Controlling Beer Oxidation

By George J. Fix (Brewing Techniques)

Control Beer Oxidation from Kettle to Bottle

The effects of the introduction of oxygen to wort or beer on flavor stability have been widely debated over the years, and the subject has been the target of much recent research. The compounds that contribute to staling flavors cannot easily be determined because of the complex nature of beer chemistry, but one fact is clear: Brewers can exercise a great deal of control over flavor stability by observing proper brewing techniques.

Perception of Oxidized Flavors

Following C.D. Dalgliesch’s well-known article on flavor stability, three stages in beer flavor evolution can be defined as shown in Figure 1. Stage A is the period of stable, “brewery-fresh” flavor. Stage B is a transition period in which a multitude of new flavor sensations can be detected. Dalgliesch cites a decline in hop aroma, a decline in hop bitterness, an increase in “ribes aroma” (or sometimes “catty” flavor) and an increase in sweet/toffee-like/caramel tones. The terms “ribes” (or “currant”) and “catty” are widely used in the United Kingdom and Scandinavia. The terms are intended to recall overripe or spoiled fruit or vegetables. Some tasters cite a “black currant” tone. In truth, these terms describe a wide spectrum of negative flavors when beer is in Stage B. Toffee or caramel flavors can come from many sources, but those associated with staling will invariably have unattractive cloying notes. These effects are enhanced by residual diacetyl, and also by excess heat treatment of wort. Boiling in pressure cookers will develop flavor tones like these along with other negative ones.

The Stage C products are the classic flavor tones involved in beer staling. These flavors are described in subsequent sections of this article in conjunction with the part of the brewing process in which they are created.

Although it is not treated in the references cited above, a Stage D, also exists. In this phase, Stage C flavors evolve into a kaleidoscope of flavors, which in very special formulations (Rodenbach’s Grand Cru comes to mind) recall the subtlety and complexity of great wines. It is to be emphasized that this process takes years, not months, of maturation. This effect deserves a paper all its own, and hence falls outside the scope of this article.

Do we prefer stale beer? Barry Axcell and Phill Torline, in an interesting if provocative article, argue that most beers are consumed during Stage B. During this period beer flavors are undergoing discernable change, and the authors suggest that the changes are at the root of consumer dissatisfaction. Among other things, they cite the so-called import paradox as partial evidence of this theory, the paradox being that a definite proportion of the beer-consuming population actually prefers beers in Stage C. (Here, “import” means any beer consumed at a significant distance from where it is brewed.) These authors noted the stability of flavor in Stage C and “learned prejudices” (such as prestige of the beer and packaging) as the keys to this paradox. Statements like “If Pilsner Urquell has HSA, then I want my beer to have HSA,” often seen in the internet discussion groups devoted to beer, illustrate the apparent validity of this point. Stage B flavors, on the other hand, appear to have very few advocates.

M.C. Meilgaard, for one, is sharply critical of Stage C flavors because they are one-dimensional. He states, “I think it ranks as an all-American scandal that fully half of all the interesting and unusual packaged beers that are on the market get to us so oxidized that stale-ness and cardboard are the main flavor tones.” I feel this is a very important point, and this article is based on the premise that the best strategy for brewers (amateur or commercial), is to produce beers in which Stage A flavors are as stable for as long as possible. If they, or their consumers, prefer Stage B or C (or D for that matter), then putting the beer aside for the required time is all they need to do. Staling, like entropy, can go in only one direction!

Cold-Side Aeration

Oxidation on the cold side is the easiest to understand, and until the last decade has occupied all of the attention of those concerned about this issue. The major mechanisms are given in Table I. Flavor thresholds in parts per million (ppm) were taken from Meilgaard.

Oxidation mechanisms: Ethanol. The first oxidation-reduction (or redox) reaction in Table I is the start of the classic “vinegar process.” The oxidation of ethanol to produce acetaldehyde marks the second appearance of acetaldehyde in the brewing process. Acetaldehyde is first encountered in the main fermentation pathway, in which it emerges as a reduction product of pyruvic acid and a precursor of ethanol. At that stage it has a flavor that recalls freshly cut apples. As an oxidation product of ethanol its flavor changes to an “old apple” tone. (The stronger, more negative flavor results from higher concentrations and concomitant compounds.) It may also recall “rotten apples,” although this effect is not nearly as strong as the flavors associated with Zymomonas infection.

Acetaldehyde. The transformation to vinegar, or acetic acid, continues with the oxidation of the acetaldehyde in reaction 2. But because of the high flavor threshold of acetic acid, that reaction will rarely give a full vinegar taste. Generally, sherrylike tones emerge instead. Sizable amounts of oxygen and/or acetic acid bacteria (Acetobacter) infections can, however, lead to stronger vinegar flavors.

Hop constituents. As noted above, the loss of hop flavors (taste and smell) associated with Stage B in Figure 1 is the easiest way to detect the start of staling. By the time stage C is reached, the oxidation products of alpha-acids (the valeric and butyric acids noted in reaction 3 of Table I) will start to appear. “Cheesy” is a widely used descriptor for these flavors. The use of old hops at elevated levels will intensify this effect. The flavors from hop oils, on the other hand, will generally just disappear in Stage B, although sometimes residual tones recalling hay, sagebrush, or grass can be detected. Taste panels differ considerably in their response to grassy flavors. Many tasters do not see these flavors as a defect, and highly valued hops like Columbus or Kent Goldings can display these tones in attractive ways.

Fatty acids. The final CSA mechanism is often underestimated because unsaturated fatty acids from trub in wort play a highly beneficial role in the fermentation. Their value on the cold side, however, is the exact opposite. They are highly reactive with oxygen, and at this stage oxidize to a broad range of products, none of which are attractive. The soapy or goaty tones from oleic acids are examples. These effects are usually not seen in beers produced from reasonably clarified worts. The relatively low flavor thresholds of these oxidation products are a special concern.

Preventing CSA: Clearly the most straightforward way to prevent CSA is to prevent oxygen ingress during the postfermentation processing of beer. As far as beer transfer is concerned, the first important step is to remove as much air as possible from the receiving tank. Purging the tank with carbon dioxide is a start. If, in addition, the beer is transferred by applying CO₂ pressure to the sending tank and allowing a slight gas bleed in the receiving tank, one can eliminate oxygen pickup. In fact, I have found that the use of CO₂ pressure to transfer beer throughout the postfermentation process can result in null dissolved oxygen readings, at least according to standard meters. The next best option is to transfer by means of gravity flow. Even in this case, it is crucial to keep a carbon dioxide cover over the beer to protect against oxygen ingress. Maintaining a smooth laminar flow to avoid mixing air and beer is also important.

In commercial brewing, the above two procedures are not always an option. Carbon dioxide is expensive, and a horizontal brewery layout precludes gravity flows. Thus, the use of pumps is a necessity. To say that there are good pumps and some bad pumps is a mild understatement. Operator error may also play a role. For example, the operator must adjust the flow to avoid cavitation. Pumping beer through filters requires some special attention as well. For example, it is important that the bell of a diatomaceous earth (DE) filter be purged of air before the flow starts. Brewers also should take care to prevent air from entering the bell during filtration. The ideal liquid for dissolving DE before pre-coating is beer, but its use can lead to foaming. Thus, many brewers instead use water — but it should first be deaerated. Reference 5 describes this process in detail.

In spite of these other, earlier potential sources of oxygen pickup, the greatest concern about CSA has been correctly pointed at the packaging of beer. The deleterious effect of headspace air has been evident ever since the human race started putting beer into small bottles. Fortunately, modern technology has made it possible to dramatically lower oxygen pickup during filling. For example, double pre-evacuation bottle fillers routinely keep bottle air levels below 0.2 mL air per 1/3 L beer. These levels are strikingly low compared to the situation that existed a few decades ago, at which time air levels as high as 1.0 mL per 1/3 L were considered acceptable. In fact, even most of the good homebrew hand fillers on the market today, if properly operated, can get below 0.5 mL per 1/3 L. (Having beer foam displace the bottle’s headspace just before capping is very important.) On the other hand, cheap homebrew fillers, and many reconditioned commercial fillers, invariably give air levels above 1 mL per 1/3 L, and in adverse cases 2 mL or more.

As important as headspace air is, its indicted co-conspirator in the staling process is thermal abuse. Storage at high temperatures can dramatically shorten the beer’s shelf life, even at very low headspace air levels. The data in Table II illustrate the synergy of the two factors as beer progresses through Stage B. (These data were obtained from test brews that were carefully controlled with respect to HSA and other forms of CSA.) It is remarkable that beers with very low headspace air levels that have been subject to thermal abuse are no more stable than beers that have been afforded proper storage but have the high air levels one expects from cheap homebrew hand fillers.

Hot-Side Aeration

New thinking: The first hint that oxygen uptake during wort production could be a problem was found in the fundamental work of N. Hashimoto, one of the leading experts on beer staling, in the early 1970s (see reference 14, for example). His prime concern was volatile aldehydes such as 2-nonenal. This compound has a characteristic papery or cardboard flavor that sometimes is accompanied by leathery or woody tones during Stage C of the staling timeline in Figure 1. These flavor notes are of great concern because of their very low flavor thresholds, which for most people fall in the range of 0.1–0.4 parts per billion (ppb). The defining step of Hashimoto’s work came from the observation in his classic 1975 paper that “molecular oxygen does not take a part [in forming nonenal] in bottled beer”. In later work he identified oxidative processes during wort production as a source of nonenal.

The idea that wort production processes could contribute significantly to oxidation was a new finding at the time. The only previous published study involving oxygen uptake during wort production was conducted by Jean DeClerck in 1957. He found that the total elimination of oxygen during wort production increased the finished beer’s tendency to form chill haze. A follow-up study demonstrated that massive oxygen uptake prior to sparging greatly enhanced colloidal stability. This study did not, however, take into account finished beer flavor, and in fact illustrates the point that the promotion of physical stability is not always in harmony with the improvement of flavor stability.

After Hashimoto’s work appeared, manufacturers of brewing equipment (notably in Germany) funded additional research into this topic. They were concerned about the relevance of HSA to German beers, which are quite different from the light Japanese lagers used in Hashimoto’s experiments. Dr. Ludwig Narziss and his colleagues and students at the Technical University of Munich–Weihenstephan were leading figures in this research. They found that, if anything, HSA is even more important for the bigger beer styles, a fact that is not too surprising in retrospect since HSA’s off-flavors derive from the breakdown of malt-based constituents. In any case, this group quickly became the strongest and most influential advocates for the “low-oxygen brewhouse.” Reference 19 contains a good survey of their findings, and additional references can be found in references 5 and 20. See also the recent reference 21, in which the authors showed that nonoxidized materials from specialty malts can play a highly beneficial role in flavor stability. I am not aware of any articles in professionally peer-reviewed brewing journals that are contrary to either Hashimoto’s findings or those of the Weihenstephan group with respect to HSA.

This research had a profound impact on the design of brewing equipment in Europe. For example, the evolution of Huppmann, one of Germany’s most prestigious brewing equipment manufacturers, with respect to HSA is documented in reference 22. This company has developed notably gentle, low-oxygen systems for transferring beer and wort between brewery vessels.

Oxidation mechanisms: The first mechanisms to be studied involved malt-derived compounds like polyphenols and melanoidins. Oxygen uptake on the hot side immediately sets in place a number of redox reactions. Temperature is crucial, because the speed of these reactions increases exponentially with temperature. For example, introducing, say, 1 mL of oxygen per liter of wort at 158 °F (70 °C) will start reactions that take place in seconds. Doing the same for wort at 68 °F (20 °C) will have virtually no effect. At this temperature, the oxygen fraction will typically remain an inert gas until consumed by yeast. The studies have thus shown that the oxidation of melanoidins and polyphenols is an important consequence of HSA. These oxidized compounds are normally held in check during fermentation by the strong reducing power of yeast metabolism, but during maturation and Stage A (see Figure 1), these compounds undergo a complex series of electron exchanges. The net effect is the oxidation of beer alcohols and the creation of volatile aldehydes. This process is sometimes called “oxidation without molecular oxygen” because these reactions can take place without oxygen being present. The aldehydes produced do not display their presence until a lag period is over (typically 3–4 weeks). This seems to be because of bonds between the aldehydes and natural sulfur compounds from yeast metabolism. These bonds, alas, are temporary. When they are broken, a wide range of flavors appears. All have a grainy astringency associated with them, and metallic undertones are often present. Sherrylike notes have sometimes been identified as well, and this is one flavor note that is commonly attributed to both CSA and HSA.

Unfortunately, these mechanisms do not explain how the all-important 2-nonenal is created, because it has been demonstrated that there is no alcohol relevant to beer that is a precursor to 2-nonenal. Current research points to fatty acids in wort, along with malt-based enzymes such as lipase and lipoxygenase, as being potential precursors. Heat and oxygen stimulate the enzymatically induced creation of the hydroperoxides that are the precursors of 2-nonenal. In any case, oxygen uptake in the brewhouse remains an obvious culprit in the production of staling aldehydes, along with a still-to-be-determined effect from malting.

Preventing HSA: HSA is easy to avoid in home brewing because it will arise only from very sloppy brewing practice. It has been my experience over the years that advocates of such procedures rarely stay with the hobby. Thus, on the homebrew level, HSA is a problem that seems to take care of itself.

Some sources of HSA are easy to avoid in commercial brewing. An example is the minimization of shearing forces from excessive raking or stirring, a remarkably important effect.

A more perplexing problem is the HSA created by splashing during hot wort transfer. Following a presentation I gave at the April 1998 National Craft-Brewers Conference in Atlanta, I was approached by someone who has been very skeptical about the importance of HSA. He correctly noted that there are a lot of commercial systems in operation that have very high oxygen uptake owing to splashing. He then asked if HSA is sufficiently important to require the purchase of an entirely new brewhouse. I feel, at least for commercial brewers, that the answer is definitely yes, and the issue is one that also should be taken up by the manufacturers of the problematic equipment. The comparative advantage of craft brewers in the market place is the perception of a dedication to excellence in all aspects of brewing. It is very hard for even ardent supporters of craft beers to maintain this ardor in support of beers whose “primary flavor tone is cardboard” (to quote Meilgaard).

Moreover, corrections are not always expensive. A particularly elegant “fix-up” can be seen in the pilot brewery at Coors in Golden, Colorado. The once 100% copper brew kettle now has a stainless steel tube leading from the original kettle inlet at the top of this vessel to its bottom. What was once a splashing flow was replaced with a gentle laminar fill. I do not know how much the modification cost, but it clearly is a minute fraction of the price of the kettle.

Detecting Oxidation

The amount of headspace air in bottled beer is probably the single most important data point to determine in trying to control oxidation, particularly for home brewers, and also happens to be the easiest. For commercial brewers producing bottled beer, such monitoring is essential.

Measuring headspace air: The most cost-effective, and the most widely used, instrument for measuring headspace air levels in bottled beer is the Zahm & Nagel (Buffalo, New York) air tester. Their 7000 series model provides carbon dioxide levels at the same time air levels are measured, and the instrument itself is trivial to operate. The device pierces the cap, and CO₂ in the beer pushes head-space gases into a caustic solution. The solution entrains CO₂, but allows air (oxygen and nitrogen) to escape into a burette, where its volume is measured. (As a rough rule of thumb, the oxygen fraction of air is around 21%.) This unit costs $ 900, which may be steep for the average home brewer, but not necessarily for the larger homebrew clubs. As noted above, an instrument like this is essential for commercial breweries.

It is important that headspace air be measured as soon after filling as possible, because it will gradually be absorbed into the beer and react in the ways cited in Table I. In fact, measuring attenuation of air levels as the beer ages gives a nice way of tracking its evolution by way of staling. One important exception is a situation in which headspace air levels increase owing to air ingress through imperfectly sealed bottle crowns (or PET bottles). This can dominate any effect due to air levels at filling. Table II dramatically shows the combined effects of high headspace air levels and thermal abuse on beer.

When planning a headspace sampling program, keep in mind that neither commercial nor home bottle fillers fill bottles uniformly. Samples should be taken from the middle of the bottling run, from bottles that foamed over properly when filling.

Measuring dissolved oxygen: It is also important to determine dissolved oxygen levels during beer processing on the cold side. Respectable dissolved oxygen (DO) meters cost $400–$ 500. The recently published Laboratory Methods for Craft Brewers gives an excellent set of protocols for taking these measurements. As noted above, oxygen pickup during filtration is a special concern. It has been my experience that it is possible and desirable to keep dissolved oxygen in beer below 0.05 mL/L during processing.

DO meters are not suitable for measuring HSA, since on the hot side dissolved oxygen will react with wort constituents before it can be measured. The classical Indicator Time Test (ITT) is a good way to measure oxidation caused by HSA. The test measures the time, in seconds, for a solution of dichloro indophenol (DCI) to become discolored when beer is added. DCI exists in a highly oxidized state and will react with a color change when combined with a substance in a lower oxidation state. The speed of the color change is a measure of the substance’s degree of oxidation and varies from seconds for highly reduced solutions to hours for those having a significant amount of oxidized components. Special note should be made of the improved media recently recommended for this procedure.

Stress tests: In my own brewing, I have found that stress tests for beer have been the most effective means of detecting CSA and HSA. My procedure requires the knowledge of beer air levels at fill. Samples are held at 86 °F (30 °C), and are tasted twice a month. Using the information in Tables II, III, and IV, it is possible to approximate the source of the oxidation.

Find out What Works for You

Discussion of oxidation has been bedeviled by sweeping generalizations ranging from “The flavor stability of beer is determined in the brewhouse,” to “No other oxidative effect is as important as headspace air.” My own work has been sharply criticized, perhaps with some justification, for overemphasizing HSA. My posture came about primarily because of the dramatic effect that I observed when I eliminated HSA. In particular, the consistency of the performance of my beers in competitions dramatically improved when I eliminated HSA from my brewing process. Others may have a different experience. The best way to deal with real and relevant problems associated with oxidation is to get the necessary data from your own beer so you can determine for yourself what you need to do in your own system.

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