Dissolved Oxygen in Beer: How It Compares to Total Package Oxygen

When it comes to questions about oxygen in beer, I think the one I’m asked most often is, “What is the difference between dissolved oxygen and total package oxygen (TPO)?”  The main source of this confusion is that when measuring O2 in packages, the O2 in the headspace is often overlooked. If you don’t take headspace oxygen into account, then you are measuring a partial concentration, period. So let’s talk about the differences and what each one tells you.

A significant number of craft brewers have a dissolved oxygen (dO2) analyzer they use to measure the dO2 content of their beer in process. The most common point of measurement is the finished beer tank. The beer in a finishing tank will have O2 pickup from the empty vessel and from the filtration process, plus it will pickup more O2 as it goes through packaging.

Once the beer is packaged, however (assuming good packaging,) rapid O2 pickup from outside sources all but stops. So what can we tell about how much oxygen actually made it into the package?  It is not a simple matter of measuring the O2 in the beer.  The package must be shaken to equilibrate the oxygen in the beer and the headspace before the 02 in the beer is measured, and that number must then be used to calculate your TPO. Let’s think about what it is possible to measure and what each thing tells you.

Package dO2 –

The easiest measurement to take on packaged beer is the dO2 of a package just off the filler without shaking the beer. It is important to measure as quickly as possible, so the product does not “consume” the oxygen in the beer. (Residual or live yeast may be hungry, plus oxidation by trace metals, etc.) In some packages there is a measurable difference within five minutes and in other packages the rate of oxygen consumption takes significantly longer, sometimes hours. It is always best to measure as quickly as possible.

This unshaken package measurement represents the combination of the dO2 of the beer at the base of the filler and the oxygen pickup of the filler. Oxygen picked up at the filler can be quite variable. Most fillers run at about 25 to 50 percent deviation, but in some cases it can be up to 100 percent deviation. The best way to measure the percent deviation is to determine the dO2 at the base of the filler and then measure six to ten packages and determine the variation of each package as compared to the average of all the containers. But remember: this measurement only tells you what is in the liquid. When measuring unshaken packages, any gas in the headspace is left uncounted.

Shaken Package dO2 –

When you shake a package of beer so that the partial pressure of the oxygen in the liquid is equal to the partial pressure in the headspace, it changes the characteristics of the oxygen partitioning in the package. If most of the oxygen in the package is locked in the liquid, then shaking the container will move the O­2 from the liquid to the headspace until equilibrium is reached.

So, you have measured the dO2 and then shaken the package. Now what do you do with the data? If you really want to quantify the TPO of the package you have to take into account the headspace oxygen. To do this accurately you need to know the headspace volume and the package temperature.

Total Package Oxygen –

When using the dissolved oxygen measurement, the TPO can only be calculated from a shaken package. To do this calculation you also need to know the headspace volume, liquid volume and the package temperature. The temperature and the headspace volume are critical values and small inaccuracies can alter the results significantly, but the liquid volume may be estimated by using the average fill volume. Once you have your figures, then you can use a TPO calculator to determine the concentration from your initial DO2 measurements.

My final thought is to not skimp on how much you shake the packages. Cold containers should be shaken for five minutes and room temperature cans or bottles need about three minutes. If you’d like a copy of a TPO calculator built into an Excel spreadsheet, then please click here to request one.

 

 

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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.

Pictorial Instructions for Creating a TPO Validation Standard

The most difficult thing about using Total Package Oxygen instrumentation is creating a proper validation standard. In other words, you need a solution with a known concentration of oxygen in order to validate your TPO instrumentation, but how do you get that?

Since the concept of TPO was first published widely, the most referenced paper on the subject is the 1985 article in Brauwelt by Carlos Vilachá and Klaus Uhlig, “The Measurement of Low Levels of Oxygen in Bottled Beer.” It covers the best calculation to get TPO, but doesn’t talk about how to validate the results.

Your TPO validation standard must be repeatable (for statistical purposes you’ll be using multiple packages of your standard,) but it’s not easy to repeatedly put a known dO2 concentration in a package and have it be stable over time. You can package water, but microbes or can corrosion can decrease oxygen after awhile, plus it’s hard to find a can filler capable of getting the oxygen levels in the headspace low enough to reflect the values found in most freshly filled beer packages. (That’s because when beer foams, it displaces the oxygen out of the headspace, and water doesn’t do that.)

So, the best idea that I know of for dealing with this problem is to use old pasteurized beer and inject the cans with a known concentration of air. The method is fairly simple, but there are a few tricks that make getting accurate recovery levels possible. Here’s how it works:

  • Use room temperature beer that is at least 30 days old and preferably pasteurized, but not can-conditioned.

Spike -5

  • Place a sticky-back septum (most are pretty small, less than an inch in diameter) on the can and hold the septum on the can with a large hose clamp. I recommend putting the septum on the upper edge of the can, where the can begins to neck into the seam, because the can is stronger in that location and will flex less.

Spike Clamp-1

  • Tighten the hose clamp so you can still rotate it if you use some force. Make sure to center one of the hose clamp holes over your septum.

Spike -1

  • Using a gas-tight syringe, pull 200 to 300 μL of deaerated water into the syringe making sure that any air bubbles work their way to the tip of the barrel and can be expelled. Then push the plunger until there is 100 μL of water in the syringe and the needle if full of water.

Spike -3

  • Next, pull the syringe until the tip of the plunger reads 300 μL. The syringe and needle will contain 200 μL of air, even though the plunger shows more water in the syringe. Note that this extra volume is from the now evacuated needle. Make sure the water stays touching the plunger.

Spike -7

  • With the syringe, pierce through the septum and into the can and inject the air and water into the container.

Spike -8

  • Then rotate the hose clamp so it blocks the hole where you injected the air and shake the can.
  • After shaking the can for three minutes, measure the dO2 and calculate the TPO.
  • Next, calculate the mass of the oxygen in the air you used (based on whatever volume you used.) This will give you a value to compare to your TPO.

Hach sells a kit that contains all the pieces you need, including detailed instructions for the procedure, as Part Number DG33373. (Here’s a link to the syringe kit). You can also get septums, syringes, and needles from distributers like VWR and the hose clamp from your local hardware store.

My final thought is two-fold. First, make sure you test your ability to do this procedure on multiple packages and get statistics on your percent recovery. If you’re getting great repeatability but not good accuracy, it may be the calibration of your instrument or your experimental technique. Second, some TPO instrumentation that places the oxygen probe directly in the headspace of the sample may not get valid or repeatable values due to the foam. While these instruments work very well on water samples, foam can affect their overall accuracy.

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