The following post is the second in a series on Total Package Oxygen (TPO).
We’ll cover two topics in this blog post: 1) The parameters you need to do a TPO calculation and 2) Best practices for getting packages to equilibrium.
First, TPO calculation parameters. It’s a paradox, but the actual calculation for TPO is both complex and simple. The complexity part arises from bits of Henry’s Law, Boyle’s Law, physical properties of water vapor, and atmospheric barometric pressure. But these complexities are incorporated in the calculations, so the simple part is that in order to do our calculations, all we really need are these easily obtained variables:
Dissolved oxygen content
The package must be shaken, so the gases in the headspace and liquid are at equilibrium.
Accuracy of liquid volume is not very critical, so using the average package fill will give a statistically valid TPO.
The more accurately we can determine the headspace volume the more accurate the measurement. Knowing it to within 1 mL is best.
There are a lot of variables that rely on the package temperature built into the TPO calculation, so it is best to measure to within 1 oC.
So now we know our important package parameters. But how do we know we’re measuring our packages at equilibrium, so our calculations will be correct when we plug them into our formula?
We shake (equilibrate) packages in order to mix dissolved gases in the liquid with air trapped the headspace after the closure has been applied to the package. The question is: how long to shake the package?
People have tried all sorts of things through the years to determine the proper amount of shaking time. In the end it turned out that one of the best methods was to take old cans of beer with less than 2 ppb of dO2 and inject known volumes of air into the cans. By doing that and then shaking for various amounts of time, it could be determined how long it takes to be able to account for 100% of the oxygen when measuring TPO using the dissolved oxygen concentration and the other parameters outlined above.
Using a platform shaker at a minimum of 180 revolutions per minute, room temperature packages take 3 minutes to equilibrate. Packages at 5 oC, on the other hand, do best with 5 minutes of shaking. The basic rule of thumb is that the colder the package, the longer the shaking. When shaking cold packages, it is also important to remember that the temperature of the package is constantly trending toward room temperature, so the best practice is to shake continuously until you are ready pierce the package.
One short note on bottle conditioned beer: I’ve seen yeast so active upon packaging that they can consume statistically significant amounts of oxygen in the time it takes to shake a bottle, so I’ll have some hints about how to deal with that next time.
My final thought is to always shake thoroughly and consistently. Using some sort of a mechanical shaker will decrease operator error and insure against inconsistent results.