Package Headspace Volume and its Effect on TPO


I’m at the CBC this week, so come by and say “hi” if you’re there. I’ll be in the Hach booth at the Brew EXPO.

Most total package oxygen measurements are determined by the dissolved oxygen concentration in a shaken package. A straight shaken dissolved oxygen measurement can be a great part of your brewing quality toolkit, and it never hurts to have it. But it doesn’t reflect the oxygen in your headspace and its potential effect on your package. For that you need to calculate the TPO.

Headspace oxygen concentration can actually be more than twice your dissolved concentration before shaking. So I thought I’d show an example of how much the TPO can vary on two packages with the same dO2, especially if their volumes are slightly different.

Let’s start with two cans of beer, each with 100 ppb of dO2. (That means we’re looking at dO2 in the liquid only, before shaking.) The first has a 15 mL headspace due to a slightly high fill and the second has 25 mL because of a slightly low fill. While both packages have 100 ppb of dO2, the first has a TPO of 227 ppb and the second has a TPO of 312 ppb. For some this may be considered above their total package threshold.

Headspace Example

My final thought is that you should always consider the headspace volume when measuring dissolved oxygen in packages. You don’t need to know the exact TPO of every package, but by being aware that low fill measurements have less latitude than high fill measurements, you can help keep your package concentrations as low as possible.

Validation Techniques for Optical Dissolved Oxygen Sensors in Beer

I’ve recently spent a few posts discussing measurement validation, mostly with a focus on situations where instruments were reading properly and there was a problem somewhere in the brewing process. Now I’d like to continue the discussion with an example of the reverse: the beer was actually okay, but sensors were in need of validation and calibration.

Last week I visited a brewery where they were getting unusually high portable dO2 measurements on beer that had been fermenting for a week. After a day of sitting in a conical fermenter, beer usually has a dissolved oxygen concentration of less than 5 ppb, but in this case their optical oxygen analyzer was reading 30 ppb. The brewer was wondering if there was an issue with his fermentation, so here’s what we did:

  • We measured the beer in the fermenter with a second instrument that we could easily calibrate and validate, should the reading fall outside the expected range.
  • Then we measured 99.999% N2 calibration gas on both instruments.

Here’s the reasoning behind these steps:

  • Optical oxygen analyzers will drift upward over time due to photo bleaching of the fluorescent matrix.
  • Using a 99.999% N2 or CO2 calibration gas to check the zero of the instrument is a way to validate low-level (less than 200 ppb) measurements. It is important that the pressure of the gas flowing through the instrument be as low as possible. (Do this by controlling the gas flow at the inlet to the instrument and opening the analyzer flow valve completely. Ideal gas flow should produce about 2-5 bubbles per second when the outlet tube from the analyzer is placed in a few inches of water.)
  • If instrument readings using calibration gas are above 3 to 5 ppb, then it’s probably time to calibrate the instrument.

In this case we discovered that both instruments needed calibration. On the beer, one was reading 8 ppb and the other 30 ppb. On the calibration gas, the first instrument read 3.5 ppb and the second read 16 ppb. We didn’t proceed to calibrate right then, but knew that once they were calibrated they would agree.

My final thought is to validate whenever a reading does not seem logical. In this case, the brewer was wondering if there was an issue with his fermentation, but it wasn’t his process, it was the need for instrument maintenance.

Frivolous Friday – Tutankhamen Beer Fetches a Royal Sum

I’m sometimes amazed at what connoisseurs are willing to do for a unique experience. Would you pay $7,686 for a bottle of beer? This one happened to be the first filled with someone’s best interpretation of what might have been cooked up in King Tut’s kitchen. Subsequent bottles didn’t fetch nearly as much, but it still sounds like it was an interesting experiment. Here’s the full story:

Happy Friday!

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