Bread dough - CO2 cannot itself form bubbles, it must diffuse into existing bubbles?

I was discussing bread making with someone on another forum - he’s a fairly experienced baker, by all accounts - he stated that carbon dioxide produced by yeast cannot, of itself, form bubbles in the dough - it can only diffuse into existing bubbles of ordinary air that have been incorporated during initial mixing.

The assertion comes supported by citation of this book - which looks like it’s pretty established science.

So my question is: why is this the case? Why cannot CO[sub]2[/sub] excreted by yeast, in dough, produce its own bubbles?

Well CO2 forms plenty of bubbles when I open a can of soda, and I don’t think there’s any ordinary air there. It could contain some dissolved oxygen as well I guess.

Sure, but that’s not yeast making the bubbles - it’s coming out of solution.

My first thought is that it’s not so much that the CO2 can’t form bubbles, more that it doesn’t do it very well on its own (at least for the purposes of making bread).

As far as I know, however, CO2 may not be able to make the bubbles in bread on its own, but my intuition is that the situation is closer to the one I stated.

This book goes into a little more detail:
Technology of Breadmaking

Carbon dioxide’s high solubility and relative inability to form gas bubbles are directly related to its propensity to form carbonic acid in water. (see this post in the thread " Pressing Scientific Issue: Do Pump-Caps Preserve Soda Fizz?, Part II")
The slow CO[sub]2[/sub]/carbonic acid equilibrium essentially buffers the carbon dioxide concentration at saturation.
Since bubbles form in super-saturated solutions, CO[sub]2[/sub] isn’t as quick to form them as most other gases.

The problem might be one of bubble nucleation, the formation of initial, very small bubbles. There is a substantial energy hurdle to get over for small bubbles to form.

This is related to the phenomenon of superheating, where clean water heated in a smooth-sided vessel such as a test tube can be raised substantially above boiling point without forming steam. When a successful nucleation event finally occurs, a whole lot of water turns to steam at once, “bumping” the water. To prevent this from happening, “anti-bumping” stones can be added to provide bubble nucleation sites, so the water boils continuously rather than going through superheating and bumping cycles.

Yeast forms CO2 slowly, and it will initially go into solution in water in the dough. When the water is saturated with CO2 it will “want” to come out and form bubbles. But if bubble nucleation is hard, it will require a degree of supersaturation before the bubbles can form. If the CO2 is diffusing into existing bubbles and making them grow, the supersaturation will never be achieved and so CO2 won’t form its own bubbles.

I guess this is important because it’ll affect the texture of the bread. The more pre-existing bubbles there are to act as nuclei, the finer the bubble distribution will be, which is probably a matter of how much kneading was done.

The tendency for dissolved CO2 to form carbonic acid is a complication, but I’d bet on bubble nucleation being the most important factor here.

let’s try that again:

I expect there’s more than one correct answer here. Bread dough is a complicated system, and it’s easy to find critical variables.
Here’s a bit on CO[sub]2[/sub] supersaturation:

I’ve made bread of both hole types. The trick is in controlling it so you get the size distribution you want.
Apparently, even vacuum cycling can be useful in making some types of bread doughs. It’s time to drag the vacuum pump out of the basement. :wink:

So is this one of those ideal gas law/partial pressure things? - that the CO[sub]2[/sub] exists in solution at a concentration not sufficient to emerge as its own bubbles, but more than sufficient to compel it to mix with existing bubbles that contain a low concentration of the gas?