Wort Dissolved Oxygen – Part Art and Part Science

After I wrote last week about my recent experiences measuring dissolved oxygen in wort, I started thinking about how what ultimately matters is being able to replicate measurement parameters that tell you what you need to know, even if the numbers at certain points in the process don’t match what you are “supposed” to be achieving.

Back in the late 1990s I had the opportunity to work with some excellent British brewers. When it came to their wort measurements, most of the dO2 readings from their inline instrumentation were significantly lower than expected compared to their fermentation vessel dO2. Yet they had still managed to determine their “ideal” inline process dO2 value. They did this by creating a five-way matrix based on liquid flow, gas injection volume, inline dO2 readings, tank dO2 readings, and the ultimate quality of the fermentation. They’d come up with those parameters because they didn’t always have a good place to put an inline process O2 probe. So they just put the probe in a convenient place and took the value – even though it was too low to be ideal by standard metrics — and then used what they knew, from experience, would give them the numbers they wanted to see in the fermentation vessel.

For example, it was not unusual to see values of 5 to 7 ppm dO2 in their process pipe, but 9 to 12 ppb dO2 in the fermenter. That’s a huge difference, but given that not all of the gas had a chance to dissolve into the wort before reaching the O2 probe, it’s a completely probably reading.  So let’s go back to the five metrics that I mentioned above.

The liquid flow and the gas injection volume, or gas pressure, are the two factors over which you should have the most control. If the liquid flows at a specific rate and the gas is injected in a similar manner, the action should be completely repeatable.  If you then measure the dO2 in the process pipe you should get a repeatable value if the first two factors are the same. If this can then be correlated to the dO2 in the fermentation vessel, you should be able to use the inline value to tell you when you have reached your target, even if the number seems improbably low.

If factors like liquid flow and gas pressure are so easy to control and repeat, and if being sure of those values is such a link to your ultimate fermentation vessel dO2 goal, then why measure inline dO2 at all? I’d say we measure for the same reason we perform any quality parameter.  If our overall goal is to maintain a consistent flavor profile, this is just one more way to insure that gas really is getting into the solution properly, and that the liquid flow is not out of whack. A system of reliable, repeatable monitoring will always be your best hedge against human error (hey, it happens!) and equipment failure.

My final thought is that creating perfect fermentation is part art and part science.  We use science to predict and check certain facts, but in the end the values we are “supposed” to be achieving may need the nuanced interpretation of the brewing artist.

Wort Dissolved Oxygen Measurements Simplified

As a brewer, you already know the importance of controlling dO2 in your finished product. But did you know that it’s just as important to measure dO2 in wort? The right O2 balance – usually following your yeast manufacturer’s recommendations — will keep your yeast happy, so they don’t impart unwanted off-favors to your beer. However, if you’ve ever tried to measure the dO2 of wort using the same portable O2 analyzer you use for beer, then you know the challenge of minimizing flow chamber clogging from bits of trub and hops.

Years of dealing with this problem have lead to a simple solution, and it really works: run a sample of aerated or oxygenated wort (chilled!) from a sample valve into a small Erlenmeyer flask or beaker while holding a small probe from a hand-held analyzer in the flask. There are a few tricks, but for the most part this is a very straightforward way to get your measurements.  Here are the main tracking points:

  • Use an optical — also called Luminescent Dissolved Oxygen (LDO) – probe, so the flow rate of the sample can be low. Electrochemical probes require a fairly high flow rate.
  • Run the wort from the sample valve and through a tube into the bottom of the flask. Use a flask with a reasonably narrow opening. Have the probe in the flask at the same time you are flowing your wort into the flask.
  • It’s important to have flow into the beaker. It can be low, but put the outlet of the tube near the sensor, so the sensor responds well.
  • Whenever possible, take your wort sample – the one you will use in the flask – from the process pipe before the yeast is pitched.
  • Likewise, take the wort sample as far from the air or oxygenation point as possible.
  • If you take your wort sample from the fermentation vessel, then you need to be on your toes, even taking the Erlenmeyer flask to the vessel so that you are measuring as close to real-time as possible. This is because yeast will quickly gobble oxygen, and that in turn can lead to significant underestimation of your starting oxygen value.

My final thought is that wort dO2 measurements can be as important as beer oxygen measurements.  Keeping the yeast happy is your first step to creating a fantastic brew.




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