The following post is the first in a series we’ll do covering Total Package Oxygen (TPO).
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.