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.

TPO – The Importance of Shaking Packages When Using a Dissolved Oxygen Monitor

When a dissolved oxygen monitor is your only option for measuring package oxygen content, the best way to calculate the total package oxygen (TPO) is from a shaken  (equilibrated) container. In my last blog post I wrote about what you can learn by measuring unshaken packages. This time we’ll focus on equilibrated packages, which are containers that have had sufficient shaking to bring the gases in the headspace and liquid to equilibrium. First, let’s define equilibrium.

The gases in the headspace and liquid of a package are in equilibrium when their partial pressure (also called percent concentration) is the same. The only way to create equilibrium after filling is to shake the package, so that the gases move from the area of higher concentration to the area of lower concentration and are finally distributed equally.

Packages can be shaken by hand, but if you’ll be shaking a lot of packages then you’ll probably want to use a rotary platform package shaker.

Packages must be shaken for different lengths of time, depending upon their temperature. This is because warm packages reach equilibrium faster than cold packages. As a general rule, warm containers need about three minutes of vigorous shaking and cold packages need about five minutes. When shaking cold packages, it is important to keep shaking the container up to the point of use and not let the package warm after the shaking is completed. If the package warms between the time it was shaken and the time it’s measured, then the oxygen partitioning in the package will have changed and you may underestimate the TPO.

What does a shaken package tell you? Since packages come off fillers with different amounts of gas in the liquid and headspace, the most practical way to calculate the total gas content is to equilibrate them and then make your dissolved oxygen measurements. If you measure the dO2 of a freshly filled package without shaking, then you’re only determining the oxygen content of the liquid, without any feedback as to whether there was sufficient fobbing of the headspace.

Knowing the TPO not only helps you determine exactly how much oxygen is trapped in the container and can react with the beer, but it also allows you to calculate the headspace contribution to the package oxygen concentration. The headspace oxygen of a package is the TPO minus the dissolved O2:

Headspace O2 = TPO – unshaken dO2

My final thought is that if you want to measure the TPO of your packages using a dissolved oxygen sensor, you must shake the packages and measure the dissolved oxygen in as short a time frame as possible. Since different beer types have different residual package O2 consumption rates, understanding your specific beer will also help you know just how quickly this needs to be completed. We’ll talk about that in a future post.

For a review of a previous post on what you need to know to measure total package oxygen, follow this link.

Gas Phase Measurement Units

In the past couple of weeks I’ve received several questions about measurement units and how they differ from one another. Have you ever tried to keep bar, mbar, atm, Kpa, %Vbar, %, torr, ppm and ppb straight? If you’re listening to someone in speed mode (I plead guilty) it can be a challenge to follow.

So let’s start by looking at the way different units present at 1 bar, the unit of pressure sometimes also referred to as “atmosphere.”

1 bar =

  • 1000 mbar
  • 750.1 Torr
  • 750.1 mm Hg
  • 29.53 inches Hg
  • 0.987 Atm
  • 14.50 psia
  • 100 kPa

As a brewer you probably won’t see much of units like Torr or mm of mercury (mm Hg), but there’s a unit called %Vbar or ppmVbar that may be helpful. I use them a lot and they can easily be interchanged with percent, but there is a specific distinction in that it is tied to atmospheric pressure and thus stands for “Percent Volume Barometric and “PPM Volume Barometric”. “

So why use Vbar instead of just percent? If you’re at a high elevation and want to specify that that the percent of the gas you are measuring is being measured at atmospheric pressure, then Vbar is your unit. For example, Denver Colorado is roughly 5280 feet. At that elevation there are about 15 percent fewer atmospheric gas molecules —  855 mbar – versus the 1013 mbar you would find at sea level in San Francisco. The Vbar units confirm that the instrument is at atmospheric pressure while the sample is being measured.

This table compares different gas percentages using some of the most common units you may encounter:

Unit

mbar

Bar

Atm

Percent (absolute)

%Vbar

PPM

100% gas

(at sea level)

1013

1.013

1.000

100.0

100.0

1,000,000

100% gas  (atmospheric at 5280 feet)

855

0.855

0.844

84.4%

100

1,000,000

1.000 % gas

10

0.010

0.010

1

1

10,000

0.100 % gas

1

0.001

0.001

0.1

0.1

1,000

0.010 % gas

0.1

0.0001

0.0001

0.01

0.01

100

0.001% gas

0.01

0.00001

0.00001

0.001

0.001

10

0.0001 % gas

0.001

0.000001

0.000001

0.0001

0.0001

1

My final thought is to understand the units available to you. If you are purging down a tank with CO2 and want a specific percentage of CO2 purity, use the units that will equate back to what could dissolve in your beer if the purge didn’t exhaust all of the contaminating gas in the tank.

%d bloggers like this: