by Scott Bickham (Brewing Techniques - Vol. 6, No.2)
No beer lives forever. Like chemical clockwork, staling compounds begin winding your beers down toward an unsavory end from the moment you call your work done. This installment studies their dour-tasting epitaphs: the catty, papery, musty flavors of oxidized beer.
Last issue’s Focus on Flavor column introduced readers to the four basic tastes of bitterness, sweetness, saltiness, and sourness and the compounds that produce these characteristics in beer. We can, for the most part, control the aspects of the brewing process that determine these flavors, but as we will see, brewers are often helpless in protecting the stability of their beer after it is bottled. As a beer ages, it develops stale or oxidized flavors ranging from catty to papery to musty, depending on the types of compounds that are oxidized. All are detrimental to the character of the beer and should be recognizable by a trained beer taster. This installment focuses on these off-flavors.
A Survey of Staling Chemistry
Definition: Staling compounds are primarily formed through oxidation–reduction (redox) reactions in packaged beer. The resulting flavors are characterized by unpleasant adjectives such as those of the Class 8 descriptors of the Beer Flavor Wheel listed in Table I. Each will be described in turn, but let’s begin with some background on redox reactions and their relevance to beer. Bear in mind that these reactions are much more complex than is possible to get into here. George Fix’s Principles of Brewing Science (1) does an excellent job with a more thorough explanation.
Oxidation. A compound can become oxidized in several ways, either by losing an electron, by losing a hydrogen atom, or by gaining an oxygen atom. All of these processes are defined as oxidation, even though oxygen is not always explicitly involved. For example, the oxidation of hop alpha-acids can occur through the addition of oxygen to the molecule, but it can also result from the loss of a hydrogen atom.
Reduction. The converse of these processes is called reduction. A sealed bottle of beer is a closed system, so the oxidation of one compound can only proceed if another is simultaneously reduced. It is beneficial to keep as many compounds as possible in the reduced state. For example, melanoidins in the reduced state are flavor protectors (known as reductones), but in their oxidized forms are often able to oxidize other compounds into staling compounds.
Although the beer may seem quiet from our perspective, these redox reactions are continually taking place at the molecular level. The conditions under which one compound becomes oxidized or reduced vary greatly in the course of the brewing process. A beer’s flavor when the bottle is opened will therefore be a function of the beer’s composition at bottling time, the amount of oxygen in the headspace, the time since bottling, and storage conditions. The accompanying box, “A Technical Interlude on Beer Chemistry,” provides an overview of some important compounds involved in these redox reactions.
Origins: Oxygen or oxidizing agents may be introduced into beer at several points in the brewing process. The most obvious is air trapped in the bottle’s headspace, but aeration of the beer while racking or bottling will have the same effect (2). Oxidation earlier in the brewing process can be just as detrimental, however, such as the effects of hot-side aeration (HSA), as described below.
Oxygen in the headspace. Most of the air we breathe consists of nitrogen, which is relatively inert, but molecular oxygen makes up an important 21%. This oxygen trapped in a bottle’s head-space readily dissolves into beer and may oxidize a variety of compounds.
The amount of air in the headspace of a bottle can vary considerably, ranging from the scant 0.1 mL achievable with the use of commercial fillers to the 2-mL level of some hand-operated devices (3). The latter level is excessively high, since only 1 mL of air is sufficient to oxidize all of the reductones in a typical beer (4). This reaction is not instantaneous, but proceeds more rapidly with higher levels of air in the headspace and increased storage temperatures.
Because headspace oxygen can never be completely eliminated, some brewers have resorted to using oxygen-barrier caps. The linings of these caps are impregnated with an antioxidant so that, in theory, some of the oxygen in the headspace will be eliminated.
It should be noted that of all the oxygen in the headspace, only a portion of it will lead to the stale flavors listed in Table I. An experiment in which a radioactive oxygen tracer was incorporated into headspace air found that only 30% of the tracer oxidized carbon-containing compounds into staling aldehydes and other volatiles, whereas 65% formed polyphenol complexes and the remaining 5% was incorporated into oxidized iso-humulones (5). These oxidized polyphenols and isohumulones generally have astringent flavors and will be described in a future issue when we discuss mouthfeel.
Hot-side aeration of melanoidins. Another important source of oxidizing agents is the hot-side aeration (HSA) of melanoidins. These compounds are amino acid–carbohydrate complexes formed by Maillard reactions during the malting, mashing, and boiling phases of beer production. Melanoidins can include compounds with oxygen (furans, for example), nitrogen (pyrroles, pyridines, and pyrazines), or sulfur (thiozoles).
In their reduced state, melanoidins are probably best known for lending positive bready, toasty, caramel, and roasted flavors to malt and beer, which are desirable in intensely malty beers such as Doppelbocks and imperial stouts. Melanoidins are primarily produced by the maltster during the malting and kilning processes, and because they are formed during Maillard reactions that occur at high temperatures, brewers can also increase their amounts by decoction mashing or by caramelization during the boil.
Melanoidins are also strong oxygen acceptors and act as antioxidants to improve a beer’s stability.* They can only postpone the inevitable for so long, however. In the presence of heat and light, even their effectiveness is reduced. Furthermore, they can become oxidized by air in the bottle headspace. Because they are present on the hot side of the brewing process, they are also very susceptible to oxidation through aeration of the mash or hot wort and, perhaps more importantly, may in turn oxidize other compounds, depending on wort conditions. This process converts the melanoidins back into reductones, but not until the damage has been done by acting as a middleman in other staling processes.
*Jim Busch gives an excellent description of their benefits in his article, “The Magic of Munich Malts” (BrewingTechniques 4 , pp. 28–32 [September/October 1996]).
The oxidation of melanoidins themselves will have different effects on different beers. They almost always have a negative flavor effect.† Although dark beers such as stouts contain more potentially oxidizing melanoidins than light beers, the antioxidant effects seem to dominate. When dark beers eventually become stale, they tend to have musty flavors; lighter beers, on the other hand, develop sweet, papery, and metallic notes. In both cases, these stale flavors develop at the expense of positive characteristics such as alcohol, maltiness, and body.
†Oxidized melanoidins can take on flavors that are characterized as almondine or sherrylike, accepted and even desirable in English old ales, barleywines, and strong Scotch ales. These effects, along with other positive flavor effects, are grouped in Classes 3 and 4 of the Beer Flavor Wheel and will be discussed in a future article.
Careful mashing to reduce oxygen pick-up will minimize the negative impact of melanoidins. Keeping the beer from direct light and storing it at cool temperatures will slow the rate at which they degrade inside the bottle.
Filtration and pasteurization. Yeast, with its need for oxygen for its growth stages, is a powerful reducer and can help to keep staling reactions in check. Yeast-free beers are therefore more at risk of oxidation than bottle-conditioned beers. Other antioxidants such as melanoidins can also be stripped out by ultrafiltration.
Class 8 Flavors up Close
Catty flavors: These unpleasant flavors have been correlated with the amount of oxygen in the headspace (3). In beer, this flavor is usually associated with the oxidation of some melanoidins; of particular importance are heterocyclic nitrogen compounds such as 2,5-dimethylpyrazine, which has a low flavor threshold, though it can often be present at twice that level (5). This particular flavor has been related to the taste of tomato plants and ribes (the leaves of shrubs in the blackcurrant and gooseberry families), which explains why these synonyms for “catty” are often used in sensory evaluation. A similar aroma is exhibited by isovaleric acid, an oxidized hop compound that is often described as cheesy or sweaty. This component is volatile in steam, however, and will contribute cheesy off-flavors only if the hops are added later in the boil or for dry-hopping.
Papery and leathery flavors: These flavors are primarily attributed to long-chain aldehydes with more than four carbon atoms. These aldehydes include pentanal, whose flavor has also been compared to grass and unripe bananas, and hexanal, which imparts unpleasantly bitter and vinous notes. The thresholds for these compounds are on the order of 1 part per million (ppm), but typical concentrations are rarely more than half of this level.
In comparison, the aldehyde trans-2-nonenal has a threshold of less than 1 part per billion (ppb), and typical concentrations range from 10 to 200% of this value (5). Because of its high relative concentration, this aldehyde is the most likely culprit of these off-flavors, though its flavor properties depend on its concentration. At the threshold concentration, it is characterized as papery, but at twice that value it has cucumbery and green-malty notes. At three times the threshold value, it is perceived as fatty and leathery.
Higher aldehydes (aldehydes with more molecular carbon) such as decanal also have low thresholds on the order of a few parts per billion. Decanal flavors are described as bitter, metallic, or like an orange peel. Aldehydes have sensory characteristics that fit the trend observed by Morten Meilgaard (one of the people responsible for developing the Beer Flavor Wheel) for carbonyl compounds in beer — the threshold decreases and the flavor strength increases with the number of carbon atoms until flavor reaches a maximum at 8 to 10 carbons (6).
The compounds described above are aliphatic aldehydes, which have an open-chain structure. Another important class of hydrocarbons in beer are cyclic, or ring-shaped, compounds. These include melanoidins, phenols, and some esters, aldehydes, and alcohols. The heterocyclic aldehyde furfural and some oxidized melanoidins also produce the papery and stale flavors, but they have higher flavor thresholds than trans-2-nonenal and are usually found at very low levels. Heterocyclic aldehydes therefore are not regarded as significant contributors to papery flavors, though they can contribute a background staleness to dark beers.
The origins of aldehydes in beer. Aldehydes find their way into beer in one of five primary ways (1,5).
· Auto-oxidation of unsaturated fatty acids by molecular oxygen. It is thought that trans-2-nonenol originates from the oxidation of linoleic and linolenic acids. These fatty acids are derived from malt and will always be present in beer, so the only way to limit their formation is to limit the beer’s exposure to air as much as possible.
· Oxidation of higher alcohols by melanoidins. Hot-side aeration can leave melanoidins in an oxidized state, and these compounds can later transform into their reduced analogs by in turn oxidizing fusel alcohols into long-chain aldehydes during fermentation.
· Oxidative degradation of isohumulones. The alpha-acids in hops may become oxidized before they’re added to the beer by exposure to oxygen and elevated temperatures during storage. They may also be oxidized in the finished beer by losing a hydrogen atom, often reducing previously oxidized melanoidins in the process. Melanoidins are beneficial to beer stability while in their reduced state (as previously mentioned), so the oxidation of humulones may actually protect the beer from further damage.
· Strecker degradation of amino acids. Strecker degradation refers to the oxidative transformation of amino acids into aldehydes. This is an important step in the formation of melanoidin compounds during malt and wort production, but it can also occur in the post-production phase. According to George Fix, this process’s contribution to beer staling is not clear (1).
· Aldol condensation. The short-chain aldehydes are not very stable and can combine to form volatile aldehydes such as 2-butenal and trans-2-nonenal. These compounds generally have lower thresholds and more unpleasant flavors than the short-chain aldehydes.
Of the five oxidative processes cited above, the first is the most prevalent source of aldehydes, but the second is an important contributor to the staling of dark lagers. All of these may be eliminated, or at least minimized, by preventing aeration of the mash, hot wort, and finished beer.
Moldy flavors: This group of flavors encompasses the second-tier descriptors “earthy” and “moldy.” Earthy flavors usually have a more pronounced aroma than flavor. One compound responsible for earthiness is acetoin, which has a high threshold of 17 ppm and typical concentrations of 1–10 ppm (5). Acetoin does not come from molds; it is an intermediate product of the reduction of diaceryl to 2,3–butanediol during fermentation, but oxidation can reverse this reaction in the finished beer and form elevated levels of acetoin. Moldy flavors were more common when wooden fermentors and casks were used. Molds can grow on damp malt, and these flavors are undoubtedly carried over into the finished beer, where they may also contribute to gushing and clarity problems.
Musty flavors usually originate from the oxidation of melanoidins in the pyrazine and pyradine groups. The specific compounds include methyl pyradine and 2,3-dimethylpyrazine, which have thresholds of 1.0 and 0.02 ppm, respectively (5). Typical concentrations for both are on the order of 0.01 ppm, so the latter is more likely the culprit.
Mustiness can also arise from the oxidation of the hop oil humulene. In this case, sulfury undertones will distinguish it from the musty flavors produced from melanoidins. Although normally undesirable, an oxidized form of humulene is the likely origin of the earthy, tobacco-like quality of some English hop varieties. Rule of thumb: The most common source of musty, moldy, or earthy flavors in modern brewing is oxidation or contamination of the raw ingredients. If this can be ruled out, then microbial spoilage of the beer is the next most likely possibility.
Oxidation and the Senses
You can train yourself to identify these off-flavors by finding a sample of tainted beer. Because commercial examples of contaminated or extremely stale beers are (mercifully) rare, you may have to resort to brewing a bad batch yourself or intentionally exposing a sample to the air. It is quite easy to prepare a spectrum of stale flavors without explicit doctoring. Below are some suggestions to help you to find or create oxidized samples for evaluation.
Catty samples: Catty flavors can sometimes be found in old or mishandled hoppy ales. If locating an affected beer in the stotes seems like a crap shoot, try brewing a catty beer of your own. Use cheesy-smelling or old hops in the recipe as late hops (or dry hop with them). Boiling releases the compounds that cause the off-flavor, so late additions make it very likely that cattiness will be carried over to the finished beer.
Papery/leathery samples: Papery and leathery flavors are more prevalent in light lagers, which is one reason why many domestic breweries encourage distributors to pull potentially stale products by providing bottling or expiration dates. Most imports are also good candidates for oxidation because they are not monitored as carefully and have already suffered through a long voyage by the time they hit the shelves. Scottish ales seem particularly susceptible to acquiring papery and winey notes, probably because of production methods and the resultant low levels of reductones present.
Low levels of oxidation can also be produced in selected dark and light beers by subjecting them to a 120 °F (49 °C) heat bath for a few hours or by storing them on top of the furnace or hot water heater for a few days to speed up the degradation processes. In a preliminary experiment of my own with an American light lager, both techniques resulted in sweet, winey, and papery flavors. Stronger flavors were obtained by opening and recapping a bottle of the same beer to allow additional air into the headspace (a few seconds is all it should take). The oxidative process not only occurred more rapidly, but the flavors were so evident they were described as offensive by some local judges. Further oxidation could be promoted by purging the headspace with oxygen before recapping, but this is probably unnecessary. If you want to try this experiment, I recommend including a hoppy pale ale in addition to light and dark lagers to provide a complete range of stale characteristics for a tasting group.
Moldy/musty samples: Although papery, catty, and leathery flavors are generally undesirable in any beer, low levels of mustiness are acceptable in the Bière de Garde style. These beers are usually corked and cellared, which sometimes produces some earthy characteristics. Musty flavors can also be produced by purposely aerating the hot wort when brewing a Doppelbock. Stale flavors should be evident after fermentation and bottling when compared with a normally produced version of the same beer. Very old malt extracts may also produce these flavors as a result of
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