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

When an industrial supplier sets a minimum purity for the CO2 they supply to your brewery, you need to be aware of the ramifications of that purity and whether there is any chance it will increase the dissolved oxygen concentration of your beer. CO2 specified at 99.5% or better may sound very pure, but when we do the math we find this is actually a problem. This post is specifically about carbon dioxide that is “injected” into beer. Another post will address CO2 that is “sparged” into beer.

If your CO2 has a 99.5% purity, then the impurity is 0.5%. For purchased CO2, the assumption is that the impurity is always air, so only 1/5 of the impurity – 0.1% — should be oxygen. If you were to add to your beer one Volume of CO2 with an impurity of 0.1% O2, it would increase your oxygen concentration by a whopping 1,420 ppb.

In 1985, Nick Huige and his Miller Brewing Company co-workers published a paper on this subject in the MBAA Tech Quarterly. Their findings on the affect of injecting impure CO2 are astounding and can been seen in the table below.

 

Amount of added CO2

Concentration of O2 impurity in CO2

0.001%

0.005%

0.02%

0.5 V/V

7 ppb

35 ppb

142 ppb

1.0 V/V

14 ppb

71 ppb

284 ppb

2.0 V/V

28 ppb

142 ppb

567 ppb

Dissolved oxygen added to the beer

 

So if you want to inject CO2 into your beer, you need to be mindful of the actual purity. I know a brewer whose CO2 specification from his supplier was 99.5%. Most of the time the supply was much purer >99.998%, which is excellent. But when they had an unexpected increase in their dO2 levels, the cause was eventually traced back to the CO2. What was the purity of the “problem” C02? A number that still sounds good on a cursory level – 99.97%! – but was not acceptable in the context of dissolved oxygen in the product.

Most brewers specify a minimum CO2 purity of 99.990%. This equates to an oxygen impurity of about 0.002%. If one V/V of CO2 with this oxygen content were injected into beer, the resulting increase to the beer dO2 would be about 28 ppb.

My final thought is that if you are injecting CO2 into your beer, be sure your purity specification is not too low. You don’t ever want to be in a position where you’ve been getting great CO2 but then have one “bad” batch – still within the manufacturer’s specification – adding too much oxygen to your product.

 

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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.

Package O2 Measurements – What Can You Learn From Unshaken Containers?

 

When using a portable dissolved oxygen analyzer to measure package oxygen concentrations, you have two options:

  • Measure the package directly off the filler.
  • Shake the package until the liquid and headspace gases reach equilibrium and then measure.

Let’s dive deeply into interpreting the results of unshaken packages and learn what it tells you.

Since we aren’t doing anything to the container to equilibrate the headspace gas with the dissolved gas in the liquid, an unshaken package gives us a snapshot of these three oxygen influences:

  • Dissolved gas in the liquid right as it enters a filler.
  • Dissolved gas pickup during filling due to air in the package that has not been cleared before the package is filled.
  • Fill bowl O2 pickup in rotary fillers

The differentiation of these is easy to quantify. One is the dO2 at the base of the filler and the other is the dO2 measured in the package minus the dO2 at he base of the filler. Here’s an example:

  • dO2 of an unshaken package = 63 ppb or 0.063 ppm.
  • Base of filler dO2 = 18 ppb or 0.018 ppm.
  • Filler dO2 pickup = 45 ppb or 0.045 ppm.

Since it is relatively easy to measure just before the filler and just after filling, let’s discuss what influences the results of each.

The dO2 concentration of beer at the base of the filler is usually easy to control and is based on just a few potential influences. High values can be caused by:

  • High residual in the finished beer tank.
  • Oxygen pickup from a pump between tank and filler.
  • O2 pickup from a valve or fitting between tank and filler.

Likewise, if you make it a practice to regularly measure the dO2 at the base of your filler and then calculate the filler valve pickup, it can give you great feedback on when to service your filler. If the O2 at the base of the filler is low, but the unshaken dissolved O2 is high, then perhaps there are ways to alter your filler system to achieve lower values. Here are some potential areas of oxygen pickup:

  • Purging on rotary bottle fillers as impacted by vacuum pumps, CO2 purge duration, fill tube lengths, filler speed, and fill bowl characteristics.
  • Effectiveness of CO2 purge pressure and flow on an inline batch bottle filler.
  • CO2 purge time, fill tube lengths, filler speed, and fill bowl characteristics on rotary can fillers.
  • CO2 purge pressure and flow on an inline can filler.

So we can learn a lot from measuring gas content in unshaken packages, although it’s important to remember that an unshaken package won’t tell you if you’re picking up oxygen from a can-seamer or while fobbing bottles, since they can contribute to oxygen in your headspace.  Total Package Oxygen (TPO) takes into account headspace oxygen and is the only calculation that can give you a complete picture of the oxygen in your finished product.

My final thought is that understanding unshaken package dO2 measurements will help you troubleshoot some sources of package oxygen contamination. Next time we’ll examine shaken package measurements and tie it all back total package oxygen.

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