# Henry's Law

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#### Pommy

##### Well-Known Member
I had a brewer explain to me that due to Henry's law there must always (in normal circumstances) be the required amount of oxygen in the wort because as it cools it has no option but to enter solution. Does this apply to brewer's wort with it's increased gravity?

http://en.wikipedia.org/wiki/Henry's_law

He argued that for normal gravity beers (~1.050) yeast will have the recommended levels of O2 as the amount in water and the recommended levels in wort are both around 12ppm, and although the oxygen is boiled out of solution it will return as the wort is cooled.

I'm no physicist and although his explanation made sense it would go against what we are usually taught, "feed your yeast"

Please keep this simple for me..

One key point is nothing happens instantaneously.

In "equilibrium," yes, there will be a specific amount of a gas dissolved in a liquid as a function of the partial pressure of the gas, the type of liquid it is sitting on and the temperature of the system.

Given time to reach equilibrium, there will be a certain amount of O2 dissolved in H2O. Or, a certain amount of O2 dissolved in a wort.

Don't know what that is, gravity, temp, etc.

I don't know how long it takes to reach what level of O2 in a wort or if it's enough for the yeast...

So, I add a drop of olive oil to give the little buggers their fat.

Cheers!

A little knowledge is a dangerous thing.
Henry's Law, one of the many ideal gas laws, gives the equilibrium condition for a gas dissolved in a liquid under constant pressure. What Henry's Law does not address is the time frame it takes to reach equilibrium.
After you boil wort, most of the oxygen has been knocked out of solution. (There's around 1 ppm oxygen gas dissolved in just-boiled water.) If you cool your wort down to ale fermentation temperatures, Henry's Law would predict that the wort should hold something around 9 ppm (or whatever, the argument doesn't change as long as this number is bigger).
However, it takes time for oxygen to dissolve. Your wort would reach a higher level level of oxygen if you let it sit at atmospheric pressure for a long time (days?), but by then it would be contaminated. That's why brewers force oxygen into the wort with aeration stones.

Chris Colby
Editor
beerandwinejournal.com

The oxygen will find its way back in, but it takes a while. Gases mix quickly, but diffusion into liquid from a gas is very slow, if the interface is static.

A barleywine ages for months. A shook-up beer oxidizes in hours.

Simple enough?

OK I get that. Is there any way to know how long it would actually take, I'd be quite interested to read up on this concept? I assume that the speed of equilibrium could be increased by splashing, or would I be right to assume that although it would mix no much O2 would actually diffuse into the wort?

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