Pure O2 Breakout During In-Line Oxygenation?

[Matt@Bauhaus]

We currently crack the butterfly valve on the CIP arm of the receiving fermenter when knocking out in order to prevent excessive pressure buildup in the tank. I'm wondering if we are allowing O2 to break out of the wort – and thus underoxygenating – by venting in this manner. We have noticed some variability in our attenuations – mostly underattenuation. While there are plenty of factors that could be contributing to this variability, I want to at least remove oxygenation from the pool of variables.

We don't have a DO meter. Rather, we use a sanitary wort flowmeter in conjunction with a rotameter for air (http://www.mcmaster.com/#flow-rate-meters/=vf6lxh) to determine our oxygenation rate. For worts < 16P, we typically oxygenate to around 10 mg/L. For worts ≥ 16P, we oxygenate with 1 mg/L oxygen for every degree P. We're generally knocking out at around 20C for all of our worts (both lagers and ales).

Do you guys vent your receiving FVs when knocking out?

[Rooh]

explosion

Seems like the alternative is an exploding fv. I imagine the pressure in the receiving tank would far exceed the manufacturers pressure rating if you didn't at least let some of the air out of the tank.

[Matt@Bauhaus]

Seems like the alternative is an exploding fv. I imagine the pressure in the receiving tank would far exceed the manufacturers pressure rating if you didn't at least let some of the air out of the tank.

Right, and if we didn't vent, our PRV would do it for us. Maybe my question should be whether anyone compensates for any O2 breakout during knockout by increasing the oxygenation rate.

[DageraadBen]

I think that no matter what you do, a significant portion of the O2 you put into the wort will never dissolve.

In Chris White's book "Yeast" there's a table of O2 rates and durations used by a dozen breweries and the actual DO levels each of them achieves. Analyzing this data shows that most of the breweries only get between 2.5% and 7% of the O2 they pump into their wort to actually show up as dissolved oxygen in the fermentor (a couple of the breweries are outliers at 0.75% and 17%). That means that about 95% of the oxygen you pump into the wort will blow off at some point. I doubt that any amount of head pressure you could practically apply would significantly change that.

On my 12 hl (10 bbl) system I oxygenate at 3.5 L/min for the duration of a 40-45 minute knockout. Oxygen weighs 1.43 g/L at normal pressure, so I've pumped about 225 grams of O2 into the wort, which would theoretically achieve 192 ppm. I hope that about 5% of it actually dissolves into the wort, giving me 9.5 or so ppm of DO. I wouldn't say I'm doing everything perfectly, but my yeast's performance over multiple generations has been pretty good.

[Matt@Bauhaus]

Thanks for that info, DageraadBen! Very helpful. I have the Yeast book, so I'll take a look at that data too.

[dick murton]

It depends whether you are using pure oxygen or air to oxygenate the wort. If you are using air, then you will only achieve about 8 ppm with 16 P wort - it simply will not dissolve much if any more than that. 10 ppm is about what you could get in water, at about 1 deg C. If you are using pure oxygen, then you could get close to 30 ppm in water, probably closer to 27 ppm in 16 P wort.

However, in all cases, you need for most rapid solution, high turbulence, by having a correctly designed mixer, or supply just before a centrifugal pump, or plate heat exchanger, plus high pressure - say 4 bar, (60 psi)

So the critical factors are low temperature, turbulence / small gas bubbles and high pressure. If any of these are not met, then you are simply wasting gas. With a decent system shouldn't need to overdose. You should be able to get 100%. So all those samples quoted are simply grossly overdosing, or (and probably) have poor solution conditions.

[Matt@Bauhaus]

Dick,

We're injecting inline with pure O2 via a sintered stone just after the wort passes through our single-stage, plate HX. I've been relying on the estimated gas flow rate efficiency factors published in Volume 2 of the MBAA's Practical Handbook series (it's the one on Fermentation, Cellaring and Packaging Operations) in order to hit a particular dissolved O2 target.

[MN_Nice]

We have our blowoff hose hooked up to our PRV arm and it's open the entire time during knockout with the blowoff bucket bubbling vigorously the entire time. That's pretty standard practice so I doubt that venting your FV is the issue.

One key variable that hasn't been mentioned is knockout hose length. Keep an eye on your sightglass at FV block and bleed using a short hose and then again using a longer hose (or series of connected hoses). You'll notice that in the longer hose lengths there's less bubbles present, which means that there's more O2 absorption happening along the longer hose length vs the shorter hose length.

Long story short, use a long hose for your KO line and you'll get more O2 pickup. Also invest in a DO meter and take a reading at the block and bleed connected to your FV. It's worth the investment if your beers are suffering from under attenuation. 1 PPM per degree plato O2 is what you're aiming for

[Matt@Bauhaus]

Also invest in a DO meter and take a reading at the block and bleed connected to your FV. It's worth the investment if your beers are suffering from under attenuation.

It's on my list - I know we need one! Do you have any suggestions as to a make/model? It would be great to have a meter I could just plug into a 1.5" tee.

FWIW, I increased our O2 rate in a few of our recent knockouts and we have achieved complete attenuations. So, we may have just been erring too far on the side of not wanting to overoxygenate (if that makes any sense!).