hey guys im a new brewer and I wanted to ask if during primary fermentation if oxygen is required for the yeast I know that the yeast is a facultative anaerobe so it prefers oxygen but does not need so why then do we limit exposure during primary fermentation with the gromett and air lock? Also how does this effect secondary fermentation.
new brewer fermentation question
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Yeast needs oxygen to reproduce so we make sure there is plenty of oxygen in there at the start. The yeast will quickly scavenge all the oxygen in the container, multiply, and then start making beer. At this point we want to limit oxygen as once the yeast run out of food, they will stop scavenging oxygen and any left over will oxidize your beer.
If you are bulk aging your beer for months instead of weeks, you might want to move it to a container with zero headspace. There is no fermentation going on at this point. True 'secondary' fermentation doesn't start until you add more sugar, like priming sugar at bottling time. That will cause the yeast to re-activate, scavenge any oxygen in the bottle, and carbonate the bottle.
If you are bulk aging your beer for months instead of weeks, you might want to move it to a container with zero headspace. There is no fermentation going on at this point. True 'secondary' fermentation doesn't start until you add more sugar, like priming sugar at bottling time. That will cause the yeast to re-activate, scavenge any oxygen in the bottle, and carbonate the bottle.
An "air lock" cannot prevent O2 ingress...
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Could you expand on this idea? If I understand correctly, the air lock doesn't prevent O2 but slows it way down. Is that about right?
Yes. During fermentation, the airlock allows c02 to be expelled since it is produced as a byproduct of fermentation. But once fermentation slows and stops, small amounts of oxygen will come through the water in the airlock. That's why it's advisable to have very little to no headspace in secondary, if using a secondary clearing/aging vessel.
As Yooper alluded, "air locks" offer no absolute control of ingress. And I doubt they can "slow down" physics. I mean, you have some fluid between two open ports, and you have atmospheric partial pressure of O2 that's driving molecules into whatever fluid is inside the air lock, and you have a fermentor vessel that isn't being held in perfect temperature balance to the outside ambient, so it's going to "breath", and those molecules will end up in the fermentor to keep the universe in balance.
If one is actually concerned about keeping things out of a fermentor one should totally isolate said fermentor from the source of those things. For the utterly OC solution, once active fermentation has ceased I connect my fermentors to very low pressure CO2 (~ 0.4 psi) from that point on. Others rack to a keg with a few points of fermentables extant, seal it up with a spunding valve attached, and let nature take its course without fear of O2 creeping in...
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When your beer is done fermenting it contains an excess of CO2 that it cannot hold in solution. For quite a long time that CO2 is being released and will have a slight pressure on the airlock, thus slowly bubbling it. That keeps oxygen out, not completely, but mostly. Oxygen only slowly dissolves into water unless it is agitated and then would be very slow to move into an area with a higher pressure of CO2. That's why people who leave their beer in the fermenter for 4 weeks do not have undrinkable beer, it simply does not oxidize much.
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Another little tidbit, CO2 is heavier than just O2 so it will have a tendency to sink. After primary fermentation is done and there is no more turbulence, the headspace in a fermentor will be very still (as long as you don’t touch the Carboy). In that environment, CO2 will settle and form a protective blanket above beer. In theory that should greatly reduce oxygen coming into contact with your beer.
Sorry, that's a complete myth that defies molecular motion that assures fairly rapid diffusion.
Further, CO2 moving out won't stop O2 moving in. Again, there are gas laws in play...
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Further, CO2 moving out won't stop O2 moving in. Again, there are gas laws in play...
Cheers!
The fact that the oxygen has to move through two liquid surfaces does slow it down a huge amount. If not, all beer would be terribly oxidated but I can leave a beer in primary for 6 to 9 weeks and still get the same flavor. If I were to leave it exposed to air for that time it would be so stale as to be undrinkable. Try leaving half a glass of beer sitting on your counter overnight and tell me about the aroma a flavor.
