Ask Mr. Wizard

Trying to Gauge the Speed of Oxidation


Richard Swent — Palo Alto, California asks,

While reading AN article in A recent issue of BYO I was left with a question about how long it takes oxygen to cause oxidation. I know it depends on temperature, but give me some ranges. For example, when packaging bottle-conditioned beer there is some oxygen exposure but the yeast rapidly consume any oxygen they can. Is the beer oxidized before the yeast can act or does the yeast consume the oxygen before the undesired reactions have time to take place? I cold crash my beer (with a CO2 source for suckback) and bottle while still quite cold (40–45 °F/4–7 °C). This would presumably slow any oxidation reactions but it also slows the yeast metabolism. Which one wins the race as the bottled beer warms up or does it affect both equally?


This is a terrific question and is well suited for a terrifically short answer. Beer oxidation can occur shockingly fast, especially if a beer is the sort to easily show off oxidized aromas. And the rate of oxidation doubles when the storage temperature of beer is increased by about 15 °F (8 °C); for example, beer stored at 39 °F (4 °C) that retains 90% of its freshness after 30 days will have a similar level of oxidation in 15 days when stored at 54 °F (12 °C). Yeast can prolong freshness, but the notion that yeast quickly consume oxygen is not true if the yeast is old and tired, especially if the cell density in the package is low.

The best way to appreciate the speed of oxidation is to perform a simple trial. Grab two or three bottles of a subtle beer, such as Budweiser. Open one bottle, gently fan some air into the headspace to help move the carbon dioxide blanket out of the bottle, recap, and give the bottle a few shakes to help dissolve the headspace gas into the beer. If you want to see how yeast may slow oxidation, shoot in a milliliter of slurry (assuming 1 mL of slurry contains one billion cells, this will give you 2.8 million cells/mL, which is a healthy density). Incubate the sample(s) in a warm environment for 24 hours, transfer to the refrigerator where the control beer is stored, and do a side-by-side tasting in 2–3 hours after the beers are both the same temperature. You should be able to easily differentiate the control from the experimental beer without yeast (not sure about the yeasted sample) and detect the tell-tale signs of oxidation in the experimental sample.

The method described above is pretty extreme, but not uncommon. Growlers, unfortunately, are a great example of mistreated beer because the manner in which they are often filled is sufficiently abusive to oxidize beer within hours of filling. I personally dislike growlers for multiple reasons, but the effect they often have on beer freshness is my #1 complaint about them.

The best way to appreciate the speed of oxidation is to perform a simple trial.

Filling bottles with flat beer is a challenge and should be done with special care. Adding a fresh dose of yeast can help if the fermentation may have been stressful or the beer was aged. Commercial bottle filling (or growler filling using a proper filling device) is a different story because bottle fillers are designed to purge air from the bottle, counterpressure-fill beer into the bottle, and then gently release pressure after filling. A properly filled bottle will be quiet after the fill tube is removed and the beer can be fobbed (intentionally foamed) by knocking the bottle with something like a plastic screwdriver handle or by squirting a small volume of water onto the surface of the beer before capping on foam. Commercial bottling lines are indeed equipped with water jetters or bottle knockers before the capper so that beer is capped on foam. Another thing that makes a good bottling operation better is the use of oxygen barrier caps that have special liners to absorb oxygen that is either in the bottle headspace after filling or diffusing into the headspace through the crown seal during storage.

Response by Ashton Lewis.