When beer goes bad: Acetaldehyde

Acetaldehyde smells and tastes like fresh green apples in beer. Open a bottle of Budweiser to taste it as Bud contains a little acetaldehyde which adds a fresh, crisp flavour. If it’s present in stronger beers or in higher volumes then it might smell like bruised old apples, solvent or paint.

At the beginning of fermentation the yeast gets to work converting sugar to ethanol and acetaldehyde is formed as a precursor to the booze. The yeast will usually then convert the acetaldehyde into ethanol in the latter stages of fermentation. Too much acetaldehyde is usually a sign that the beer is ‘green’ and the yeast hasn’t had a chance to do what it needs to do (this could be if it’s taken off the yeast too soon, crash cooled or filtered out too early). To avoid it the key is to allow the beer to properly condition for the yeast to convert all the acetaldehyde.

If a beer becomes oxidised post-fermentation then the formation of ethanol can be reversed and create acetaldehyde, which can then become acetic acid and turns that fresh apple flavour into funky cider vinegar and that’s definitely not good in a beer.

If your pint smells like apples then the brewery were probably in a rush to get the beer made and it’s a sign that the beer is a little too ‘green’. If it smells like paint or bruised apples then there are bigger problems. If it smells and tastes like cider then send it back. It’s worth noting that some particular yeast strains give off more acetaldehyde than others but this is a personal concern of the brewer, not the drinker. Unless it’s present in very high volumes then it’s not unpalatable and some like the flavour.

Is acetaldehyde something you’ve noticed in beers before? Ever had one like solvent or cider?

This is one of those flavours which I don’t mind in small volumes, but it is one which is easily fixed so should never really be there (unless deliberate) as it should naturally disappear if you give the yeast enough time. If there’s any more simple science stuff to add then please put it in the comments – I’m not a brewer or scientist so I’m trying to keep it simple.

This is the last off-flavour post for now. We’ve had Diacetyl, Autolysis, Light-struck, Phenolic, DMS and Oxidation. I’ve got a few more that I want to go over later in the summer but these are the big few that it’s good to know about.

I used this and this to help write the post. I also used Randy Mosher's Tasting Beer for all of them. And thanks to Mark from Beer. Birra. Bier. who looked over the posts for me and spotted some terrible errors. 

When beer goes bad: Oxidation

When oxygen gets to a beer after fermentation then the beer is in trouble. The typical flavours you can expect are cardboard, paper and wood, while extremely oxidised beer can become sherry-like and sour. In a fresh beer these stale are not good characteristics; in an aged beer, particularly vintage ales and old ales, these can be good things. It’s a matter of taste and of relevance to the style.

While the brewers need oxygen to stay alive, if it gets into the beer it can have the opposite effect. Oxygen can get to beer at many different stages during beer production, some when brewing and some with filtering and packaging, but it becomes most relevant after fermentation. In fresh beer oxidisation is bad. I find it one of the least enjoyable flavours, imparting a dry mouthfeel like you’ve got a piece of paper stuck on your tongue. It can also evolve the malt flavour in ways the brewer didn’t intend – pale beer can mutate into honey or sherry qualities while dark beer can get these sherry notes plus dried fruit, which many actually enjoy, but it can also reduce the depth of malt flavour and leave it thin.

In aged beer oxidation can be a good thing, especially if the beer is stored correctly, as it can add complexity and depth. It can also be a bad thing in aged beer if it’s turned sour and lost all its flavour. Oxidation in aged beers is a complicated one and very much an individual taste depending on individual beers.

The oxidation process can also bring back foes from fermentation such as acetaldehyde (which can then turn into acetic acid), while diacetyl can speed up the staling process.

Any oxygen in beer after fermentation is bad news but heat and motion can speed up the oxidising process. Shipping beers internationally in non-chilled containers can result in the beers arriving at their destination oxidised (so if you’ve ever been disappointed by an exciting new import from afar then consider the distance it’s travelled and the cargo conditions). Likewise, kegged beer sitting in direct sunlight can suffer the effects of oxidisation (the light is ok here, the heat isn’t). Beer needs to be kept cool and still for it to have the best chance to stay fresh.

Beer oxidises. Thankfully there are ways of softening the blow of opening what you think should be a great beer only to find that it’s going bad. While it may develop interesting depth in some beers, it’s not necessarily a good thing, particularly in fresh beers. Do you like the flavour of oxidised beer?

I used this post to help me write this blog.

When beer goes bad: DMS (Dimethyl Sulfide)

DMS (Dimethyl Sulfide) smells and tastes like sweetcorn or stewed vegetables and is most susceptible and perceptible in light lagers and pilsners.

Stuart Howe describes the formation: “DMS is formed during fermentation from dimethyl sulphoxide which is formed during kilning from the precursor S-methlymethionine [SMM]. The reason it is found more commonly in pilsners is that the more gentle kilning of lager malt removes less S-methylmethionine than that of ale malts.”

SMM is a product of malt germination and the more kilned and roasted the malt is the less SMM there will be in them, this is why pale lagers are more susceptible to DMS than dark ales (interestingly, in the case of a beer like Carling, the malt is kilned to specifically allow a certain higher-than-usual amount of DMS into the beer). When brewing, the malt gets mashed with hot water to create wort. Once the wort has been transferred (with the SMM coming with it – there’s a direct relationship between the levels of SMM and DMS) it is boiled and DMS is produced and then typically it is evaporated out with the boil. If the boil is covered then DMS struggles to escape, so will stay in the wort. If the boil is short then some DMS may still be present, so a long and vigorous boil, uncovered, is the best way to see off DMS. If the beer cools too slowly then more DMS could also be created.

Some people are more sensitive than others to the flavour; I don’t mind it in a lager, where it can be appropriate, but only in small volumes and only if it’s corn-like. If your beer tastes like cold, stewed cabbage then that’s never a good thing. Like most of these characteristics, DMS is naturally present in beer but it’s only when it hits higher perceptible levels that it becomes a problem.

Ever tasted sweetcorn/DMS in a beer?  

There’s lots that can go wrong when making beer but thankfully most of them can be controlled. If I’ve missed anything important about DMS then it’d be great if you add it below. Also, this shouldn’t be confused with beers made with corn as an adjunct as that is something different (although DMS can still be present from the usual brewing method). I used this and this to help me out with the post.

When beer goes bad: Phenolic

Plasters, Islay whisky, TCP, smoke, clove; very few beers should taste like this.

A phenolic aroma and flavour should be present in some beers, including wheat beers and some Belgian ales, where it gives a faintly smoky or clove-like aroma, but if you find it in a pale ale, lager or bitter then things aren’t quite right. And this is another off-flavour which some people are more susceptible to tasting than others.

The phenolic flavour most often comes from a reaction between chlorine and the phenolic acid naturally found in malt; this could be from the chlorine content in the water (especially if it’s untreated) or from a cleaning solution used in the brewery which contains chlorine.

This aroma and flavour can also come from the yeast (wheat beer yeast, for example, will produce it, and I’ve had a few wild beers which have smelt like smoky swimming pools). Peated malt will also have this flavour but it will often add a different character more along the lines of charred toast than TCP. 

If a phenolic flavour is appropriate to the style then it’s fine. I’m sensitive to the flavour in beer and I don’t like it at all – I like wheat beers but not ones which are heavy on the clove. I also sometimes taste it as flinty or like concrete. Interestingly, however, I love massive smoky whiskies (and I really like stouts aged in smoky whisky barrels - if the flavour is from a barrel like this then it isn't an off-flavour). When it shouldn’t be there it dives out the glass at me and I find it undrinkable.

Is this a flavour which you can easily detect in beer? If so, do you like it or hate it?

I only realised recently that it was the clove-like flavour which I didn’t enjoy in wheat beers and some Belgian ales. I typically described it as ‘spicy’, which does no justice to what it is; it was just the word which meant to me that it tasted a bit funny.

For this post I used this and this for references.

When beer goes bad: Light-struck or Skunked

Beer contains hops (it also contains sulphur). Hops contain alpha acids. When hops are boiled the alpha acids get isomerized and become isohumulone. When light hits the isohumulone compounds it breaks them down and causes a reaction with the sulphur in the beer and produces some of the same stinky chemicals that skunks spray. Hence the term ‘skunked’ beer.

Brown bottles offer the best protection from those beer-harming UV rays, green bottles don’t help much and clear bottles offer no protection against light. Open a beer in a clear bottle and it will have a similar dusty, funky character to any other beer in a clear bottle; that's not some unique British hop you're smelling, that's skunked beer. All bottles let through some light and therefore all are susceptible to being light-struck. Casks, kegs and cans are safe from the light.

Beer can be light-struck almost immediately in direct sunlight; it takes a little longer in non-direct daylight and a little longer still in fluorescent light (the sort of light that brightens the supermarkets which have aisles filled with beer...), but it can happen quickly. And it’s the hops which make it go bad; Coke or wine in clear glass bottles aren’t affected the same way.

I’ve never smelt a skunk but I have smelt and tasted skunked beer and it’s bloody horrible. I now won’t buy beer in clear bottles because I hate the aroma and flavour of light-struck beer. Sometimes it’s just a little moldy and funky, like over-stewed vegetables, dust or damp cardboard, other times it’s like sewage or really bad breath. Sometimes it’s like all of the above. Never is it nice.

I don’t understand why any brewery would make beer and then put it in clear bottles when they know it will be affected by light. I’m guessing it’s a marketing choice but if that decision is done on aesthetics while knowingly spoiling decent beer then what’s the point? Some well known beers have an essential quality of being light-struck (Newcastle Brown Ale, Shepherd Neame and Becks to name just a few) so the brewers are making decent beer and then having to let the light do its damage to create the flavour. Why?

I also think beer in clear bottles looks cheap and horrible.

It is possible to use clear bottles without it getting light-struck but it involves using tetra-hop extract, a lab produced hop replacement that is pre-isomerised and doesn’t get affected by light. The trouble is that it doesn’t taste much like real hops (the beers it gets used in don’t taste much like real beer though...).

Beer in clear bottles is bad. If you want to taste it for yourself then buy two beers in clear bottles (although they will already be affected by the time you get them), leave one in the light for a week or two and the other in a dark cupboard and then try them side-by-side (for more extreme results leave it in direct sunlight but watch out because it’s horrific).

Do you buy beers in clear bottles or not?

If I’ve missed any interesting science stuff then let me know below. If you know why a brewery uses clear bottles then also please tell me. Of all the off-flavours I think this is the stupidest because it can largely be avoided simply by using a different coloured bottled. And sure, most people are used to that flavour in the beer, but that doesn't make it right, does it?

I used this and this to help write the post.

When beer goes bad: Autolysis

Have you ever had a beer which tastes like marmite, burnt rubber or soy sauce? If so, what you are tasting is yeast cells which have died, ruptured and spilled their beer-spoiling guts into your brew...

Autolysis will be most common in old beers and I’ve had it often in aged bottled-conditioned strong ales but it’s also possible in fresh beer, where it will produce a burnt rubber aroma and taste. As a flavour I don’t mind it in strong, dark beers, finding that it adds a depth of complexity to the beer if it’s not too overpowering, but given its umami, marmite-like flavour (marmite being yeast extract), I’m guessing it’s a love-hate thing with other people.

What causes autolysis? Firstly, unhealthy yeast cells are more prone to it than healthy ones because they are weaker. It’s also due to stresses put on the yeast during fermentation. It can be caused by too-rapid warming or cooling of the beer during fermentation; a fermentation temperature which is too warm; exposure to high temperatures after fermentation (keep that bottled beer somewhere cool!); the hard work of trying to ferment a strong beer can leave the yeast cells dying in the beer; and strong bottle-conditioned beers rely upon the secondary fermentation to keep them going and if the yeast inside gives up then autolysis comes along.

I asked Mark from Beer. Birra. Bier. to look over these posts (because he homebrews and knows lots of stuff about beer) and he adds the following about autolysis: “A big thing here is oxygen. If oxygen isn’t present when the yeast cells are multiplying then the cell walls of the yeast cells that result will suck. Leaving beer on yeast cells for long periods of time will also cause autolysis and crappy unhealthy yeast cells are just more likely to want to explode and die.” (I love the last line and have visions of crazy, lazy kamikaze yeast cells self-destructing because they’ve had enough!)

This piece by Moritz Kallmeyer is very informative. Interestingly, as well as imparting those flavours to beer, the autolysed yeast cells also release “proteolytic enzymes which degrade beer-foam proteins and also increase protein and carbohydrate hazes,” it releases lipids (fats) and will increase the pH value, affecting the perceived flavour. None of these are good things.

Autolysed beer is generally not good because essentially a major part of it has died. Have you tasted beers which have these flavours? Is it something you like or dislike in a beer? Have you ever had a fresh beer with these characteristics?

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This is another flavour which won’t do you any harm and which can add an interesting complexity to strong, dark beers, although it can kill fresh beers and leave them undrinkable. As with all the off-characteristics it’s down to the perceptible level and individual taste. Autolysis is also something which I knew nothing about until a few months ago; I’d tasted it but had no idea where that marmite flavour came from (glutamic acid is a breakdown product of the yeast). If anyone has anything to add to the science stuff which I may have missed out then please do.

And this is a question which I could use some help with (because I'm writing these posts so that I can learn stuff): In very hoppy beers, often ones around 7-8% which aren’t exactly fresh but also aren’t old (let’s say 5-8 months old), I’ve tasted rubber bands and assumed it’s come from tangy, intense old hops. Is this autolysis or is it from the hops? My instinct, because it doesn’t taste great, it towards autolysis...

When beer goes bad: Diacetyl

If you’ve ever experienced a butter or butterscotch flavour in your beer than you’re tasting diacetyl. The interesting thing about diacetyl, I think, is that not everyone can detect it as a flavour; of those who can detect it, some like it or don’t mind it, while others can’t stand it.

Diacetyl is a natural product of fermentation and is created and given off by the yeast cells early in fermentation. The yeast cells then take it back within themselves towards the end of fermentation and convert it into something almost tasteless – if a brewer stops the fermentation process before the yeast has finished its conversion, or if the fermentation stalls, then diacetyl will still be there (stopping it early could be the result of a need for brewing speed or just a slack move by the brewer; if it stalls early then the yeast is probably not healthy enough).

Lower temperatures towards the end of fermentation could see lazy yeast leaving behind diacetyl. One way to combat any leftover diacetyl is to include a diacetyl rest in the fermentation process, which involves slightly raising the temperature to poke the yeast into a final burst of action and leaving it for a further day. Diacetyl is an easy thing to prevent or control unless you’ve got a yeast infection.

An excess of diacetyl can be the result of a contaminated yeast which isn’t up to the job of converting the buttery flavour into a flavourless one, so the yeast needs to be fit and healthy. It could also be the result of an infection or bacteria in the yeast or the presence of wild yeast – some of these produce diacetyl. However, some strains of yeast just throw off more diacetyl that others when they ferment (pale ale and lager yeasts often do).

Beer shouldn’t taste like butter but small amounts are appropriate to some beer styles – pale lagers and some pale ales. In larger volumes diacetyl can give a fatty, slick mouthfeel (as if there is some melted butter in your pint...) which is very unpleasant. If your pint tastes like butter then you might not want to finish it, but that’s down to individual tastes and it’s not a fault that will do you any harm.

Diacetyl is one of the off-flavours which many find easy to recognise and can sniff it out an arm’s length away. I used to enjoy the flavour in beer (mmm, this has a delicious buttery taste!) until I realised that it shouldn’t be there and now my inner beer geek just screams out DIACETYL! whenever I taste it. I now only get on with it if it’s in small volumes. Are you sensitive to diacetyl? Do you like it, tolerate it or hate it?

This is the first in a series of posts looking at when beer goes bad. Being neither a brewer nor a scientist I’ve tried to keep it simple and from a drinker’s point of view. If I’ve missed anything interesting or important out then please add it below.

I used Beer Sensory Science (a great blog!) to help with this post.