Over-Oxygenation of Wort
TroubleShooting
Andrew Willing - Minneapolis, Minnesota asks,
In the May-June 2013 edition of BYO, Jamil Zainasheff discusses the Belgian blond style. At the very end of the article, he discusses "oxygenation" (i.e., with pure O2, as opposed to "aeration" with air) and states that (a) over-oxygenation of wort at pitching can result in fusel production, presumably outside of any other factors or variables; which obviously also suggests THAT (b) over-oxygenation of wort is even possible. Jamil goes on to suggest an oxygenation rate of 1 L/min for one minute.First, is (b) true? Is over-oxygenation of wort even possible? I've read on a number of forums and other places — though, unfortunately, none available to cite at the moment — that speculate that over-oxygenation of the cooled wort (at pitching time) is *not* a reasonable concern. The thought is that the solubility of O2 in the wort is limited, and that any possible over-saturation would likely leave solution within short order.Second, if (b) should be true, I have never read anything (else) that suggests a link between fusel production and oxygenation. Everything I've read suggests fusel production is a result of poor fermentation temperature control. Is there any support for Jamil's oxygenation claim? Is there any known oxygenation rate (i.e., in contrast to the cited 1 L/min for one minute) that will lead to noticeable fusel production, even with tightly controlled fermentation temperatures?I ask because I do oxygenate my wort, and have for some time, but my regulator does not have a gauge by which I can measure flow. If this is more than a theoretical issue, I may need to invest in some kind of flow-metered regulator.
I have been writing this column for nearly 18 years and I think I have been writing about the importance of wort aeration for nearly the same time period. While oxygen is not a brewing ingredient, the addition of oxygen, either through aeration or injection of oxygen, is as important as any other brewing ingredient. Given the choice of forgetting to add a specialty grain to a brew or forgetting to aerate my wort, I would pick the former any day of the week.
So is it possible to add too much oxygen to wort? The answer is … a resounding maybe … when the source of oxygen is from a bottle. Higher oxygen levels can be achieved when pure oxygen is used, and that is the issue. Oxygen solubility depends on wort gravity, but for normal gravity wort in the range of 10-15 °Plato (1.040 – 1.060) aeration produces up to about 10 ppm of dissolved oxygen, compared to about 30 ppm when oxygen is used. Brewing scientists agree that high rates of oxygenation can cause oxidative damage to intracellular membranes, especially when oxygen is added to propagation vessels. But the problems associated with this typically do not manifest themselves until the yeast is harvested and re-used after fermentation, and in practical terms not all scientists have the same opinions about this topic. Since most homebrewers do not re-use yeast for more than one or two batches, there is not much written about over-oxygenation in the homebrewing literature. Given the importance of aeration, many homebrewing advisors would rather a brewer err in the high side rather than err on the low side, or worse yet, simply forget to add oxygen to the wort.
The easiest way to control the amount of gas that dissolves into wort is to limit the flow rate and time of the process. In Jamil’s article about Belgian blond ales he recommends 1 L/min for one minute. Without getting too geeked out with this topic, it is fairly easy to calculate the amount of gas being added during this one-minute period. One mole of an ideal gas (as in the ideal gas law) expands to 22.4 L at atmospheric pressure, so 1 L of oxygen is equal to about 0.05 moles or 16 grams, and corresponds to about 75 mg/L in a 5-gallon batch. Since the solubility of oxygen is limited, a little less than half of this gas is dissolved into the wort. Brewers who use pure oxygen and are concerned about adding too much simply dial back the flow rate or duration of oxygenation. Commercial brewers commonly use air instead of oxygen because the amount of oxygen that dissolves into wort when using air is limited to 10 ppm or less and there is no concern about oxidative stress.
What does this have to do with higher alcohols (also called fusel alcohols and fusel oils)? Higher alcohols are derived from amino acids. When yeast absorb amino acids from wort some are used for building new yeast cells and others end up as carboxylic acid chains with a couple of biochemical possibilities. If these organic acids are decarboxylated and reduced they end up as alcohols. The most common organic acid to follow this path is pyruvate and the resulting alcohol is ethanol. Organic acids can also be combined with ethanol within the yeast cell to form esters and the most common ester is ethyl acetate. Ester production is higher when wort oxygen levels are low, and the corollary is an increase in the levels of higher alcohols when wort oxygen levels are increased.
From a practical view there are a few take-home messages related to this topic. The first is that aeration and oxygenation are important, and the notion that wort can be over-oxygenated is certainly true. Whether this is critical for the homebrewer is debatable, but process control is not debatable. If you are adding oxygen to your wort, you should have a regulator that allows you to adjust the pressure. Even if you do not have a gas flow meter, you can be consistent if you use a given pressure for a given time. If you think you are adding too much or too little, you can adjust the time of oxygenation as needed. Higher alcohols and esters are influenced by aeration and this just highlights the importance of this process since both compounds are flavor-active. The thing about higher alcohols to remember is that they are derived from amino acids, and when you increase the original gravity of wort you should expect an increase in the amount of higher alcohols in the finished beer. Adjuncts like sugar, rice and maize do not have much protein and do not increase higher alcohol levels as much as malt; that’s one reason they are commonly used as adjuncts.
