Bottle bombs and Le Chatelier's Principle

[BOBTHEukBREWER]

If a chemical reaction is increasing the volume ie giving off CO2 for us brewers, reducing atmospheric pressure will speed the reation up. Conversely as the pressure builds up in a bottle the reaction will slow down to the point where it has in effect stopped. This assumes constant temperature.

 

[paulster2626]

I was just thinking this myself...

 

[Guess42]

and?

 

[seabass07]

Are you talking about reducing pressure outside of the bottle? It would have no affect on what is going on inside the bottle. But it would lower the internal pressure required for a bottle bomb.

 

[Guess42]

Ok avoiding being a dick from before, other things need to be taken into consideration. First, the CO2 moves out of the cell and therefore the concentration of CO2 around the reaction would remain fairly constant, thus relieving the equilibrium pressure. Second, the numerous reactions in a biological metabolism are more complex than merely saying sugar->CO2. Third...swear I had something here.

 

[zeno27]

As I understand it, Le Chatelier's principle is important for chemical equilibria. Since yeast can't generate sugar from CO2, increasing the concentration of CO2 or pressure won't have any effect on the reaction. (i.e. sugar -> CO2, not sugar <-> CO2)

Now, increasing the pressure might change the yeast metabolism, and thus have a kind of second-order effect, but I don't think Le Chatelier's principle is important.

 

[seabass07]

Since decreasing pressure means releasing co2, doesn't that defeat the purpose of bottle carbonating?

 

[bowiefan]

If a chemical reaction is increasing the volume ie giving off CO2 for us brewers, reducing atmospheric pressure will speed the reation up. Conversely as the pressure builds up in a bottle the reaction will slow down to the point where it has in effect stopped. This assumes constant temperature.

But the CO2 produced in a bottle of beer is one byproduct of a complex biological process not a reversable chemical reaction, and a bottle has a stress limit which is not theoretical. While it is possible to stop the process with pressure alone, yeast have a wider survivable pressure range than does a glass bottle. Also, as the biological process is one way (ie: yeast do not turn CO2 and alcohol back to sugar) a state of chemical equalibrium is never reached. The yeast just eat until there is no remaining fermentable sugar, or the environment becomes toxic due to alcohol levels.

Edit: zeno posted while I was typing.

 

[BOBTHEukBREWER]

or the yeast die of old age with clogged up arteries due to the stress of being under high pressure......

 

[AntX]

Yeast will NOT die of high pressure in glass bottles... just saying. 4 volumes is not enough, and that's why you can reuse/wash bottle yeast.

 

[BOBTHEukBREWER]

but it could become dormant, maybe....

 

[SickTransitMundus]

What bowiefan said. Not reversible. Le Chatlier's principle does not apply.

 

[BOBTHEukBREWER]

ok so it might be dormant because of the pressure...

 

[SickTransitMundus]

No, not the pressure. Yeast go dormant because they have exhausted their available food supply - sugars - and because the ethanol they have produced is toxic.

 

[BOBTHEukBREWER]

sictransit, do you absolutely know that high enough pressures do not cause yeast to go dormant - as it is true that increasing pressure slows the reaction......

 

[Guess42]

Not seeing the point in this discussion, as even champagne bottles would explode long before the pressure would have an effect on the yeast.

 

[ayoungrad]

Even assuming that a high enough CO2 pressure could cause the cessation of yeast CO2 production, the concept is irrelevant to beer production...

The pressure of CO2 that would have any relevance if CO2 pressure could cause cessation of CO2 production is the CO2 pressure in the beer itself, not in the headspace of the bottle. It is true that as pressure in the headspace increases, more CO2 will be dissolved in the beer but the pressure of CO2 in the beer will never approach levels even close to that needed to stop CO2 production. i.e. The bottle will burst before any equilibrium is reached.

I also don't understand this thread. If the theory were true, over-carbonated bottles would never burst. But they can. And they do.

 

[SickTransitMundus]

This is basic first-year chemistry and physics. Increasing pressure does not slow the reaction because it's not reversible. You can't push ethanol and CO2 into yeast and expect it to crap out maltose.

Since you seem to want more than a theoretical explanation of why this is not an issue for homebrewers, here's a paper on the effect of high pressure on yeast.

http://cat.inist.fr/?aModele=afficheN&cpsidt=17034616

They report that pressure stress does not kick in until 500 atmospheres. Let's assume that you've carbonated your beer to 13psi, and that hydrostatic pressure from keg/bottle height is negligible. So the yeast under 13psi=0.8atm of pressure on top of the yeast + 1 atm at sea level gives 1.8 atm of pressure to the yeast. That's 0.4% of the pressure required to affect yeast, according to this paper.

Champagne bottles break around 10 atm. Therefore, bottles will bust at 2% of the pressure that would affect yeast.

To conclude (yet again), pressure is not an issue.

 

[BOBTHEukBREWER]

as the OP I declare this thread done well and truly thank you all.....