Science of Oxidation
Throughout history it has been the goal of commercial brewers to extend the shelf life of their product. At one point in time, protecting beer from the effects of bacterial contamination was the key. The next limiting factor proved to be haze stability, which specifically means the hazes caused by proteins and tannins in the beer. These days we are left with the most difficult problem of all, that of flavor stability. We can make bacteria-free beer that stays clear for months, but we cannot ensure that the flavor remains consistent for much over 120 days. The main culprit for this flavor instability is oxidation of the beer’s flavor compounds. Ironically the majority of the compounds that are most susceptible to oxidation are derived from malt and hops, so perhaps the mainstream brewers are on to something brewing with all that rice and corn.
Mechanisms
Recently — in different articles in two different brewing technical journals — two different opinions on oxidation were offered. One group claimed that the stale flavor was mainly caused by oxygen uptake by the beer on its way from the storage tank to the bottle. The other group maintained that flavor stability is not related to beer packaging, but rather to wort preparation. This shows researchers working in brewing labs in Germany and Belgium reaching different conclusions as to where the oxidative damage is done to the beer. Both agreed the oxidizing agent was oxygen, but both contradicted each other as to the source of the oxidized compounds.
Thus the lines are drawn in the professional ranks, so it’s no wonder there’s confusion in the home brewing literature. It can be agreed that contact with oxygen throughout the process of beer production is largely detrimental to the beer’s final flavor, the degree to which that is affected by contact with oxygen early in the process rather than the finished beer is still a subject for debate.
While the greatest damage can be done to your beer by contact with oxygen, it should also be remembered that oxygen does not directly have to be involved for a compound to become oxidized. Beer is “born” with a certain “reducing power.” Depending on the ingredients used to brew it, it has a certain finite ability to resist the effects of oxidation. Improper handling of the beer, the raw materials and even the mash or wort can use up this capacity for self-protection, leaving the beer more vulnerable to the effects of oxygen later in the process. In Japan research is focusing on measuring the beer’s total reducing power (ability to resist oxidation) when it is packaged in order to predict its shelf life. Historically brewers have used ‘forcing” tests, in which the package is exposed to extreme temperatures for a week or so to mimic what the beer may encounter in the trade over a longer period. Essentially the degradation of flavor due to oxygen or bacterial contamination is sped up so it can be discerned in the brewery before the beer is too long on the liquor store shelves. A drawback to this is that the chemical reactions responsible for oxidized beer flavor are different at higher temperatures from the normally-stored product. So, the forced sample may not give a true indication of the actual aged beer flavor. Of course, if “normal” storage includes abusive conditions such as a summer trip through death valley in a black-painted, unrefrigerated truck with faulty shocks, then these tests are not out of the scope of reality.
The dominant oxidized characters in pale, light-tasting beers is a papery or cardboard-like flavor caused by a class of compounds known as aldehydes (often referred to as carbonyls in the literature). Acetaldehyde is the most common one in beer and is produced in prodigious amounts by yeast during primary fermentation only to be reabsorbed during maturation. It can, however, combine with sulfur dioxide in the beer and survive into the package where it may break down again to result in acetaldehyde in the beer. Another famous one is trans-2-nonenal and is largely responsible for the papery, cardboard character in stale, old beer. Carbonyls find their way into beer from a variety of sources:
• Short chain fatty acids (small organic acid molecules) derived from malt can oxidize to their respective aldehydes.
• Smaller aldehydes can combine to form larger unpleasant tasting compounds in a condensation reaction
• Hops contribute isomerized alpha acids to beer, which can become oxidized via fatty acids to stale tasting aldehydes.
• Higher alcohols produced during fermentation oxidize in beer to form these same stale tasting aldehydes
There is well-documented evidence that melanoidins produced during malting are powerful, natural anti-oxidants in beer. Melanoidins are produced during malt kilning and crystal malts and particularly darker more roasted malts contain a lot of them. However, if they become oxidized earlier in the process they can actually mediate the oxidation of other beer components, mainly higher alcohols, to their corresponding aldehydes. All of this is what worries proponents of “hot side aeration,” a term coined to describe the influence oxygen encountered early in the process in the brewing vessels has on final beer flavor. Researchers have explored this subject with great zeal in recent years in an attempt to find the answer to flavor stability but find a number of contradictory factors at play. For example total exclusion of oxygen from the mash produces a beer with marginally better flavor stability but poor haze stability. Well-modified, well-kilned malts such as British malts have much lower levels of the lipoxygenase enzyme implicated by some researchers in the early oxidation of short-chain fatty acids to aldehydes in the mash. Cardboard flavor is not the only oxidation characteristic common in our beers. Darker, fuller-flavored beers and strong beers all exhibit oxidation characteristics in different ways. Alcohol itself can oxidize to acetaldehyde, which may impart a rotten apple flavor. Amber ales and amber lagers tend to become sweeter and develop cloying, toffee-type characters. I would go as far as to suggest that a good Marzen should demonstrate a toffee sweetness that comes from long periods of aging in the brewery. Stouts are loaded with melanoidins from the roasted malt and so are a little more stable, and also so rich in flavor that a lot of faults remain hidden. The main fault very dark beers exhibit is cheesiness from the oxidation of hop acids. This can happen during storage of hops, so take care to seal up those bags. The biggest advantage home brewers have over their commercial counterparts is that the majority of their beer is bottle conditioned. The yeast is a great natural anti-oxidant and mops up any oxygen in the bottle preventing oxidation reactions.
Implications
Recently a paper published in the MBAA Technical Quarterly by researchers at South African Breweries put forth the following proposal. Beer flavor is stable for only a few weeks at best, then undergoes a rapid change as oxidized characters develop. This is followed by a long period of very gradual change during which the beer is relatively consistent. They suggested that larger domestic brewers with their “born on dating” and refrigerated storage attempt to market “fresh beers,” while imports have already reached the point where they would be considered oxidized by the time they appear on American store shelves. (They must also be implying that it is those oxidized flavors that are desired in imported beers.) Having tasted both fresh cask conditioned Bass Ale with its soft, sulfury, delicate hop aroma, and smooth malt balance and compared it with the toffee-sweet, overly-malty version I find in this country, I must say I feel the researchers are on to something.
Most microbrews are on the shelves during their period of most rapid change. In your local brewpub however, you are far more likely to find fresh versions of the classic world beer styles. As such, they may not actually closely resemble what you think of when you compare them with the imported original. It’s the fresh version you should be attempting to emulate.
Avoiding oxidation
There is only one point in the brewing process where oxygen is a good thing. Freshly-cooled wort should be injected with air or oxygen to provide the yeast with oxygen for cell membrane synthesis during the initial stages of fermentation. After about a day, the dissolved oxygen in the actively fermenting wort is less than 30 parts per billion, which is as low as it will ever be.
Avoiding oxygen pickup
Brewhouse Malt that has been pre-milled is more likely to produce stale flavors that whole malt milled just before it is needed. Avoid splashing wort all over the top of the mash while recirculating or vorlauf. Run wort down the side of the mash tun or gently pour the wort through a flexible hose onto the surface of the mash bed. Do not allow wort to fall to the bottom of the kettle foaming up in the process. Pumps, if you’re using one, with leaking seals will actually draw oxygen into the flow of wort, causing oxidation. And, obviously, don’t aerate the wort on the hot side of the heat exchanger.
Fermentation After around 4 hours, almost all of the oxygen added to the wort will have been mopped up by the yeast. Evolving CO2 bubbles will quickly strip away the excess. At the end of fermentation, however, there should be a blanket of CO2 above the beer that will protect the beer from air contact. However, when you transfer the beer from the primary fermenter to your secondary aging tank, O2 pickup is likely. Care must be taken to purge the receiving vessel with CO2 prior to transferring the beer across. A good way to do it is to fill the receiving vessel with sanitizer then push the it out using CO2 pressure from a gas bottle, leaving the vessel filled with CO2.
Packaging Professional brewers fill kegs and bottles regularly and so have access to cleaning/filling machines that will take care of the air in the package for them. The homebrewer’s practice of siphoning primed, yeasty beer into an air-filled bottle does a couple of things. First, the beer picks up a lot of dissolved air, but then the yeast in the beer absorbs it all. Brewers who filter then bottle face the same issues as commercial brewers with regards to oxidation characters. Filtration is a big area for oxygen pickup. It’s important to realize that this will not result in the immediate appearance of these characteristics, but will hasten their inevitable development. Filling the filter and all the hoses with water and removing it by pushing it out with CO2 will help reduce the air in the filter itself. Large breweries use de-aerated water to chase beer through lines. The receiving tank should have been flushed /purged with an inert gas prior to filling. The surest way to do this is to fill it with water or sanitizer then push the liquid out with CO2.
By now all brewers should understand the importance of minimizing air in a bottle, since as little as 1 mL of air in the headspace of a bottle is enough to oxidize the entire contents. Purging the bottles with CO2 prior to filling, gentle filling and capping on foam will go a long way toward minimizing air pickup. Low air at bottling however does not mean your bottles will last forever, partly because of the oxidation your beer has already been exposed to and also because air will leak into the bottle around the seal of the cap.
And, if I may add a final note…
The best way to avoid oxidation characters in your beer remains to keep it cold and drink it quickly, as the most effective way to stop a beer from oxidizing is to drink it.
