We’ve all encountered stale-tasting beer at some point, unmistakably distinguished by a cardboard, wet newspaper, or even vinous, sherry or vinegar flavor. And while we might have assumed the beer had long passed its sell-by date or a poorly fitted seal had allowed air to creep into the package, it’s possible that the beer actually left the brewery containing too much oxygen.
Oxygen — while imperative to yeast growth at the beginning of fermentation — becomes a scourge to beer if introduced on the cold side. Though it’s pretty much impossible to keep out all excess oxygen during brewing, brewers work diligently to minimize it, lest it spoil the beer, reduce flavor stability, darken the color or make it undesirably cloudy.
The Brewers Publication’s text Water: A Comprehensive Guide for Brewers notes, “Current industry maximum acceptable levels of oxygen are always less than .05 ppm, usually less than .03 ppm, and the goal for many brewers is .01 ppm.”
A 1990 study conducted by Heineken and published in the Journal of the American Society of Brewing Chemists linked the principle cause of stale beer flavoring to the molecule (E)-2-nonenal that gets released when oxygen degrades unsaturated fatty acids shed by hops, particularly when beer ages at warm temperatures. American Chemical Society research from 2019 strongly suggests that other organic aldehyde compounds, generated by the natural removal of hydrogen during ethanol formation, also produce staling flavors.
A 2008 issue of the Journal of the Institute of Brewing posits that activated oxygen interacts with metal ions like iron and copper to create highly reactive peroxide free radicals that begin cumulative damage. For example, these radicals can oxidize hydrogen peroxide during this process, which can oxidize the polyphenols in the beer. In Charlie Bamforth’s Beer: A Quality Perspective, author Aldo Lentini explains that the hydrogen peroxide darkens the polyphenols, which gives the beer a darker look. When the polyphenols polymerize and adhere to proteins, haze forms and taste suffers.
Scott Janish, author of The New IPA and co-owner of Sapwood Cellars in Maryland, says, “Oxidized polyphenols can lead to a black-tea-like astringency which can (other than a color change) impact the beer negatively.”
Because chemical reactions continue during beer storage to intensify this activity, preventing oxygen contamination becomes especially important in beers that may not sell right away. And as if that’s not enough bad news, you can’t fix an oxidized beer.
However, the good news is you can install oxygen sensors throughout your system, and taking multiple readings throughout the brew cycle can help you catch elevated levels before they become unmanageable. Additionally, you can take many precautions to keep levels low throughout the cold side of the brewing process.
As mentioned, yeast utilizes oxygen (O2) at the beginning of fermentation. Taken together, at least four notable scholarly articles published between 1987 and 2007 explain that at this stage, yeast draw on oxygen to create a class of lipids called sterols that form and maintain new yeast membranes in their proper liquified state so they can divide as required.
According to Chris White, co-founder of White Labs and co-author of Brewers Publication’s Yeast: The Practical Guide to Beer Fermentation, an ideal reading in wort of dissolved oxygen (DO) — which simply refers to oxygen that’s dissolved in water — ranges between 8 and10 parts per million (ppm). Within approximately 30 minutes of pitching, the yeast consumes most or all of the oxygen and can begin what Louis Pasteur called “anaerobic fermentation,” meaning fermentation that occurs in an oxygen-free environment.
Additionally, beer naturally protects itself from unwanted oxygen that enters after this phase by forming a layer of carbon dioxide on its surface. Though White advocates aerating high-gravity (greater than 15%) beers after about 12-to-18 hours to assist in attenuation, other brewers agree it’s best not to upset the CO2 layer or otherwise introduce oxygen by aerating or agitating your beer, even to jolt a stuck fermentation back to life.
White says, “Brewers put their beer in a fermenter and generally don’t touch it. This is unusual in science. For instance, in the pharmaceutical industry they add nutrients and carbohydrates and make all sorts of pH adjustments. Brewers are just so scared of the oxygen, and rightly so.”
Yet even with careful handling, O2 can seep into a fermenter through attachments that aren’t air-tight or a pressure valve that pops open. It can also get carried in along with any post-fermentation ingredients — whether it comes in with late-addition hops or a dilution that uses water that hasn’t been deaerated.
Among the most significant challenges in dry hopping is avoiding the introduction of DO. One way to circumvent this common problem is by dry-hopping during — instead of after — fermentation to give the yeast time to metabolize the O2.
If you prefer the results of dry hopping after fermentation, invest in a fermenter with a dedicated dry hopping port so you don’t need to open a portal that exposes the beer to oxygen. To be even more cautious, invest in a part called a hop doser, hop cannon or similar, which securely attaches to the hop port.
Janish suggests shooting 15-20 PSI of carbon dioxide (CO2) into the headspace of the vessel through the spray ball while adding in the hops to fortify the protective CO2 blanket on top of the beer.
“This method,” Janish writes, “has been tested and shown to reduce variability in the aroma intensity of dry hopped beers caused by oxygen.”
Any good homebrewer knows the most fraught time for introducing oxygen into beer is while transferring it from one vessel to another; despite dedicated equipment for such purposes, commercial brewing isn’t immune. Before each transfer, it’s wise to check every connection to make sure it’s as tight as possible.
Fill your lines with sterile, de-aerated water and flush them with CO2 before you transfer, and ideally, you’ll want to push the beer from one tank to the next with CO2 instead of relying on pumps or gravity. And don’t push the beer through too fast — high velocity may save you a few minutes but cost you headaches if turbulence generates any O2.
Beware the carbonation process! Oxygen loves to creep into your beer during carbonation, and not just through any loose-fitting hoses — even if you’ve purged the tank beforehand. Anything less than the purest CO2 tank contains trace amounts of oxygen that can add up.
Writing in Brewer World, Bijay Bahadur, author of Brewing – A Practical Approach, says, “Carbonation of beer by one volume of CO2 gas containing 0.05% air can theoretically contribute 0.2 ppm of oxygen (if all oxygen becomes dissolved in beer).”
A discussion about oxygen and filtering can prove a confusing one, as some filtration systems actually oxidate the liquid to remove unwanted elements to make them easier to filter out. Further, filtration systems vary greatly one from another.
That said, it’s still important to use deaerated water to flush your system before running beer through it. Better yet, invest in a centrifuge if you can afford it or rely on a closed filtration system like a lenticular filter if you can’t.
As beer stabilizes, its molecules change in a way that makes it more susceptible to the effects of oxygen. On top of that, commercial stabilizers themselves can introduce oxygen when they (along with other additives) are prepared in water that hasn’t been de-aerated.
“If you’re adding carbon dioxide into a tank to remove the oxygen, you’re already thinking about removing the air intake but look at your water intake, too, and be aware of what it touches and where it gets left behind,” says White.
If your beer stays below 50 ppb in the brite tank, you’re in good shape … until you start packaging!