Total Package Oxygen 101 – TPO Defined

 

The following popular post was the first in a series covering Total Package Oxygen (TPO). The blog is still on vacation, but I’ll be back at it Friday with a new Frivolous Friday post. 

In the late 1980s I worked on a research and development team — funded by a crown manufacturer — with the goal of creating oxygen-scavenging polymers for beer bottle closures. The project went through numerous challenges (think scorched plastic) and dicey prototypes (think raised eyebrows at the FDA). In the end we were successful, but by far the biggest and most important challenge was learning how to quantify the efficiency of our oxygen scavenging polymers. In other words, exactly how much oxygen did the polymer coated closures remove from bottled beer?

It turned out that the ticket was to measure Total Package Oxygen (TPO). We’ll talk about our method in a minute. But first, what exactly is TPO? Simply put, TPO is defined as all of the oxygen in the package: headspace and liquid. We can also just say that TPO is all of the oxygen available to react with beer in a package. In order to get an accurate TPO, we need to know four things: dO2, headspace volume, liquid volume, and temperature.

We’ll cover the importance of volume and temperature in a future post, but for now let’s talk about the first parameter: dissolved oxygen. For our project, we knew that measuring headspace oxygen alone wouldn’t do, because experience had taught us that beer itself has a really fast oxygen uptake, and we needed to know how well our polymers were scavenging.

So we thought a bit more about the oxygen content of a package just after filling. We already knew we had gas in the headspace of the bottle or can, and gas dissolved in the liquid. The dissolved oxygen was whatever was present in the beer before filling, plus any oxygen picked up during the filling process. The headspace oxygen, on the other hand, was air left over in the headspace after a closure was put on the bottle or can.

So there we were, with two distinct partitions of oxygen in the package — dissolved and headspace – and yet we wanted to measure using a portable dO2 analyzer. How were we to do that and still take into account the headspace O2? It turned out that the answer was a bit James Bond: the package needed to be shaken. (Not stirred!) We found that if we shook the packages properly, the gas in the liquid and the headspace would equilibrate to the same partial pressures. Once the gas was at equilibrium, we could use our portable dO2 analyzer to measure the dO2 in the liquid.

Since then, many TPO techniques have been developed that measure variations of dissolved oxygen, headspace oxygen, and a combination of headspace and dO2.  Different techniques have different advantages, but for now I just want to assure craft brewers that it is possible to do accurate dO2 measurements in pursuit of a TPO goal using one of your simplest tools: a portable dO2 analyzer.

My final thought is to emphasize the importance of making sure your packages are at equilibrium. If a can or bottle is not properly shaken, then your TPO may be either under or over estimated.

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2 responses

  1. Chez, we met a few weeks ago while visiting our brewery. Can you suggest a way of standardizing the shaking method? Is there such a thing as an endpoint when you know the gas in the liquid and the headspace would equilibrate to the same partial pressures?

    Trying to create an SOP for our operators. I think beer temperature would effect saturation rates.

    -Ryan

    • Hi Ryan – The best way to standard the equilibrium endpoint is to use a mechanical shaker. The work we did with a platform shaker shows that it takes about 2 minutes to reach 95% equilibrium at room temperature and three minutes to reach 99% confidence. At 3 degrees C, it take about 5 minute to reach equilibrium. I have done some experimentation with hand shaking and have had good results using a vortex method and thirty seconds of shaking. The problem is that this is very hard on your hands and wrist and should be avoided. It’s also has the issue of having person to person variation. To perfectly replicate shaking requires a mechanical shaker. – Chaz

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