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Currently studying for the General Certificate in Brewing from the IBD.

The section on wort oxygenation/aeration states that a disadvantage of in-line cold wort aeration is the

need to increase solubility by injecting small bubbles or ensuring vigorous mixing and injecting when wort pressure is high

Wouldn't the relatively cool temperature of the wort actually increase the solubility of the oxygen, and so reduce the need for vigorous mixing?

Or is this actually a reference to the aeration taking place inline, and so being at relatively higher pressure than when in the fermentation vessel? If so, I wondered why it isn't listed as a disadvantage for inline hot wort aeration as well.

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    Cold wort would be denser than hot wort. I would expect that to effect the solubility of O2 in wort as well. It does in water. Just a thought. – brewchez Feb 22 '17 at 10:50
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'Wouldn't the relatively cool temperature of the wort actually increase the solubility of the oxygen, and so reduce the need for vigorous mixing?'

For sure. Looking at those revision notes, I think they are probably trying to emphasize that it's important, regardless of when oxygenation occurs, to provide some aid to the oxygen in dissolving; it's one of the very few advantages of aerating hot that the pressure and (more importantly) the turbulent flow of the heat exchanger make gas dissolution, as a percentage of total gas injected, very high.

Even with the increased solubility at lower temperatures, if you do not provide some sort of restriction, dispersion or turbulence between the fermenter and the oxygenation point, a lot of that oxygen can arrive at the fermenter not fully dissolved, thus being wastefully added but also breaking out and causing foaming in the fermenter (which can strip volatiles and foam-positives out). This is especially likely to happen early in knockout when the liquid level in the fermenter is low and therefore provides little hydrostatic back-pressure. In my experience it's incredibly common to see even larger more established (pro) brewers practicing aeration so vigorous that most of the gas injected just comes right out before it has any chance to dissolve. If you can see enormous bubbles in a sight-glass, it's a pretty good guess that that oxygen isn't getting dissolved!

I will say I've studied quite a bit through the IBD and I find their information great in general, though they can be a bit obtuse at times.

Good luck with the exam!

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    One source of confusion is the (common) misuse of the the term "solubility". It really should refer to the "rate of dissolution". Solubility refers the the maximum concentration of solute in a solvent AT EQUILIBRIUM. In the case you reference the objective is to reach equilibrium as quickly as possible i.e maximise the rate of dissolution. Cooling the wort will increase the solubility, but for a given target ppm of oxygen a higher temperature and corresponding pressure will give an increased rate of dissolution. – Kevin Sharp Mar 27 '17 at 4:53

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