How the Purity of Sparged Carbon Dioxide Affects the Oxygen Concentration of Beer

 

My last blog post discussed the importance of carbon dioxide purity when using injected CO2 to increase the CO2 concentration of your beer. The thing we learned is that the purity of CO2 must be very high (99.99% or better) when using injection, or you will at the same time significantly increase your dissolved oxygen levels. However, injection isn’t the only method for adding CO2 to beer. Sparging, in which CO2 is bubbled through beer (usually in a tank with slight over-pressure) is another common practice for boosting CO2. So in this post we will explore how sparging a finished beer tank with carbon dioxide impacts the final oxygen concentration of your product.

First, let’s do a quick review of what happens when you inject CO2. When you inject gas (usually into a pipe,) you are forcing a given weight of gas into the liquid under pressure. All of the carbon dioxide, plus any trace oxygen and nitrogen, gets pushed into the beer and dissolves completely, allowing you to calculate the weight of gas used and extrapolate from there your various gas concentrations.

On the other hand, when you sparge gas into a liquid the dissolved concentration of gas will be bound by Henry’s Law. Henry’s Law tells us that the amount of gas that will dissolve in a liquid will be proportional – at a constant temperature – to the partial pressure of gas in equilibrium with the liquid.  This means that the gasses dissolved in your beer will never be more concentrated that the partial pressure of the gas you are using to sparge.

The consequence is that, with any given CO2 concentration outcome desired, you will have significantly lower oxygen concentrations for sparged beer then for injected beer. For example, in injected beer the oxygen pickup from injecting one volume of 99.95% CO2 (at 0oC) into the beer when the oxygen concentration is 0.01% is 143 ppb. But a theoretical sparging of that same CO2 into the beer at atmospheric pressure would follow Henry’s Law, and your oxygen pickup would be about 7 ppb.

In real brewing situations, however, most brewers use tank overpressure to help get sparged CO2 into solution, so you would probably be picking up about 2 times the above amount, or 14 ppb. The table below shows the expected oxygen pickup given varied percentages of O2 traces (in your CO2) when measured at sea level and at 0oC:

Sparged CO2 at 1 V/V

0.001% O2

0.005% O2

0.01% O2

<1

3

7

My final thought is that CO2 purity isn’t nearly as important if you are sparging rather than injecting, since the amount of gas that will dissolve into your liquid is much lower. This also applies to the purity of the gas you use to flush air from tanks before filling.

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