A Practical Guide to Off Flavors

January 27, 2017

By Miles Liebtag, January 27, 2017

There’s a specter haunting craft beer: the specter of quality.

It’s become a shopworn canard to observe that “there’s never been a better time to be a beer drinker,” which, yes, if consumer choice be the metric of good times, these are very good times indeed: 2016 saw America pass the 5,000 mark for breweries, with many more in the planning, preplanning, and notional stages.

There is beer everywhere: stuffed onto dusty warm shelves at your local supermarket, moldering in greasy keg coolers at strip mall sports bars, flying out the door, unit by expensive unit, at your neighborhood bottle shop.

The proliferation of choice is staggering; you almost need specialized knowledge in order to even begin to understand it all. After five years in the industry, I’m not sure I understand it.

Brands rear up, roar in, fizzle out and disappear with almost daily regularity, and old stalwarts with decades of proven consistency and growth now struggle to find homes in shoppers’ carts and fridges before the clock runs out on their date codes. Brand loyalty is almost unheard of, and novelty reigns: as the man said, the best lack all conviction, while the worst are full of passionate intensity.

Because that’s what this was all supposed to be about, right, once upon a time? Passion?

It was passion for brewing, for beer, that drove all those homebrewers out of their basements and backyards and into warehouse spaces and disused factories in order to hone and develop and strive for perfection in their craft. Better beer was the object: beer better than your dad or his dad drank or was reduced to drinking; beer better than the seltzer-like deracinated pilsners of American big business tradition; better than what was available.

Better beer was the object: beer better than your dad or his dad drank or was reduced to drinking.”

So what makes beer better? Flavor, certainly, a depth and intensity of flavor seems a hallmark of craft brewing. Balance, maybe, depending on whom you ask; the predilection of American tastes in the craft segment seems to suggest otherwise, but most, I think, would call balance a virtue if pressed. But what makes a beer worse?

Put another way, what qualities should a good craft beer not possess? Easy, right? Off flavors!

What is an off flavor? Let’s define it, loosely, as any undesirable sensate element the brewer or producer did not intend for the beer. Obviously there’s a lot of latitude in there: intentionality is a moving target, as is the particularity of what may be “undesirable.”

I once heard Garrett Oliver, doyen of Brooklyn Brewery and then-speaker at a Craft Writing conference in Kentucky in 2014, relate a heartfelt anecdote about ‪famous beer writer Michael Jackson:

Jackson, as a child, had often been sent to the corner pub to fetch his father home for dinner. The pub was a place of wondrous adulthood, full of mysterious masculine ritual and a delightful smell that Jackson associated with that pub, his father, and his childhood, well into his adult life. As he became the worldly beer aficionado, he realized what that smell was: diacetyl. A flaw.

Nostalgia notwithstanding, very few brewers want diacetyl anywhere near their finished beers. It's part of the core of common off flavors, and is, along with oxidation qualities, probably the most common. As the number of breweries in America has exploded, so too has, anecdotally at least, the incidence of commercial beer exhibiting what one might characterize as serious flaws.

Paul Gatza, director of the Brewer’s Association, put a shot across the bow of the industry with a 2014 address at Craft Brewers Conference focusing on quality, and subsequently, Dick Cantwell, late of Elysian, was named the first Craft Beer Quality Ambassador at the BA.

Received wisdom about the previous craft beer bubble in the 1990s holds that many of the fledgling breweries were brought low by consistency and quality problems that grew to plague the wider public perception of the industry as one that was immature and unable to set its own standards of quality.

Matthew Rogers / Good Beer HuntingA technician at yeast producer White Labs works precisely.

The below is a synthesis of study, a few years of market experience, and many tasting panels. I’ve tasted a lot of beer in my time, and I’ve tasted a lot of bad beer.

Still, even when you are self-possessed in palate and knowledge, you might hesitate: should you send that buttery pint back?

I’d argue that, yes, you certainly should: an educated consumer who demands a high quality product is essential to the longevity and continued vitality of this industry. Pulling punches when it comes to bad beer is bad for everyone: the brewery, who presumably (hopefully) is unaware that they have a deficient product in the market; the retailer, who is serving something (again, hopefully) below her standards and which will discourage multiple pints, repeat visits, etc.; and ultimately the consumer, who is tacitly sending a message that subpar quality is a-okay.

So I’d argue that it’s incumbent upon you, the consumer, to speak up.

It’s not incumbent upon you to be a dick about it, however; as with all things, there are tactful ways to affect one’s displeasure. “This had a bad flavor; you may want to have your manager try it,” is certainly preferable to “This beer SUCKS and this brewery SUCKS” / *duly registers outrage on Untappd*.

Someone (hopefully!) worked hard to put that beer in your glass; giving them the benefit of the doubt is always worthwhile.

It’s not incumbent upon you to be a dick about it, however.”

Make an effort, too, to understand intent and assume good faith — if a beer has an oxidized, leathery note, for example, does that result from extended barrel or wood aging? Ask questions, be curious — what does the staff know about a particular beer? Is it supposed to taste as it does?

If the knowledge at a particular establishment seems lacking, a private note or direct message via a brewery’s social media channels is typically appreciated whenever there’s a perceived problem. In my experience, breweries (the good ones, at least) don’t have a thin skin about quality questions; if there’s a problem, they want to know about it.

By being a judicious and thoughtful consumer, you can help your favorite breweries step up their game. In the coming years of slowed growth and increased competition, everyone’s gonna have to.

Finally, a note on tasting: individuals are idiosyncratic and genetically predisposed in their ability to apprehend certain flavors. It’s something you usually don’t notice unless you’re in tasting panels fairly frequently, but everyone has a different level of sensitivity as regards certain chemical compounds. It’s always better to get another opinion. And take another sip.

Let’s take a look at the the three most common off flavors. Studying what they taste like, why they happen, and what can be done about them, will make us better beer drinkers.

Matthew Rogers / Good Beer HuntingTeacher always taught us: check your work. With lots of beakers in this case.


The presence of excess levels of diacetyl in a beer is marked by a buttery aroma — like movie theater popcorn butter or butterscotch — and a slick mouthfeel accompanied by a buttery sweetness. Sometimes referred to as “the brewer’s original sin,” diacetyl is, as noted above, probably the most common off flavor in craft beer next to oxidation.

As a byproduct of a healthy, normal fermentation, it is present in all beers to one extent or another. In the initial stages of fermentation, diacetyl compounds are produced by brewers’ yeast; later, yeast cells reuptake these compounds and metabolize them, greatly reducing the impression of butteriness higher levels of diacetyl would cause.

Some yeast strains produce more diacetyl than others; “high flocculation” strains, that is, strains in which yeast cells have a higher propensity to clump together and easily fall out of solution toward the end of fermentation, tend to produce diacetyl in greater concentrations.

Many English ale yeasts are highly flocculative, and diacetyl is (as the Jackson anecdote would suggest) fairly common in English beer; in fact, a certain amount of diacetyl is acceptable or even desirable in certain ale styles, English especially.

In other beer styles, however, it typically signals a problem, and certainly isn’t appetizing — anyone who’s ever been treated to a buttery West Coast style IPA knows how off-putting the combination of citrus and pine with butter can be.

Anyone who’s ever been treated to a buttery West Coast style IPA knows how off-putting the combination of citrus and pine with butter can be.”

Diacetyl is common in craft brewing in part because of the nature of these small businesses. All but the most fortunate and well-funded startup breweries face constraints of budget, time and equipment — sometimes beer needs to be ready to package now, in order to fulfill a purchase order, or brite tank #3 needs to be empty now, in order to accommodate the next batch of seasonal beer, etc.

Diacetyl can be largely eliminated or at least controlled through time and temperature; at most breweries, a “diacetyl rest” is often employed, a period during which temperature is raised above cold conditioning levels in order to encourage the yeast to reuptake diacetyl.

Sometimes an excess of diacetyl is caused simply by separating the young beer from the yeast prematurely, before the yeast has had a chance to metabolize the chemical compounds that result in a butter bomb. Unhealthy fermentations — caused by unhealthy yeast, incorrect fermentation temperature or poor temperature control, or the use of poor quality malts — can also result in increased levels of diacetyl.

Diacetyl can also present in beer through no fault of the brewery’s. Microbial contamination (or bacterial infection, if you want to get graphic about it) can also result in a buttery pint; both pediococcus and lactobacillus, types of lactic acid bacteria, can and will produce sharp, sour-tasting diacetyl under the right conditions.

The bacteria are anaerobic, meaning they require no oxygen to thrive, and alcohol-tolerant, meaning they can live in draught beer systems and impart a distinctive buttery quality to beer otherwise free of flaws in the keg from which it’s dispensed. I gather this used to be a much more common problem; with modern draft equipment and the legally mandated regular cleaning of draught lines and equipment in most states, contamination at the site of service is, in my experience, relatively uncommon – at least in decently run, clean establishments.

In my experience, there’s a divergence in attitudes regarding diacetyl between brewers and other beer industry folk and the average consumer. Brewers tend to take a whiff of diacetyl pretty seriously, in part because its presence suggests sloppiness or inattentiveness to process and quality control; most brewers worth their salt will conduct in-house lab tests for diacetyl and record the results to ensure the compound is below acceptable levels before packaging and bringing the beer to market.

Consumers, on the other hand, might not perceive it as a flaw at all, depending — I’ve certainly guided more than one tasting wherein I poured a sample for a guest or patron and he raised the glass, smiled, and remarked, “Mmm, smells like buttered popcorn!” Not that there’s anything wrong with that. If the past year has taught us anything, it’s that there are no facts, only interpretations.

If the past year has taught us anything, it’s that there are no facts, only interpretations.”

If you want to experience diacetyl without the inconvenience and expense of procuring a chemical flavor spike, chances are you can find it out in the wild pretty easily — sample local, small breweries’ IPAs, pale ales, blondes or kölsches on draught, and you’re bound to taste it eventually. Lighter styles conceal less (one part of the reason there are relatively few quality craft pilsners), and hoppy styles tend to create a harsh dissonance with the buttery quality.


Probably more common than the dreaded D when considering not just craft beer, but beer overall — especially international lagers — is oxidation.

“Oxidized” is a descriptor in beer that can mean several different things; generally, when someone says a beer is oxidized, they don’t mean it as a positive quality, and they are probably referring to the flavor and aroma of trans-2-nonenal.

That chemical compound can make beer taste papery, like wet cardboard, some say. I also experience this flavor as a deadness across the palate, an inert quality I think of less as “papery” and more as “stale.”

John Mallet, in his excellent Malt: A Practical Guide from Field to Brewhouse, relates a familiar story of beer exploration, hunting obscure beers from around the world in late-80s Boston: “Although we were unsure of the origin of many of the flavors, we knew what we liked… It was many years later during the formalized flavor training that was part of my Siebel education that I found out that the ‘German flavor’ we often noted was identified with a proper name: ‘severe oxidation.’”

So why do imported lagers so often and intensely exhibit the papery oxidation quality? It has a lot to do with the life cycle of beer, and package beer specifically: all commercial beer is typically packaged in cans or bottles that have been ‘purged’ with carbon dioxide to force out excess oxygen, and then have their head space (the empty bit between the liquid and the top of the package) purged with CO2 as well, in an attempt to leave as little oxygen as possible in the final product before it’s sealed.

Some oxygen is always left behind, however, and the amount of oxygen in the beer at packaging is referred to as its dissolved oxygen (or DO) level. The amount of DO in a beer at time of packaging can determine a lot about its eventual quality when it reaches you, the consumer: beers with higher DO levels oxidize more quickly.

The traveling beer will also almost certainly experience some degree of temperature fluctuation on its long journey from central Europe to that warm shelf in an American grocery store, and those swings in temperature force the oxygen in the beer and the headspace in and out of solution repeatedly, making it taste stale, papery, and tired.

Occasionally, especially in reference to an old ass IPA, you’ll hear someone refer to a beer as “cooked,” and this process contributes to a lot of what many people think of as “old IPA”character — the oxidation process takes a particular toll on hoppy beers, and oxidized crystal malts taste especially cloying and gross. This excellent piece from the Full Pint goes into some detail about this process.

Matthew Rogers / Good Beer HuntingKeeping the beers clean and uncooked at White Labs.

Reducing DO levels in beer is relatively difficult and costly, especially for a small brewery that may not own the requisite equipment to perform the quantitative analysis. Controlling oxygen levels at time of packaging is also critical, and many smaller breweries may use older and inefficient bottle or canning lines that make controlling DO levels difficult. Some rely on mobile canning operations to package their beer, often making for squishy cans that are not terribly shelf-stable or long-lived.

Remember, the longer a beer has been around, the more oxidized it’s likely to be; the savvy consumer can make a smart purchasing decision by gently squeezing cans (just like picking an avocado!), looking for sun- or light-faded packaging, and, of course, always checking date codes. With very few exceptions, I will not buy undated beer at retail — if a given brewery doesn’t yet date their packaged beer, I say revisit them when they do.

If you want to experience oxidation as an off flavor, you have many options: many German lagers, especially from unfamiliar brands, will exhibit the papery, cardboard deadness described above. Less popular brands get imported less often, spend more time in distributor warehouses and on store shelves, and have more time to oxidize.

To experience these flavors in American craft beer, one needn’t look any further than a dusty sixpack of IPA, and I guarantee you there’s plenty out there. Poke around a little and look for pale or hoppy styles well past their date codes.

In a pinch, you can also oxidize your own: an employee at Dogfish Head once told me that when training new reps, he’d leave a case of 60 or 90 Minute IPA in the trunk of his car over the course of a sweltering summer week in Delaware, and on Friday he’d have plenty of cooked beer to taste alongside fresh in order to demonstrate what time and temperature can do. No chemical spike needed.

Dimethyl Sulfide (DMS)

I hesitated to include DMS in this small rogue’s gallery, as I myself have a very high sensory threshold for it and thus rarely experience it, as such, in the wild. DMS is characterized as an aroma of cooked or creamed corn, cooked vegetables (such as broccoli, celery, cabbage, etc.), shellfish (at high concentrations) or, my personal polestar for this off flavor, cheap tomato sauce.

For a long while I considered myself basically “blind” to DMS, as I could never pick out its supposedly characteristic corny aroma in spiked samples during sensory trainings or off flavor tastings. It wasn’t until a sensory panel at Madtree Brewing in Cincinnati that I had a breakthrough, of sorts: the fellow leading our panel mentioned that he always got DMS as Spaghetti-Os, or cheap, canned tomato sauce, and from that point on, I’ve been able to pick it out with some success.

The fellow leading our panel mentioned that he always got DMS as Spaghetti-Os, or cheap, canned tomato sauce.”

As mentioned above, everyone’s physiology is different, and everyone experiences flavors and aromas differently and at different levels of intensity, to some extent; if one descriptor doesn’t make sense to you, read through all of them until you find one that does. “The Complete Beer Fault Guide” by Thomas Barnes is an absolutely invaluable resource for this.

DMS, like diacetyl, has a naturally occurring precursor in barley malt. This precursor comes out of the malt during mashing and then transforms into DMS during the wort boil; pale malts tend to have more DMS than darker malts, and thus it is often associated with pale styles, particularly pilsners, as pilsner malt tends to contain more of the precursor.

In fact, a certain amount of DMS character is appropriate to the pilsner styles and some other European and American light lagers. In darker beers, however, the presence of high levels of DMS is particularly off-putting; porters and stouts that smell like store-brand canned spaghetti, for example.

Consulting some colleagues in preparing this piece, I conducted an informal poll: which off flavor do you experience the most, after diacetyl and oxidation? “Vegetal” or “DMS” was the clear “winner,” and I found it curious because, as I mentioned, it’s not one I experience all that often myself.

I find the incidence of solvent-like esters or fusel alcohols much more common in craft beer, especially from smaller producers, as poor fermentation control or adverse conditions can create harsh, chemical-like aromas and flavors. But DMS is a common enough fault on its own, I suppose; most likely a result of poor malt quality or insufficient boiling times/evaporation rates, since DMS is volatile and evaporates out during a vigorous boil.

To experience this off flavor, you can -- and I swear I’ve heard from several people who successfully prepared for evaluative sensory panels like this -- go sniff a can of creamed corn. If you wish. Traditional Helles lagers are also said to have a light DMS characteristic endemic to the style. Maybe you'll have "better" luck picking it out than I've had in the past. 

ZX Ventures, a division within AB InBev, is an investor in October
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