Monday, November 30, 2009

How much pressure can a fermentation achieve?

This ought to be known since, in principle at least, it is a pretty simple experiment to envision. However, I have done a bit of searching on Pubmed and found nothing on this subject. There could be a couple of good reasons for this: 1. It could be such a well known thing that a published paper would be pointless. 2. There is no practical need to find out an answer to this question. Makers of sparking wine already know how much sugar to add to get the desired level of carbonation and avoid exploding bottles, so what more is needed?

Well, I am interested out of pure curiosity. But how to do the experiment? One could fill a strong tank with juice and yeast, then close it up and keep track of how high the pressure gets. One possible complication is that the yeast may run out of sugar before the pressure stops them. Or the alcohol may get so high that this kills off the yeast. Or the Carbon dioxide in solution may cause the pH to drop to levels that kill the yeast. A couple of these things can easily be eliminated, depending on what happens: Let's say the fermentation slows to a halt at 200 psi. We relieve the pressure and then close-up the system and it again builds to 200 psi. This would show that the alcohol is not too high and the sugar is not limiting. What about the pH? When you vent the reactor the CO2 will leave and the pH should be able to climb. The only solution I see to this would be to have a neutralizing agent in the mix, perhaps a bit of Sodium bicarbonate for instance.

Part of what interests me about this is that I don't have a good feel for the relationship between delta G (or the energetics of a reaction) and pressure. Surely it takes energy to generate pressure, so at some point the reaction would stop at some pressure. There are also concentration effects in chemical reactions. We have Sugar going to Alcohol + CO2 and this reaction gives-off energy. If neither the Alcohol nor CO2 harmed the yeast then at some concentration, the mere excess of either of these would slow and then stop the reaction from moving forward. This is Le Chatelier's principle.

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