
About four times, from two doublings. Most people guess either much lower, because a twenty degree gap does not feel dramatic, or much higher, because the phrase exponential invites big numbers. Four is enough to matter. It turns a month in a hot warehouse into the equivalent of four months in a cold one.
Imported beer suffers mainly because of the distance it travels.
Answer: False
False, and the distinction is useful. Distance itself does nothing. What damages the beer is the time the journey takes and the temperature it reaches on the way, especially inside a shipping container. A beer flown in cold and sold in a week would be fine. A beer trucked across one country in July, badly, may not be.
Pair each stage of oxidation with what it smells like.
Answer: Early with A dull edge, and slightly less life than it should have · Middle with Wet cardboard and damp paper · Late with Sherry, toffee and dried fruit
It runs in that order, which is why a beer can be past its best long before anybody would call it faulty. The late stage is the interesting one. Those are pleasant words, and in a very strong dark beer the result can genuinely be pleasant. In a pale lager it is the same reaction and there is nothing to enjoy about it.
Which part of an ageing beer disappears first?
Answer: The aroma, because those compounds are the smallest and most fragile
The aroma. The compounds that reach your nose are volatile and light, which is exactly why they reach your nose, and exactly why they leave. Bitterness is much more robust and usually outlives the smell by months. Colour barely moves at all, which is part of the problem: a stale beer looks completely normal.
Which of these beers should you buy on the newest date code you can find?
Answer: A hazy pale ale sold for its hop aroma · A non-alcoholic lager · A fruit beer sold for the smell of the fruit
The first three are all bought for aroma or have very little flavour in reserve, so they have the least margin for age. The barleywine is the exception that proves the point. It is strong, dark and sweet, and the slow changes that ruin a pale ale turn into sherry and dried fruit in a beer built to receive them.
Fill the gap. Cold storage for beer means roughly 40F to 45F, which is about 4C to ____C.
Answer: 7
Seven. That is ordinary fridge or cellar territory rather than anything specialised. The number itself matters less than the habit: every ten degrees Celsius you take off the storage temperature roughly halves the rate at which the beer ages.
Why does a low alcohol beer go off sooner than a strong one?
Answer: Alcohol holds bacteria back, and a weak beer also has less flavour in reserve to lose
Two things, and they are worth separating. Alcohol makes life hard for bacteria, which is why nearly all non-alcoholic beer has to be pasteurised or made under near sterile conditions. What alcohol does not do is stop oxygen. The second half of the answer is about proportion: there was less flavour there to begin with, so the same small loss shows more.
Sort each situation by whether it damages beer quickly or slowly.
Answer: A clear bottle under a bright shop light goes in Fast damage · A shipping container crossing the tropics goes in Fast damage · A brown bottle in a cold dark cellar goes in Slow damage · A can in a fridge goes in Slow damage
Light and heat are the accelerators. A cold, dark, sealed container is the safest place beer can be, and a can in a fridge is close to ideal because it excludes light completely. The container crossing the tropics is the one people forget, because it happens long before the beer reaches anybody who cares about it.
A beer that looks completely normal in the glass cannot be stale.
Answer: False
False, and this is the reason staleness goes unreported. Colour and clarity barely change as a beer ages, and the head can look fine. The beer that has quietly lost everything it was sold for will still photograph beautifully. Your nose is the only instrument that catches it.

P2.2.2 Hot alcohol, butter and skunk

Three faults worth knowing properly, including the one that is a fault and the one that is just a strong beer doing its job.

Hot alcohol, and how it differs from warmth

A strong beer is supposed to warm you. That warmth comes from ordinary alcohol, it arrives after you swallow, and it spreads gently through the chest. It builds over a glass rather than hitting you in the first sip. Nobody should apologise for it and it is not a fault.

Hot alcohol is a different thing wearing a similar name. Yeast working too fast or too warm makes heavier alcohols alongside the ordinary one, and those taste of solvent. Alongside them sits a compound that smells of nail varnish and pear drops. The result burns rather than warms.

The smell test is the fast one. Warmth is silent, and solvent is not.
Question to ask	Warmth	Hot alcohol
Can you smell it before you sip?	No. Warmth has no smell	Yes. Solvent, nail varnish, paint
When do you feel it?	After you swallow, spreading	While the beer is still in your mouth, at the back of the throat
What does it do as the glass warms up?	Stays about the same	Gets sharper and more obvious
Where does it turn up?	Any strong beer, correctly made	Beers fermented hot or fast, and a lot of home brew

The cause is a brewing decision rather than an ingredient. Fermentation makes its own heat, and a strong beer makes a lot of it. Let the temperature climb and the yeast produces more of the heavy alcohols. Give it a cool room and enough yeast for the amount of sugar, and it does not.

A well made strong ale will warm your chest and give you nothing that smells of a hardware shop. If you are getting solvent, that is not the strength. That is the fermentation.

Covered in Module 5, Amber, brown and mild ales

Wee Heavy, a style built on warmth. Strong, malty and Scottish, and warmth is part of what you are buying. A solvent smell in one is still wrong.

Butter, and why it turns up on every tap

Diacetyl is made by the yeast itself, as a side effect of building the proteins it needs. It leaks out of the cell and turns into the buttery compound on its own. Healthy yeast then takes it back in and turns it into something you cannot taste, given a few days somewhere warm enough to work.

That is the whole story of the fault. Brewers who allow those days get clean beer. Brewers who need the tank back do not. It shows in the mouth as well as the nose, as a slick or slippery film on the tongue.

The second source is not the brewery at all. Bacteria living in dirty draught lines produce the same compound, and they produce it in whatever beer passes through. That is why a buttery note on every tap in the building points at the plumbing rather than at any brewery.

Sensitivity varies enormously between people. The commonly quoted level at which most tasters find it is around a tenth of a milligram in a litre, and some people pick it up well below that. So one person at a table can be certain the beer is buttery while another genuinely cannot find it. Both can be telling the truth.

Two styles that allow a little of it

This is one of the few faults with official exceptions written into the style guidelines.

Czech Premium Pale Lager: the 2021 BJCP guidelines call light to moderately low levels acceptable, though not required.
Sweet Stout: the same guidelines list low diacetyl as optional.
German Pils, American IPA and most modern styles: none is acceptable at any level.

So butter in a Czech lager is a judgement call about proportion. Butter in a German Pils is simply wrong.

Skunk, and what actually causes it

Every beer contains riboflavin, which is vitamin B2, and it comes in with the malt. Riboflavin absorbs light in the blue part of the spectrum, around 450 nanometres, and passes that energy on to the bitter compounds that came from the hops.

One of those bitter compounds breaks apart under that energy. A fragment of it picks up a sulphur atom from elsewhere in the beer, and the result is a compound called 3-methyl-2-butene-1-thiol. Chemists shorten that to MBT. It is close kin to what a skunk sprays, which is why the description is so consistent.

Two things follow from that mechanism, and both correct things people commonly say. First, the light doing the damage is ordinary visible blue light and not only ultraviolet, so blocking ultraviolet alone is not enough. Second, none of this needs heat, age or shaking. It needs photons.

Container	Protection	What to expect
Can or keg	Complete	No light gets in, so this fault cannot happen
Brown glass	Good	Blocks most of the wavelengths that matter. Not perfect over months in a bright shop
Green glass	Poor	Blocks some. A green bottled import that has sat in the light is the classic case
Clear glass	Almost none	Assume it has happened, unless the brewery took the step below

There is one honest exception. Some breweries use modified hop extracts that cannot form MBT at all, and those beers can sit in clear glass indefinitely without skunking. So a clear bottle is a warning rather than a verdict.

Sources disagree

How quickly beer becomes skunked in the light.

Popular beer writing. Seconds, or a couple of minutes, in direct sunlight through clear glass.
Craft Beer and Brewing, reporting BJCP exam preparation guidance. To produce a skunked sample to practise on, leave a commercial beer in a green bottle in sunlight for one to three days.
Kvasny Prumysl 2022, on beer photodegradation. The reaction begins as soon as light of the right wavelength reaches the beer. How much builds up depends on the intensity of the light, the colour of the glass and the time.

These are not really in conflict, because each one is describing a different amount of light. Treat it as a sliding scale rather than a stopwatch. Strong sun through clear glass is fast. Green glass takes longer and still gets there. Brown glass is reasonably safe for the life of the beer, and a can cannot skunk at all. The shopping rule does not change: if the glass is clear or green and the shop is bright, expect it.

Skunky does not mean the beer got warm

This is the most common wrong explanation in beer, and it survives because both things happen to badly kept beer at the same time. Heat and light usually travel together, in a delivery van or a shop window.

They do different damage. Heat gives you cardboard. Light gives you skunk. A beer can have one, the other, or both, and telling them apart tells you what went wrong and where.

Sources: BJCP Beer Faults reference · BJCP 2021 Style Guidelines, 3B Czech Premium Pale Lager · BJCP 2021 Style Guidelines, 16A Sweet Stout · Kvasny Prumysl 2022, Direct detection of beer photodegradation in commercial bottles · Craft Beer and Brewing, Off Flavor of the Week: Lightstruck

Check yourself

A pleasant warming feeling in the chest after swallowing a strong beer is a mild version of the hot alcohol fault.
Answer: False
False. They come from different compounds and behave differently. Warmth comes from ordinary alcohol, arrives after the swallow and has no smell. The fault comes from heavier alcohols made during a hot fermentation, and it announces itself as solvent before you have even sipped. A strong beer that warms you is doing exactly what it is for.
Which single question separates warmth from hot alcohol fastest?
Answer: Can you smell solvent or nail varnish before you sip?
The smell. Warmth is completely silent to the nose, so any solvent or nail varnish note settles it before you drink anything. Strength is the tempting answer, because strong beers are where the fault turns up most, but plenty of strong beers are made cleanly and plenty of ordinary ones are not.
Sort each observation by what it points at.
Answer: Only this one beer tastes buttery goes in The brewery made it · A solvent smell on a strong ale from a fresh bottle goes in The brewery made it · Every tap in the room has the same buttery note goes in The bar made it · The lager and the stout both taste faintly sour goes in The bar made it
Anything shared across beers from different breweries has to come from something those beers share, and in a bar that means the lines and the glassware. Anything confined to one product came with the product. The buttery note appears in both lists because it has two entirely separate sources, which is what makes it the most useful diagnostic fault in the ten.
Which vitamin, present in every beer, starts the reaction that makes a beer skunky?
Answer: riboflavin
Riboflavin, also called vitamin B2, which arrives with the malt. It absorbs blue light and hands the energy to the bitter compounds from the hops, and one of those then falls apart. It is a strange fact that a nutrient is the trigger, which is exactly why it is worth remembering.
A friend says their beer is skunky because the shop kept it too warm. What is the accurate correction?
Answer: Warmth gives cardboard. Skunk comes from light, and the two damage beer separately
Skunking is driven by light, so it can happen to a cold beer and cannot happen inside a can. Warmth does real damage, and the damage it does smells of wet cardboard. The reason the wrong version is so persistent is that heat and light usually turn up together, in a van or a shop window, so the same badly kept beer often has both faults.
Pair each container with how much protection it gives against skunking.
Answer: Can or keg with Complete. No light gets in at all · Brown glass with Good. Blocks most of the wavelengths that matter · Green glass with Poor. Blocks some, and the classic imports come in it · Clear glass with Almost none. Assume it happened
The ranking follows how much short wavelength light each one lets through. Green glass is the interesting case: it looks protective and is much weaker than brown. The one caveat is that some breweries use hop extracts that cannot skunk, so a clear bottle is a warning rather than a verdict.
A trace of butter in a Czech lager is automatically a fault.
Answer: False
False, and this is one of the few faults with an official exception. The 2021 BJCP guidelines call light to moderately low diacetyl acceptable in Czech Premium Pale Lager, and low levels optional in Sweet Stout. That makes it a judgement about proportion rather than presence. In a German Pils the same note is simply wrong.
Which of these point at hot alcohol rather than ordinary warmth?
Answer: A smell of nail varnish before you sip · A sharp burn at the back of the throat while the beer is in your mouth · It gets more obvious as the glass warms up
The first three. Solvent smell, burn during rather than after, and getting worse in a warming glass. The fourth is what a well made strong beer is meant to do. The reason this matters is that people write off good strong beers as harsh when the warmth is exactly what they paid for.
Every beer on the bar has a faint buttery note. What is the useful thing to say?
Answer: There is a buttery note on everything I have tried tonight. When were the lines last cleaned?
Naming what you smell and asking about the lines gives the bar somewhere useful to go. Asking for a different beer will not help, because the next one comes down the same plumbing. Saying nothing is the common choice and it is why the problem persists. Line cleaning is routine maintenance, and staff who care will not be offended by the question.
Put the skunking reaction in order.
Answer: 1. Light in the blue part of the spectrum passes through the glass 2. Riboflavin absorbs that light and takes on the energy 3. The energy passes to a bitter compound from the hops, which breaks apart 4. A fragment picks up a sulphur atom and becomes the skunk compound
Light, then the vitamin, then the hop compound, then the sulphur. Knowing the chain is useful because each step tells you a way to stop it. Block the light with brown glass or a can. Take out the hop compound by using a modified extract. Those are exactly the two things breweries actually do.

Class P2.2 done

You can explain what oxygen does to beer and why heat multiplies it, tell warmth apart from hot alcohol, and say why an old beer smells of nothing before it smells of anything bad.

The three lines worth keeping

Heat gives cardboard. Light gives skunk. They are different faults with different causes.
The smell goes first, so a stale beer looks perfect and gives you nothing.
Warmth has no smell. If you can smell solvent, that is the fermentation and not the strength.

Class P2.3

Fault, or just not for you

A lot of the beers people call faulty are correct beers they do not enjoy. The error runs the other way too, and that one is more common and costs more.

By the end of this class you will be able to

Name the style characters that most often get mistaken for faults
Run the three checks that separate a broken beer from a correct one
Spot the faults that regular drinkers have stopped noticing

P2.3.1 Things that are not faults

Seven characters that set off alarms in a beginner and are exactly what the brewer intended.

Most beginners meet their first strange beer and assume something has gone wrong with it. Usually nothing has. Several styles are built around characters that would be faults anywhere else, and every one of them has a history and a place attached.

Every row is a character that would end the conversation in a pale lager and is the point of the beer here.
What sets off the alarm	What it actually is	Where it belongs
Banana and clove	Compounds the yeast makes during a warm fermentation. No banana and no clove went into the beer.	German wheat beer, taught at 4.1
Sharp sourness	Lactic acid, made on purpose by bacteria the brewer chose and controlled.	Gose, Berliner Weisse and the sour family, taught in Module 8
Smoke and bacon	Malt dried over a beechwood fire, which is how all malt was dried before about 1800.	Rauchbier, taught at 2.12
Farmyard, hay, leather, horse blanket	A slow wild yeast that most breweries spend their lives keeping out.	Brett beers and lambic, taught in Module 8
Cloudiness	Yeast or protein still in suspension, because the beer was never filtered.	Wheat beers, unfiltered lagers, hazy pale ales
Sediment in the bottle	Live yeast that carbonated the beer in the bottle. Pour carefully and leave the last inch.	Most Belgian ales, many bottle conditioned beers
A vinegar edge in a red or brown Belgian beer	Controlled acidity from years in oak, held in balance against the malt.	Flanders Red, taught at 8.6

Covered in Module 4, Wheat beers

Hefeweizen, where banana and clove are the style. Warm fermentation and one particular German yeast. A wheat beer without them is the one that has gone wrong.

Covered in Module 8, Sour and wild beers

Gose, where salt and sourness are the style. Deliberately soured, deliberately salted, and from Goslar in Germany. Sour is never the fault in this family.

Covered in Module 2, Amber and dark lagers

Rauchbier, where smoke is the style. Malt dried over a beechwood fire in Bamberg. The smoke is a choice about kilning, not a fault in the brewing.

Covered in Module 8, Sour and wild beers

Brett beers, where funk is the style. A slow wild yeast that produces hay, leather and farmyard. Wanted here, and a disaster in a pale lager.

Covered in Module 1, Pale lagers

Kellerbier, where haze is the style. Unfiltered German lager, served young and cloudy. Haze is a look rather than a fault, and it is not on the list of ten.

Covered in Module 8, Sour and wild beers

Flanders Red, where a vinegar edge is the style. Aged in oak for one to two years, and tasting of cherry and balsamic with none of it added. The question is proportion, not presence.

The three checks

Is it in the style description? If the style is built on sourness, sourness is not the fault. This one question settles most cases, and it is the reason the style modules exist.
Is it in proportion, or is it the only thing there? A Flanders Red should taste of cherry, oak and acidity together. One that tastes only of vinegar has gone too far, and that is a real fault in a style that is meant to contain some.
Is every beer in the room doing it? A character that shows up across beers from different breweries is coming from the bar, not the style.

Run those three in order and you will be right nearly all of the time. The first is knowledge, the second is judgement, and the third costs nothing but a sip.

You are allowed to dislike a correct beer

This is worth saying plainly. If you order a Gose and find it too sour, the beer is not faulty and neither are you. Some people dislike smoke. Some people dislike banana. Preferences are preferences and there is no standard to fall short of.

The only thing worth avoiding is the habit of translating dislike into a complaint about quality. Saying that a beer is not for you is honest, useful to whoever recommended it, and costs you nothing.

These characters have addresses

Salt and sourness come from Goslar. Smoke comes from Bamberg. Banana and clove come from Bavaria. Farmyard comes from the Senne valley outside Brussels. None of these arrived by accident, and none of them is a mistake somebody decided to keep.

Check yourself

Your Gose tastes distinctly sour and slightly salty. What is going on?
Answer: Nothing is wrong. Both are the style, and the salt was added on purpose
That is the style working. Bacteria did produce the sourness, which is what makes the second option tempting, but the brewer put them there and controlled them. The test is not whether bacteria were involved. It is whether the brewer chose them. Sour is never the fault in this family, though the wrong kind of sour can be.
Sort each character by whether it is a fault or a style feature.
Answer: Wet cardboard goes in A fault, wherever you find it · Nail varnish and solvent goes in A fault, wherever you find it · Sticking plaster goes in A fault, wherever you find it · Banana and clove goes in A style feature somewhere · Smoke and bacon goes in A style feature somewhere · Sharp lactic sourness goes in A style feature somewhere · Farmyard and hay goes in A style feature somewhere
Four of the ten faults have no home anywhere: cardboard, solvent, sticking plaster and skunk. Nobody has ever built a style on them. The other four items are each the point of a well known style. This is why a beginner cannot judge a strange beer without knowing what it was aiming at, and why the style modules come before this one.
A cloudy beer with sediment at the bottom of the bottle is probably infected.
Answer: False
False. Most of the time it is live yeast, either left in on purpose or used to carbonate the beer in the bottle. Belgian ales, wheat beers and unfiltered lagers are all supposed to look like this. Infection is a taste rather than a look, and a cloudy beer that tastes clean is simply a cloudy beer.
Pair each character with the place it comes from.
Answer: Smoke and bacon with Bamberg, in Franconia · Salt and sourness with Goslar, in Germany · Banana and clove with Bavaria · Farmyard and hay with The Senne valley, outside Brussels
Every one of these is a local tradition that survived, usually because the town kept drinking it after the rest of the world moved on. Knowing the address is a good check on yourself. A character with four hundred years of history behind it is unlikely to be an accident in your particular glass.
Which question settles most fault arguments fastest?
Answer: Is this character part of the style description?
The style description, because it tells you what the beer was aiming at. Whether most people would enjoy it is irrelevant: most people would not enjoy a Rauchbier and it is a perfectly made beer. Price tells you nothing at all, and an expensive beer that has sat in a warm warehouse is still oxidised.
Which of these should you expect to find in a well made Flanders Red?
Answer: A clear vinegar edge · Cherry and dark fruit · A woody note from long ageing in oak
The first three are the style. The buttery note is the odd one out and is a fault anywhere in this family. The reason this question is worth asking is that acidity in a beer designed to be acidic is judged on balance, not on presence, while a fault like butter is judged on presence alone.
What produces the banana and clove smell in a German wheat beer?
Answer: yeast
The yeast, working warm. Nothing was added. Spices is the tempting answer because clove is a spice you can buy, and because one other cloudy pale beer style really does have coriander in it. Here the entire smell is a by-product of fermentation.
If you dislike a correct, well made beer, you have made a mistake somewhere.
Answer: False
False, and it is worth being firm about. Taste is taste. Plenty of people dislike smoke, or sourness, or the banana note in a wheat beer, and none of that is a failure of skill or attention. The only mistake available here is dressing up a preference as a quality complaint, which helps nobody.
A Flanders Red tastes of vinegar and almost nothing else. Fault or style?
Answer: Fault. The style contains acidity, but it is meant to be balanced against fruit and malt
Fault, and this is the second of the three checks doing its work. Presence is not the test in a style built on acidity. Proportion is. A beer that has lost its fruit and its malt underneath the vinegar has gone past the style rather than achieved it.
Put the three checks in the order that gets you to an answer fastest.
Answer: 1. Is this character part of the style description? 2. Is it in proportion, or is it the only thing in the glass? 3. Is every beer in the room doing the same thing?
Style first, because it is the cheapest check and it settles most cases outright. Proportion second, because it only matters once you know the character belongs there at all. The room check comes last because it costs a sip of somebody else and because it only applies to draught. Run out of order, you spend effort on questions the first check would have answered.

P2.3.2 Faults people have learned to like

The mistake in the other direction. These are real faults that regular drinkers have stopped noticing, and one of them is a whole category of imported beer.

Familiarity is a powerful thing. Drink the same slightly wrong beer for ten years and it becomes the taste of that beer, and the correct version starts to taste strange instead. That is not stupidity. It is how taste works, and it happens to everybody.

None of what follows means you have to stop enjoying anything. It means knowing what you are enjoying.

What people say	What is happening	How to check
That brand just tastes like that	The skunk in a green bottled import is light damage, not a recipe. The same beer from a can tastes different.	Buy it in a can, or from a shop that keeps it in a dark chiller.
That is the house character	A buttery or sour note on every tap is dirty draught lines. Nothing about the building makes beer taste like that.	Try two beers from two different taps. Shared flavour means shared plumbing.
That is the imported taste	Wet cardboard from months in a warm container. It is a fault, and beer in its home country does not taste of it.	Compare against the same style bought fresh and local.
It is meant to be served that cold	Beer served near freezing hides everything, including whatever is wrong with it.	Let the glass sit for five minutes and smell it again.
The head never lasts on this one	A glass that came out of a hot wash, or has grease or detergent on it, kills foam on contact.	Look for bubbles clinging to the inside of the glass. A clean glass does not have them.

Draught lines, and the number that matters

Beer lines grow a film of yeast and bacteria. That film makes the buttery compound and it makes acid, and both go into every beer that passes through. It is the single most common cause of bad beer in bars, and it is entirely a maintenance question.

The Brewers Association standard is that a cleaning log should show the lines cleaned within the last two weeks. Plenty of bars manage it. Plenty do not, and a bar that cleans monthly is common enough that you will meet the result regularly.

Covered in Module P3, Buying, keeping and serving

Glassware, and what beer clean means. Why a clean glass matters for foam, and how to spot a glass that is not. Taught once, in Module P3.

Worked example: A pint that tastes slightly sour

You are in a busy pub. Your stout has a faint sharp bite at the back that you do not remember from last time. It is not pleasant, and it is not obviously wrong either.

Wait two minutes and smell it again. A real sour note becomes clearer as the glass warms. Nerves do not.
Check the style. Stout is not a sour style, so nothing in the description covers what you are getting.
Taste whatever your friend ordered, from a different tap. If that has the same bite, the two beers share a cause.
Look at the bar. A dispense system that looks neglected usually is.

Two beers off two different taps carrying the same sharp bite is the answer, and it is the lines rather than either brewery. That is worth mentioning quietly, because a bar that works to a fortnightly schedule will want to know it has slipped.

Sour does not mean infected

Whole families of beer are sour on purpose, and the brewers who make them are proud of it. Sourness by itself tells you nothing.

The question is always which acid, and whether anybody meant it. Clean lactic sourness in a Gose is the style. A vinegar edge in a stout is oxygen and bacteria in the line. Same sensation, opposite verdicts.

Do not become the fault person

Knowing this material creates a temptation to audit everybody at the table. Resist it. Announcing that your friend has been drinking oxidised lager for eleven years does not improve the lager and does not improve the evening.

Use it on your own glass, and on the bar when something is genuinely wrong. Offer it to other people only when they ask.

Covered in Module F4, At the bar

Ordering, and pour sizes. What to ask for and how much comes in a glass. Taught once, in Module F4. Everything here is about what to do after the beer arrives.

Check yourself

If a famous imported lager has tasted skunky for as long as you can remember, that must be the recipe.
Answer: False
False. It is light damage, and it happens in the shop rather than in the brewery. The reason it is so consistent is that the same brand keeps arriving in the same green glass through the same brightly lit shelves. Buy the same beer in a can and the difference is obvious, which is the cheapest experiment in this whole module.
What is actually growing inside a neglected beer line?
Answer: A film of yeast and bacteria, which makes butter and acid in every beer that passes
A living film. It produces the buttery compound and it produces acid, and both go straight into whatever beer runs past. Mineral deposits are a real problem in draught systems too, but the flavour damage people actually meet comes from the biology. Beer is not remotely sterile enough to look after itself.
Before you read on. How often does the Brewers Association say a bar should be cleaning its draught lines?
Answer: 2weeks. It is frequent enough that a busy bar treats it as routine work rather than a project, and infrequent enough to fit a schedule.
Every two weeks. Their guidance is that the cleaning log should show the last clean inside the last fortnight. Most people guess monthly or quarterly, which is roughly what a lot of bars actually do, and that gap between the standard and the practice is why so much draught beer tastes of butter.
Sort each complaint by whether the drinker is right.
Answer: Every tap here tastes slightly buttery goes in A real fault · This imported lager smells of skunk goes in A real fault · The head vanished in ten seconds and the glass has bubbles stuck to it goes in A real fault · This Hefeweizen tastes of banana goes in Not a fault · This Rauchbier tastes of smoked ham goes in Not a fault
The first three all point at something someone could fix: the lines, the shop lighting, and the glass washer. The last two are the styles doing exactly what they exist to do. The useful pattern is that real faults usually have somebody responsible for them, while style characters have a town and a tradition behind them instead.
Your beer arrives with no head at all and there are small bubbles clinging to the inside of the glass. What does that suggest?
Answer: The glass is not clean. Grease or detergent residue kills foam and holds bubbles
The glass. Bubbles sticking to the sides are the classic tell, because they need something to cling to and a properly clean glass gives them nothing. A flat keg is possible, but it would not produce the clinging bubbles. This one is worth knowing because it is easy to check and easy for the bar to fix.
One sentence test for whether a fault belongs to the bar rather than the beer. If every ____ tastes of it, the problem is the bar.
Answer: tap
Tap, or beer, or any word that captures the idea of more than one product. Beers from different breweries have almost nothing in common except the plumbing and the glassware they meet at the end. A shared flavour has to come from a shared cause, and that is the whole test.
Which of these are genuinely faults rather than preferences?
Answer: Wet cardboard in an imported bottle · Butter on every tap in the bar
The first two. Cardboard is oxidation and butter across a whole bar is the lines. The last two are the defining characters of their styles, and disliking either one is a preference rather than a complaint. That is the entire distinction this class exists to draw, run in both directions at once.
Sourness in beer always means something has gone wrong.
Answer: False
False. Several whole families of beer are soured deliberately, using bacteria the brewer chose and controlled. What matters is which acid you are tasting and whether the style calls for it. Clean lactic sourness in a Gose is the style. A vinegar bite in a stout is oxygen and a dirty line.
Pair each excuse with what is really going on.
Answer: That brand just tastes like that with Light damage in green glass, in the shop rather than the brewery · That is the house character with Draught lines that have not been cleaned · That is the imported taste with Oxidation from months in a warm container · It is meant to be that cold with Cold hiding whatever is wrong with it
Each of these is a real thing that people say, and each one converts a fixable problem into a fact of life. The pattern is the same every time: something that has an owner and a remedy gets relabelled as a characteristic, and then nobody has to do anything about it.
You are fairly sure the draught lines are dirty. What is the useful thing to do?
Answer: Mention what you smell, quietly, and ask when the lines were last cleaned
Quietly and specifically, to someone who can act on it. Line cleaning is routine work and a bar that takes it seriously will want to know. Announcing it to the table achieves nothing except making the evening worse. Saying nothing is the most common choice, and it is precisely why the problem lasts for months.

Class P2.3 done

You can tell a broken beer from a correct one you do not enjoy, in both directions, and you know the three checks that get you there.

The three lines worth keeping

Style first, proportion second, the rest of the room third.
You are allowed to dislike a correct beer. Say so, and do not call it faulty.
Cardboard, solvent, sticking plaster and skunk have no home anywhere.

Class P2.4

The fault library, and sending a beer back

Every fault in one table, with the compound and the cause, plus six rarer ones. Then the exact words for sending a beer back, and an honest account of how that conversation actually goes.

By the end of this class you will be able to

Look up any fault and find the compound, the cause and whether a style ever wants it
Decide whether a beer is worth sending back, before you say anything
Say the words, in a way that gives the bar somewhere useful to go

P2.4.1 The fault library

The ten, with the chemistry attached, plus six you will meet less often. This is the page the rest of the course links to.

The ten. Two of them have two faces, which is why the sulphury and unfinished rows carry two smells each.
What you get	Name	The compound	What caused it	Ever wanted
Wet cardboard, wet paper, then sherry and old honey	Oxidised	A family of aldehydes, the best known being trans-2-nonenal	Oxygen picked up at packaging, then time and warmth. Warmth roughly doubles the rate for every 10C, which is 18F	Only as sherry, in an old strong dark beer
Almost nothing. The aroma has left	Stale	Loss of hop oils and other volatile compounds	Age, warmth, and light. The smell goes months before the taste does	Never
Butter, butterscotch, cinema popcorn, and a slick feel	Buttery	Diacetyl	Yeast taken off the beer before it cleaned up after itself. Or bacteria growing in draught lines	Low levels in Czech Premium Pale Lager and Sweet Stout
Skunk, burnt rubber, cat	Skunky	MBT, short for 3-methyl-2-butene-1-thiol	Blue light near 450 nanometres through clear or green glass, working on hop compounds by way of riboflavin	Never
Cooked sweetcorn and canned vegetables, or struck matches and eggs	Sulphury	Dimethyl sulfide for the sweetcorn. Hydrogen sulfide for the eggs	Sweetcorn survives a short or gentle boil, or slow cooling. Eggs come from young or stressed yeast	Traces of sweetcorn in some pale lagers. Eggs only in very young lager, and it should clear
Green apple and cider, or raw grain and honey	Unfinished	Acetaldehyde for the apple. Unfermented sugars from the wort for the grain	Yeast pulled off early or too stressed to finish. Or a fermentation that was deliberately cut short	Never, though the grain version is the standing problem of non-alcoholic beer
Vinegar, a sharp clean sour bite, or cheese and baby sick	Infected	Acetic acid, lactic acid, and butyric acid for the cheese	Bacteria in a line, a tank or a cask. Vinegar means oxygen reached the beer as well	Lactic acid, in the sour styles, where the brewer chose it
Nail varnish, solvent, and a burn at the back of the throat	Hot	Higher alcohols, plus ethyl acetate for the nail varnish	Fermentation that ran too warm or too fast, or not enough yeast for the amount of sugar	Never. Warmth without the solvent smell is a different thing and is fine
Plastic, sticking plaster, mouthwash, swimming pool	Medicinal	Chlorophenols	Chlorine in tap water or in a sanitiser, meeting compounds produced by the yeast	Never
Drying, chalky, like stewed tea	Astringent	Tannins from grain husk and dark malt	Grain rinsed with water that was too hot, milled too finely, or too much roasted malt. Also cacao, coffee and fruit stems	Never, though a light grip is normal in a hard roasted stout

Six you will meet less often

What you get	Name	What caused it
Burnt rubber, meat stock, soy sauce, Marmite	Autolysis	Dead yeast breaking open and leaking its insides into the beer. Old bottle conditioned beer, or beer left sitting on its yeast far too long
Iron, blood, coins	Metallic	Minerals in the water, or corroding equipment somewhere in the process
Fresh cut grass, green leaves	Grassy	Hops or grain that were not dried properly, a very heavy dry hop, or hops that have aged badly
Soap, or a fatty film	Soapy	Beer left standing on the dead yeast at the bottom of a tank. Sometimes detergent left in the glass
Blackcurrant leaf, tomcat	Catty	Certain hop varieties, and much more likely once those hops have oxidised
Damp cellar, mould, wet cardboard box	Musty	Contaminated corks or cellar materials. Related to the fault that ruins wine, and it comes from the packaging rather than the beer

How little it takes

Order of magnitude figures. Published thresholds vary between studies and between people, sometimes by a factor of ten. What they establish is the ranking, not the exact numbers.
Fault	Roughly how little	What that tells you
Skunk	Low parts per trillion	A few billionths of a gram in a litre. One of the lowest thresholds anybody has measured in food
Cooked sweetcorn	Around 0.02 to 0.1 milligrams per litre	Published ranges differ by a factor of five between people
Butter	Commonly quoted near 0.1 milligrams per litre	Sensitivity varies enormously, which is why two people at one table disagree
Green apple	Commonly quoted in the tens of milligrams per litre	Far more than the others, so finding it means a real problem rather than a trace

That ranking is the useful part. The skunk compound is detectable at a level roughly a million times lower than the green apple one. It explains why a single afternoon in the light ruins a beer, while a slightly rushed fermentation often passes without comment.

Who can do something about it

The brewery. Sweetcorn, green apple, raw grain, hot alcohol, medicinal, astringent, soapy, metallic. These arrive in the package.
Whoever moved and stored it. Oxidised, stale, skunky. The brewery may have done everything right.
The bar. Butter and sourness from the lines, no head from the glass, and skunk if the bottles sit somewhere bright.
Nobody, this time. Autolysis in a ten year old bottle you chose to keep is a decision you made.

Worth remembering

Package, journey, plumbing.

Three places a fault can be born, in the order the beer meets them. Package faults are in every bottle of the batch. Journey faults are about time, warmth and light. Plumbing faults show up on every tap at once. Deciding which of the three you are dealing with tells you whether to change beer, change shop, or say something.

Sources: BJCP Beer Faults reference list · Brewers Association, Draught Beer Quality resources · Brewers Association, Draught Beer Quality Manual · Kvasny Prumysl 2022, Direct detection of beer photodegradation in commercial bottles · BJCP 2021 Style Guidelines

Check yourself

A very old bottle conditioned beer smells of burnt rubber and soy sauce. Which fault is that?
Answer: Autolysis. The yeast has died and broken open inside the bottle
Autolysis, which means dead yeast leaking its contents into the beer. Meaty, rubbery and savoury notes are the signature. Oxidation is the tempting answer because both come with age, but oxidation gives paper and sherry rather than soy sauce, and a filtered beer with no yeast in it cannot do this at all.
Pair each fault with the compound behind it.
Answer: Butter and butterscotch with Diacetyl · Cooked sweetcorn with Dimethyl sulfide · Skunk with MBT, or 3-methyl-2-butene-1-thiol · Green apple with Acetaldehyde · Sticking plaster with Chlorophenols
These five are the ones with a single clean compound behind them, which is why they are the easiest to describe and the easiest to test for. Oxidation is deliberately absent from this list, because it is a whole family of compounds rather than one, and that is part of why it presents so differently at different stages.
The skunk compound and the green apple compound become noticeable at roughly the same concentration.
Answer: False
False, and the gap is enormous. Skunk is detectable in the low parts per trillion. Green apple takes tens of milligrams in a litre, which is around a million times more. That difference explains a lot of ordinary experience: an afternoon on a bright shelf ruins a beer, while a slightly rushed fermentation usually goes unremarked.
Sort each fault by where it was born.
Answer: Cooked sweetcorn goes in The package · Sticking plaster goes in The package · Wet cardboard goes in The journey · Skunk goes in The journey · Butter on every tap goes in The plumbing · A sharp sour bite on the lager and the stout alike goes in The plumbing
Package, journey, plumbing, in the order the beer meets them. It is the most useful sorting in the module, because each bucket has a different response. A package fault means choose a different beer. A journey fault means choose a different shop. A plumbing fault means say something, because nothing you order in that building will escape it.
Which pair of faults is most often confused, and why?
Answer: Oxidised and stale, because both come with age and one of them is the absence of a smell
Oxidised and stale. They travel together, they share the same causes, and the difference is that one is a smell arriving and the other is a smell leaving. Getting them apart matters because a stale beer is simply disappointing, while an oxidised one is actively unpleasant, and the second is much easier to raise at a bar.
Which fault gives a beer a soy sauce and burnt rubber character, from dead yeast?
Answer: autolysis
Autolysis. The word simply means self digestion, and it is what happens when yeast cells die and split open. You will meet it mainly in old bottle conditioned beer and in draught beer from a system that has been neglected for a long time.
Which of these are genuine causes of an astringent, drying, tea-like finish?
Answer: Grain rinsed with water that was too hot · A large amount of hard roasted malt · Cacao nibs or coffee added to the beer
The first three all put tannins into the beer. A long boil is the odd one out and does the opposite of harm here: a good rolling boil is what drives off the sweetcorn compound. The reason the fourth option is tempting is that a long boil does darken and concentrate the beer, and darker beers really are more likely to be astringent.
Two people share a jug. One is certain the beer is buttery and the other cannot find it at all. What is the most likely explanation?
Answer: Sensitivity to that compound varies widely between people, so both can be honest
Individual sensitivity, which for this compound varies by a large factor between people. That is why published threshold figures disagree and why fault panels use several tasters. Treating a disagreement as evidence that someone is imagining things is the natural move and it is usually wrong.
Put the three places a fault can be born in the order the beer meets them.
Answer: 1. The package 2. The journey 3. The plumbing
Package, journey, plumbing. The order is worth keeping because it matches the order in which you can rule things out. If the beer came from a can you opened yourself, the plumbing is out. If the can was packaged last week and kept cold, the journey is out. What is left came from the brewery.
A musty, damp cellar smell in a beer usually comes from the beer itself.
Answer: False
False. It usually comes from the packaging or the cellar, most often a contaminated cork or contaminated material in the storage area. It is the same fault that ruins bottles of wine. The beer inside may have been made perfectly, which is why this one is worth naming rather than blaming the brewery for.

P2.4.2 Sending a beer back

When it is right, when it is not, the exact words, and what to do when the bar takes it badly.

Sending a drink back is socially uncomfortable, and pretending otherwise would not help you. You are interrupting someone at work, in front of other customers, to tell them that something they just handed you is not good enough. Most people would rather drink the beer.

It is still worth doing when the beer is genuinely faulty, and there is a way of doing it that keeps everybody comfortable. The whole trick is to make it a report rather than an accusation.

When it is reasonable

The beer has a fault you can describe, and the fault is not part of the style.
It is not the beer you ordered.
It arrived flat, warm, or with no head in a glass that has bubbles clinging to the side.
You are on the first mouthful. Raise it early or let it go.

When it is not

You do not like the style. That is a preference, and it is not the fault of the bar or the brewery.
You have drunk two thirds of it. Whatever the problem was, you have made your position clear.
You have simply changed your mind about what you fancy.
The character you object to is in the style description. A sour beer tasting sour is not a complaint.

The safest opening, if you are not certain

This is the one to learn first, because it works whether you are right or wrong.

Could I ask your opinion on this? It tastes a bit off to me and I might be wrong.

It asks for help rather than delivering a verdict, and it gives the bar a graceful way to agree with you. In practice most staff will smell it, and half of them will already know.

A faulty pint, on the first sip

I think there is something wrong with this one. It smells of wet cardboard to me. Could I try something else instead?

Smell first, name second, and a request at the end. Asking for something else rather than the same thing again avoids a second faulty pint from the same source.

A buttery or sour note across the bar

There is a buttery note on both of the ones I have tried tonight. Do you know when the lines were last cleaned?

This is a question about maintenance rather than about the beer, so it lands very differently. A bar that cleans on schedule will answer immediately and be slightly pleased you asked.

A skunked bottle

This one has gone skunky, I think it has been sitting in the light. Do you have it in cans?

Offering the explanation takes the blame off the bar, because the damage almost certainly happened before the bottle arrived. Asking for a can solves the problem rather than repeating it.

A dirty glass, or the wrong beer

Could I get a fresh glass? This one is not holding any head.
Sorry, I think this might be the wrong one. I ordered the pale ale.

Both of these are easy, because neither implies anybody did anything wrong. Use them freely.

What a good bar does

A good bar takes the glass, smells or tastes the beer, and either replaces it or explains what you are getting. Nobody argues. In the United States you will usually be offered something else without being charged. In Britain, changing a cask ale that is not right is routine and expected, because cask beer is a live product and everybody in the trade knows it.

What a bad bar does

Some bars handle this badly. You will meet the sceptical look, the shrug, the claim that nobody else has complained, and occasionally a small argument about whether you know what the beer is meant to taste like. It is unpleasant and it is not your fault.

When that happens, the useful move is to stop. You have said what you noticed, and repeating it will not change the outcome. Say something that lets everybody move on, and make a quiet decision about whether to order a second one.

When the bar pushes back

That is fair enough. I will leave it, thank you.
No problem. I will have a bottle instead, if that is easier.

Switching to a bottle or a can is a quiet way of stepping around a draught problem without anybody having to agree about it.

The honest social picture

You are allowed to simply not finish a drink and not order it again. That is a complete and dignified response, and on a first date or at a work dinner it may well be the right one.

If you are with a group, raise it at the bar rather than across the table. It keeps the evening intact and it does not put anybody who is enjoying their pint on the defensive.

And do not tip less because the beer was faulty. Whoever poured it did not brew it, ship it or store it.

The best fix is upstream of all of this. Most bars with a serious beer list will let you taste a small amount before you commit, and asking for that is entirely normal. A taster costs nobody anything and removes the whole conversation.

Covered in Module F4, At the bar

Ordering, pour sizes and asking for a taster. How to ask, what sizes exist and what is normal to request. Taught once, in Module F4.

Covered in Module P3, Buying, keeping and serving

Glassware, and what a clean glass looks like. Why bubbles clinging to the side mean the glass rather than the beer. Taught once, in Module P3.

Check yourself

You are not sure whether your beer is faulty. What is the best opening line?
Answer: Could I ask your opinion on this? It tastes a bit off to me and I might be wrong
The first, because it works whether you are right or wrong. It asks for help rather than delivering a verdict, and it gives the bar a graceful way to agree with you. The second commits you to a position you may not be able to hold. The third sounds sarcastic even when it is not meant to be.
It is reasonable to send a beer back after you have drunk two thirds of it.
Answer: False
False, and this is the one rule almost everybody agrees on. Whatever was wrong with it, you have demonstrated that you were willing to drink it. Raise a fault on the first mouthful or accept the pint. The exception is a beer that changes as it warms, and even then you are on much better ground at a third of a glass than at the end.
Put a good complaint in the right order.
Answer: 1. Describe what you smell or taste, in ordinary words 2. Offer the name of the fault if you have one 3. Ask for something specific: a different beer, a fresh glass, or a can
Smell, name, request. Ending with a specific request is the part people leave out, and it is what turns a complaint into something the bar can act on. A description with no request leaves the person behind the bar guessing what you want from them.
You order a Gose, find it too sour for you, and have drunk a third of it. What should you do?
Answer: Finish it or leave it, and order something else next. It is not faulty
It is doing what the style does, so there is nothing to send back. This is the single most common wrong send back and it is why bar staff sometimes look sceptical at the start of these conversations. You are perfectly entitled to dislike it, and saying so to whoever recommended it is useful. It is just not a complaint about the beer.
Which of these are good reasons to raise something with the bar?
Answer: The beer smells of wet cardboard on the first sip · It arrived with no head and the glass has bubbles clinging to the sides · It is not the beer you ordered
The first three are all things somebody at the bar can act on: a faulty bottle, a dirty glass, a mistake at the tap. The fourth is a mismatch between your expectation and the style, which is worth knowing for next time and is not a fault. Bitterness that surprises you usually means you have found the edge of your own preference.
Complete the line for a suspected dirty line problem. There is a buttery note on both of the ones I have tried. Do you know when the ____ were last cleaned?
Answer: lines
The lines. Naming them specifically shows you know what you are asking about and turns the conversation into a maintenance question rather than a complaint about the beer. Glasses is a reasonable guess and a real problem too, but a glass affects one drink while a line affects everything poured through it.
The bar disagrees with you and says nobody else has complained. What is the best response?
Answer: Accept it, say you will leave it, and decide privately whether to order another
Stop. You have made your report and repeating it changes nothing except the atmosphere. Nobody else complaining is weak evidence at best, since most people never complain, but arguing that point will not get you a better pint. Explaining the chemistry to a busy bartender is the tempting option and it rarely goes well.
Sort each situation by whether it is worth raising.
Answer: Wet cardboard on the first sip goes in Worth raising · The wrong beer arrived goes in Worth raising · A glass that will not hold a head goes in Worth raising · A Rauchbier that tastes smoky goes in Let it go · A pint you are two thirds of the way through goes in Let it go
The first three are fixable and someone in the building can fix them. The Rauchbier is doing its job. The two thirds finished pint is a timing problem rather than a quality one, and raising it that late tends to produce the difficult version of this conversation.
You should tip less when the beer turns out to be faulty.
Answer: False
False. Whoever poured it did not brew it, ship it, or leave it in a warm warehouse for four months. Line cleaning may not be their decision either. Punishing the person in front of you for a problem that started somewhere upstream is both unfair and useless, since it changes nothing about the beer.
Pair each situation with the request that solves it.
Answer: A skunked bottle from a bright shelf with The same beer in a can, if they have it · A buttery note on every tap with Nothing from the taps. A bottle or a can instead · A glass that will not hold a head with A fresh glass · A beer you are unsure about before ordering with A taster, which most serious beer bars will give you
Each request steps around the cause rather than repeating it. Asking for the same beer again off the same tap is the mistake here, because the second pint comes down the same line as the first. The taster is the most useful of the four, because it removes the whole conversation before it starts.

Module P2 done

You can name ten faults from smell, say what caused each one, tell a broken beer from one you simply do not enjoy, and send a beer back without making anybody uncomfortable.

The five lines worth keeping

Cardboard, butter, skunk, corn, nothing. Then apple, vinegar, solvent, plastic, tea.
Package, journey, plumbing. Three places a fault can be born.
Heat gives cardboard. Light gives skunk.
Style first, proportion second, the rest of the room third.
Smell, name, request. In that order, every time.

Come back to this in four days

Faults are learned by meeting them, not by reading about them. Between now and then, smell every beer you are handed before you drink it. Four days from now, try to write the ten down from memory before you open this again.