Flavor Fever: Cracking the Freshness Code

The processes that change beer’s flavor over time are complex, but by understanding them, we can better brew beers built to last—and we can better appreciate them when fresh.

Randy Mosher Oct 10, 2023 - 11 min read

Flavor Fever: Cracking the Freshness Code Primary Image

Photo: Matt Graves/

As brewers and beer enthusiasts, we fuss over every little thing, whether it’s the best ingredients or processes, classic or inventive recipes, or ratings, reviews, and the latest hop-du-jour. But how much attention do we pay to how fresh our beer is? Do we recognize the complex, telltale signs when a beer is past its prime?

Beer is a fragile product that’s in absolutely pristine condition for only a brief window of time. Maybe you’ve heard a friend come back from Europe and say (or maybe you’ve come back from Europe and said), “It’s so different over there; it’s not even the same beer.” It’s true. Any beer, no matter how well brewed, that’s made a weeks-long trip by ocean freighter, truck, and other means, is absolutely different from the one that left the brewery, both in terms of its flavor and its chemistry. Time, temperature, and vibration all take their toll. This is an inviolable law.

Stale beer is a big issue for the industry and for discerning drinkers. Over the decades, more research money has been poured into staling than probably any other problem in brewing. It remains the most intractable one. The issue is that beer is not, as chemists say, “in equilibrium.” If it were, its chemicals would be at their lowest energy state, with no potential for further reactions—a perfectly stable product.

In reality, however, beer abounds in highly active molecules that just love to interact with one another. Beer’s relatively low pH further facilitates reactions, while its ethanol flings itself into the chemical melee. Even after fermentation and filtration, yeast’s enzymes continue to transform the beer. Chemical changes continue through conditioning, packaging, and even pasteurization. It’s relentless.


Freshness Is Ethereal, Staling Is Not

Broadly, two things are happening. First, the beer is losing its bright, fresh character. This is a little difficult to put your finger on; you almost have to compare a brewery-fresh beer to the same one a few weeks old, in which hoppy and fruity notes will be just a less little bright and fresh. Where does the freshness go? Pleasant fruity fermentation characters such as the banana-candy note of isoamyl acetate are vulnerable to being broken apart by acid hydrolysis. Because these esters also mask stale notes, their loss is a double whammy. Fresh hop aromas are terpene hydrocarbons or their alcohols; both groups are highly volatile and reactive. Some simply evaporate, while others may chemically change.

Meanwhile as the positive aromas fade, a rogues’ gallery of new flavors is forming. The most notorious is an aldehyde called (E)-2-nonenal (brewers usually call it trans-2-nonenal, or simply abbreviate as T2N). Created from the fatty acids in malt, its highly potent “cardboard” or “paper” odor is a classic sign of staling in pale lagers, with an aroma threshold in beer of just 0.035 parts per billion (ppb). It is sometimes more evident on the palate than in the nose. While T2N is taught at brewing schools as the stale chemical, one study found it unimportant in top-fermented ales, especially darker ones. T2N also does an excellent job of masking fresh, fruity esters, giving it an extra level of awfulness.

T2N is just one among many staling aldehydes. The same Maillard process responsible for kilned flavors in malt can also occur in the cool, wet environment of a beer, although at a much slower rate. The first compounds formed during kilning are Strecker aldehydes, and they’re players here. In stale beer, chemicals such as 2- and 3-methylbutanal can give a sweet-malt “worty” odor, while methional smells of mashed potatoes. Neither is particularly pleasant. Phenylacetaldehyde brings floral and honey notes. The shape-shifting aroma of acetaldehyde (wet grass, ripe apple, flower shop, raw pumpkin) adds to the dullness.

The Workings of Oxidation

Much beer staling is driven by oxidation—an extraordinarily complex process. Oxygen-containing free radicals do the most damage, but some nitrogen compounds can have similar effects. From the mash through every step all the way to packaging, there are endless opportunities for oxygen to enter the brewing process. Combining with malt compounds and others, it creates what are termed “reactive oxygen species” that do the damage. Brewers need to be vigilant at every step, minimizing splashing, agitation, and heat input. It’s always a goal in packaging to keep oxygen levels as low as physically possible. Oxidation also can be potently catalyzed by metals, especially iron, so keeping this out of the beer is a high priority.


Mid-colored caramel and crystal malts (40–60°L) seem prone to creating leathery/oxidized notes. Similarly, related sherry- or port-like staling odors are common in stronger beers such as barleywines, adding vinous notes in this context, although it’s not clear which chemicals are responsible for these. The current best guess is a group of chemicals interacting to create unique percepts not attributable to any single molecule.

The oxidation of bitter hop iso-alpha acids is not only responsible for diminished bitterness, but also for qualitative changes. Rather than the balancing crispness of fresh bitterness, in stale beer it becomes clinging and astringent. Beer has fewer polyphenols than red wine, but what’s there are contributed by hops and barley. Oxidation makes these harsher and more astringent over time.

More Processes that Impact Flavor over Time

Staling is not just about oxidation. Other chemical changes also contribute. One that’s particularly evident in hoppy beer is beta-damascenone, which is released from its bound glycoside form—where it’s stuck onto a heavy sugar molecule—by beer’s acidity. Usually described as cooked apple and dried fruit, maybe with some black currant, it’s a specific stale note in IPAs, although 50 percent of the population is more or less blind to it.

Sulfur can rear its stinky head, too, released from sulfur-containing amino acids in yeast. Dimethyl trisulfide brings cooked onion and savory/meaty notes. Ethyl mercaptan is typically a sign of yeast breakdown (autolysis) during aging. Its garbage-truck taint is common in lagers, although it’s difficult to pick up without some experience. But remember: Many of these staling chemicals have a meaningful effect on the beer even when we can’t identify them individually.


Ester formation doesn’t stop when fermentation is over. Ethyl esters form when a fatty acid combines with ethanol. Based on acids from hops and yeast, a range of esters gradually arise, including ethyl phenylacetate (honey, balsamic, floral), ethyl cinnamate (sweet, spicy, fruity/plum), 2-methylbutyrate (sweet, waxy, apple), and others. Note the general trend to sweet, honey/floral aromas—another hallmark of stale beer.

Beers can even get so old that they look different, but by this point their aroma and taste are pretty much shot. One common symptom is the “snow globe” effect you see in a dusty bottle of European lager, literally the beer’s protein structure falling apart before your eyes. Because these proteins form the basis for beer’s head and body, those are lost, but they started diminishing long before the snowflakes appeared. Although it’s subtle, beer will darken slightly over time because of Maillard reactions.

When Does Aging Start?

Lauren Woods Limbach of New Belgium once told me she had her panels trained to recognize the difference between Fat Tire packaged that day and week-old Fat Tire. That was obviously after a lot of intensive training, but it shows just how quickly changes can occur, even in a sizable brewery with state-of-the-art production. For most of us, such changes aren’t noticed until much later. According to studies, many drinkers are oblivious.

It does matter what sort of beer it is. Hop-forward beers age fastest. As mentioned, old-school APAs and IPAs with lots of caramel malt are especially vulnerable. Darker beers tend to age more gracefully than pale ones. Lagers, because of their delicate nature, typically show their age more rapidly than ales. Stronger beers hold up better than everyday ones. Of course, how the beer is brewed makes a difference. The advantage generally goes to larger and more sophisticated operations for obvious reasons. Pasteurization trades one negative consequence for a worse one: To kill any microbes present, the beer is briefly heated, accelerating its aging just a little, and slightly reducing its bright, fresh flavors.

So, What’s a Consumer to Do?

First of all, try not to buy stale beer. With packaged beer, this puts all but the fastest-moving Euro beers off the table. Most bigger brands have cryptic date codes, so they’re not much help. Seasonals such as Oktoberfest and maibock are good bets if you buy them in season. Most U.S. craft brands have readable date codes, so take advantage of them. Buying packaged beer to-go at a brewery is usually a solid bet. Since draft beer in the United States is usually kept chilled, it’s a better choice in bars.

It’s become fashionable to insist on extremely recent code dates on hazy IPAs. While generally a good thing, this style is actually quite tasty well beyond a week or two. At Forbidden Root in Chicago, we actually prefer them when the “hop burn” fades away after a couple of weeks. For most beers, six months is too old while six weeks is plenty fresh—but that varies widely, so learn to trust your palate, brewery, style, and retailer.

Once you’ve bought the beer, keep it cold. Beer stored at 32–39°F (0–4°C) keeps four times longer than at room temperature. Besides, there’s no reason not to keep them ready to pour in an emergency. Once you crack the code on freshness, it may quickly become one of your favorite qualities. It sure is one of mine.