Last week I spent a couple of mornings measuring dissolved oxygen in wort at two different breweries. One was injecting oxygen and the other was injecting air. It was easy to get high dO2 concentrations injecting O2, but the air was another story – the O2 in the air simply didn’t want to fully saturate. Here’s a bit about the two setups.
Each gas was injected into a process pipe where the brewery had the ability to control the volume of gas injected into the wort. In both cases the gas was injected at a similar distance from the measurement point. The two measurement points were off the process pipe through a sample valve and not in the fermentation vessel. The target gas concentration for the air injection was to achieve air saturation, which was about 9.5 ppm O2. The target for the oxygen injection was 15 to 17 ppm.
The measured concentration of dO2 from the oxygen injection was as expected: between 15.0 to 17.5 ppm. But the dO2 with air was not the estimated 9.5 ppm, measuring instead around 7.5 ppm. Flow rates were similar, the distance to the measurement point was similar, the method of injection was similar, but not all of the oxygen in the air went into solution.
Let’s explore a few different ideas as to why it’s more difficult to get saturation by injecting air. I think the main reason is that there’s just more total gas volume that needs to go into solution. To inject a given concentration of the O2 in air, you must inject four times more nitrogen than oxygen.
Another other reason is that N2 isn’t as easy to dissolve into a liquid. If you dissolve air into water, about 40% of the gas that dissolves into solution will be O2, while 60% will be nitrogen. To get all of the gas into solution, the overall pressure in the process pipe needs to be high enough to dissolve not just the oxygen, but all the nitrogen. It’s not impossible to achieve, but it probably takes a little more time than if it is pure oxygen.
Once you get the gas you want into solution, you still need to keep it from coming out of the wort in the fermentation vessel. If you’ve injected air, then it is important to keep some back pressure on the tank until the tank is filled and the liquid is quiescent. If you don’t maintain back pressure then the nitrogen can degas and potentially take some O2 with it. Even if you inject oxygen, it’s important to control the flow of wort into the fermentation vessel, keeping the fill as non-turbulent as possible so the O2 will stay in solution.
My final thought is that there is a lot to keep track of, no matter which technique you use to oxygenate or aerate your wort. Be consistent, and make sure you understand your dissolved oxygen levels, so you’ll know if your yeast are getting enough to keep them happy.