Protective Layer Of C02 On Fermenting Wort

[vlbaby]

This question has bugged me for a while, and I cannot find anywhere on the net except for brewing sites where it says co2 forms a bacterial protective layer.
I personally can understand it forming a layer that will protect against oxidisation since the c02 is much heavier than oxygen. But the concept that a rogue bacteria could not penetrate into your wort because there is a layer of co2 on top, this just doesnt seem to sit right with me.
I'm aware that there are brewers that ferment with open fermenters without any problems, so is this because of this protective layer at work? If so how does it work?

Can anyone shed some light on this? Perhaps i have misunderstood this suggestion of a bacterial "shield" that many brewers seem to support.

Its no big deal that i MUST know, but i'm just curious.

cheers

VL.

 

[DJR]

It's more about the fact that the CO2 being produced by the fermenting wort only contains cells from the yeast doing the fermentation, therefore displacing all the other air around it which contains nasties. After a while nothing is left but the CO2 and yeast produced by the fermenting wort and no other nasties can get in. That's the theory anyway, in really hostile environments where a lot of wild bacteria/yeast was present in the air you could probably still get infected.

 

[PistolPatch]

Hi VL,

The CO2 layer as you mentioned is the main thing preventing oxidation of the beer. As for nasties that are heavier than air going into the brew, you are right again, they'll be going in.

Once in there however, the CO2 environment is not a ver friendly one for most organisms which need O2 to succeed. So effectively, many nasties will 'suffocate' in the CO2. This is why in open fermentations you are more likely to get a wild yeast infection than a bacterial one as yeast is one of the few organisms that can operate in a CO2 environment. I am unsure of what other organisms can. Oh, that's right - trees and plants in the daytime! I have been very lucky so far though as no trees have been managed to find their way through my airlock and into the fermenter!

Cheers
PP

 

[vlbaby]

Hi VL,

The CO2 layer as you mentioned is the main thing preventing oxidation of the beer. As for nasties that are heavier than air going into the brew, you are right again, they'll be going in.

Once in there however, the CO2 environment is not a ver friendly one for most organisms which need O2 to succeed. So effectively, many nasties will 'suffocate' in the CO2. This is why in open fermentations you are more likely to get a wild yeast infection than a bacterial one as yeast is one of the few organisms that can operate in a CO2 environment. I am unsure of what other organisms can. Oh, that's right - trees and plants in the daytime! I have been very lucky so far though as no trees have been managed to find their way through my airlock and into the fermenter!

Cheers
PP

[post="112465"][/post]​

Thanks for that Pistol patch. I think i understand a little better now. i suppose given that most bacteria need o2 to survive, then i guess a freshly airated wort that is awaiting fermentation to kick in is in its most vunerable.

Thanks again anyway. I'm just trying to dispell truth from fiction with many of the old wives tales that get around homebrewing. I have a homebrew book that says that the most important time to watch out for infection is when fermentation is completely finished not before.
Very easy to become confused with so much conflicting information.

VL.

 

[Darren]

VL,
Open ferment is very likely to cause an infection, ESPECIALLY in a converted fridge/fermenter. There is no were for the co2 to go so goes around and around. Plenty of wild yeasts living in brewers fridges.
An ale in the middle of the room you will probably get away with it. Don't open-fermet in a fridge though.

cheers
Darren

 

[PistolPatch]

Cheers VL!

I think you are right and the book is wrong! I also think that most people here would agree that the most dangerous time for infection is at the start of fermentation.

I know what you mean about it being easy to get confused though!!!

:beer: PP

 

[pint of lager]

The term of protective layer of CO2 is a big misnomer.

Air that we breath and walk around in contains CO2. If it sank and formed layers, we would all be dead.

From basic science, gases follow brownian motion, their size, when compared to the fermenter, is very very very small. They spend all their time zipping around in the fermenter bouncing off the walls and rarely hit another gas molecule. If they hit the surface of your wort, they may be absorbed,or they may bounce off.

The big important event that happens in fermenting wort is that CO2 gas is produced. Some stays in solution (we all see the bubbles when we try and take an sg sample) and the rest comes out the airlock. Alot of it. Some time back I did some estimations about how much gas is produced from a 23 litre standard brew, it was around 230 litres of gas. This will purge the headspace of just about anything else in there, so that all the headspace is occupied by CO2 rather than a mix of standard air.

The only time there is a layer of CO2, is during active ferment and there is a layer of foam on top of the wort, this would be all CO2 bubbles. This may stop briefly any falling bacteria getting to the wort, but they would probably sink in. There are plenty of advocates for both open and closed fermentation. Me, I use closed.

 

[berapnopod]

Like what PoL said.

Due to random motions of molecules, a blanket of CO2 to keep out stuff doesn't actually exist. If you have an open fermenter, its still possible to get significant oxygen into the beer just by the random motions. That includes oxygen and that includes wild yeast and bacteria. What is important is if you have a closed fermentor then all the CO2 produced during fermentation will blow out the airlock, create a positive pressure in the fermenter and stop nasties getting in. Oxygen and all.

Its this positive pressure that keeps your beer safe. Not the CO2.
BTW, CO2 is not going to kill wild yeast or bacteria for that matter. Sure, some of them like to use O2 to respire, but just like the Saccaromyces you use to ferment your beer, they happily switch over to an anerobic state to ferment. If CO2 was a problem for them then you wouldn't get a fermented beer!

You'll notice that all the commercial breweries that use open fermentation have a very short shelf life. British real ale is typically good for a few weeks. Anchor Steam Beer in the US is fermented in open style cool ships. It doesn't have a long shelf life either.
BTW, Anchor may well ferment in open vessels, but apparently you need to go through an airlock-type door to enter the fermenting room, because they keep the whole room under positive pressure.

You may well be wondering how anyone with open vessels can get away with making anything that is drinkable. Well, I think this is because the brewing yeast used in fermentation is so dominant, and ferments so quickly (compared to wild yeast and bacteria that typically work on timescales of months) that the nasties don't (usually) have a chance to take hold.

Berp.

 

[poppa joe]

Ater ferment and when you rack...
What goes on in air space..?????????????
Pj

 

[berapnopod]

Ater ferment and when you rack...
What goes on in air space..?????????????
Pj

[post="112590"][/post]​

air, silly h34r:

But now the beer has its own natural guard against nasties. :beer:

Berp.