I was helping a brewer evaluate the dissolved oxygen pickup from his filler before his can ends were applied. He was wondering if there was some sort of apparatus he should be using to cover the headspace of the can. I recommended that he just wait until the can end was applied and measure the unshaken dissolved oxygen though the closure. Then he asked the question that is the topic of this post: did he need to let the CO2 in the can “rest” prior to measuring his dO2? He had apparently been told that in order to get an accurate dO2 measurement, the can should sit open to the atmosphere for 20 minutes in order to let the CO2 do “something” – the intent was not really clear.
I had never heard of this, but my instinct told me it was a myth, and that it could in fact lead a brewer to underestimate the dissolved oxygen in his can of beer. When a fully carbonated package sits open to the atmosphere, the partial pressure of the CO2 is significantly higher than the oxygen content of the outside environment. As the beer sits the CO2 will rise out of the can and carry oxygen with it.
I wanted to be sure my thinking on this was clear, so when I got back to my lab I sacrificed a recently packaged beer in an aluminum bottle to be sure. First I removed the 28 mm closure and then I immersed a 12 mm probe. Since I was using an optical probe, I didn’t need beer flow to do my monitoring. Upon immersion the probe O2 reading quickly equilibrated to about around 20 ppb and stayed at this value for about ten minutes. As the CO2 began to escape from the can, the probe value steadily dropped to around 10 ppb, then slowly rose over the next two days to about 50 ppb. This was only one experiment, but the data was logical, and I’ve never had another reason to test this notion.
My final thought is that I believe the “resting” theory is myth, but if anyone has a different explanation, I’d love to hear it.