Bottle Cap Oxygen Ingress..Is It Real?

We have briefly touched on this subject in other posts, but we get a lot of questions on it. How can this work?

Basically gas diffusion is driven by concentrations across the boundary of the same species. In simple terms, what this means is that a concentration of 21% O2 on the outside of the bottle sees a concentration of 0% O2 inside the bottle. The diffusion process is blind to the fact that there is a 100% CO2 inside the bottle. The gradient that the O2 sees is from the 21% ambient to the 0% concentration of O2 in the bottle. Thus it will slowly diffuse across the boundary until the two partial pressures are at equilibrium. This leads to the funny result that if the CO2 could not diffuse out (which in practice it can), you could actually increase the pressure of the bottle if only the O2 could diffuse in.

It is certainly counter-intuitive at first that you can have gas flow in a reverse direction to the pressure gradient (e.g. the pressure inside the bottle is ~3 atm, while the pressure outside is ~1 atm), but it does.

Studies have shown:

“Over a 6 month period a typical crown cap will allow approximately 1500 ppb of O2 to enter the bottle. Bottles with more than 600 ppb O2 will show definite signs of staling in 6 months. The Oxygen barrier cap limits the O2 ingress to (if you do not sanitize, as the sanitizer and water will have O2 negating any positive effects.) ~125 ppb over the same 6 month period (at 24°C).” (Data from the Proceedings of the European Brewing Convention held in Oslo 1993, pg 654).

Typical daily rates for O2 ingress are generally estimated at 7 ppb per day (150 ppb/7 ppb per day = 21 days before noticeable effects). We already know our staling limits from our COpost. Some criticism we have faced is that we only post “theoretical” concepts. We always encourage folks to test what we advocate, as we do for ourselves, and Russ, a friend and valued member of modern brewhouse, was kind enough to conduct this experiment and post his results at our forum.

Here they are:

There was recently a discussion on the brewing forum about possible ways to extend the shelf life of bottled beer. The original inspired idea, proposed by TexasWine, was a method to delay staling by packing the bottles in a sealed container filled with de-oxygenated water dosed with metabisulfite. This would theoretically scavenge any oxygen intrusion into the vessel and create a buffer between precious bottles of fresh beer and flavor staling oxygen. Building on his idea, I remembered a spare 10 gallon keg that I had which might accommodate a reasonable number of bottles and then be purged with CO2 of sufficient purity and be sealed to facilitate long term storage. Bryan proposed the great idea that the keg be pressurized with CO2 equivalent to the bottles, which would greatly reduce the possibility of gas exchange by pressure differential. And so I set out to test this thesis, with the goal of reporting the results.

Bottle cap oxygen ingress and methods for reducing it in long term storage.

The process is quite simple. Place bottles in a keg, seal, purge with CO2 then pressurize for storage. Keep cold for best results.

The ideal candidate beer for this test should be a light, low gravity, delicately flavored,  brew with the fresh malt and hop flavors intact. It is often said and is quite true that there is no place in this type of beer for any off flavors and oxidative damage to hide.


– A cornelius keg or other completely sealed and pressure capable container with an opening large enough to pass bottles with connections for gas, vent and pressure relief.

Note: Placed upright, thirteen 500ml bottles will fit in a 5 gallon keg and twenty six in a 10 gallon. I did not test smaller bottles but a higher packing density might be possible.

– Blanket gas, preferably low O2 content CO2 or other beer compatible gas such as nitrogen

– And lastly for lagers or any beer you wish to further extend the storage time, a temperature controlled ferment chamber or lager fridge large enough to fit the keg is ideal.


Twenty five 500ml bottles (stacked upright in two layers) of a recently brewed bottle spunded Kölsch were placed inside a 10 gallon corny keg. The keg was then put away in chest freezer at 35°F where it was left untouched for 3 months. As a control 4 additional bottles were stored outside the keg but inside the lager chamber so all other variables would be the same.


After the 90 day rest, samples of the gas purge stored beer (on the left) and the normally stored control (on the right) were poured in identical glasses and placed in front of a white matte background to evaluate the color.

To the eye there is little color difference in these beers, with the normally stored version maybe slightly darker. I did expect to see a greater color difference but possibly the cold storage temperature inhibited that.Compare these to a picture of the same beer (in exact same lighting) 19 days after bottling and again there is little difference save the clarity which obviously improved with extended lagering.


In this third photo you will notice the normally stored bottle (on the right) is slightly less clear but that is the only obvious difference. Foam creation and stability were very similar.

Where the beers differ radically is the taste and aroma. The gas purge stored beer was very fresh with an abundance of fresh malt fragrance mixed with faint fruity esters and a proper hop aroma. The taste is bright with mildly fleeting bitterness and a sweet malt finish. A bit too light for a proper Kölsch maybe but that of course is an issue with this recipe and not the storage conditions. The normally stored bottle on the other hand was just dull with almost no ester or hop aroma remaining. The taste was overpowering herbstoffe (malt bitterness) which made it difficult to drink and so was poured down the drain.

 Conclusion: I believe this test was a success, at least for me, in proving the effects of cap oxygen ingress over time in bottles. There were stark differences in taste and aroma but with this we have the possibility to delay the inevitable staling and degradation with better storage conditions. Granted this method is probably overkill for most people especially for short term holding. But for easily damaged beers or anywhere one might wish to delay the staling effects of O2 this is an acceptable, if slightly more costly way, to get the job done.- Russ AKA Bilsch.

For more reading, we also found bottle cap ingress to be a factor with our blog post Low Oxygen Bottle Filling. Where, after the bottled beer was refrigerated the beer was tested for sulfite and oxygen consumption during different aging time frames. There is more discussion about this on our forums as well. Fascinating stuff, and gas laws are real and really a pain for us as we know.