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 not have the oxygen content change 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 foam.)
So the best solution I know 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.
- 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. 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.
- Using a 500 micro liter gas-tight syringe, pull 100 micro liters of deaerated water into the syringe and then add an additional 250 to 350 micro liters of air to the contents. Make sure the water stays touching the plunger.
- With the syringe, pierce through the septum and into the can and inject the air and water into the container. 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. If you have a TPO analyzer, the TPO result from the instrument is the proper value to use for comparisons.
- 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. You can also get septums, syringes, and needles from distributers like VWR and the hose clamp from your local hardware store. Next week I’ll post a step-by-step pictorial and a way for you to get the calculations.
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. Second, some TPO instrumentation will not get valid or repeatable values due to foam in the headspace. While these instruments work very well on water samples, foam messes them up and can affect their overall accuracy.