Why oxygen is bad for beer?

[ak47clown]

I'd like to get a better handle on the science behind brewing and was just wondering if someone could give me a better explanation on why oxygen is bad for the fermentation process. From what I've read in How to Brew and a few posts on the forum, you risk contamination and also it can stale the beer quicker. But what i'm after is why this is? I guess the contamination part is kinda self explanatory but I'd still like to learn some more about how oxygen reacting with the wort at that point would cause it to stale.

 

[albannach]

Some good info

Professor_Beer_Stale_Oxidized_Beer

www.aseanfood.info/scripts/count_article.asp?Article_code=11015522


Oxygen/aeration is good after the wort is cooled post boil to help the yeast start fermentation.

Oxygen/aeration is bad for wort when it is hot (hot side aeration) and it is bad for beer post primary fermentation.

 

[reim0027]

From what little I understand, here are the basics.
1. Fermentation where the yeast produce alcohol is anearobic process (happening in the absence of O2).
2. In the presence of O2, yeast are able to reproduce and adapt to their environment, but fermentation is impaired.

So, initially you want to aerate your wort before pitching your yeast. That way, the yeast can have a rapid growth and adaptive phase before they start fermenting the wort (so there are more yeast and they are happy in their environment).

Once the yeast are done with their growth and adaptation, you want them focusing on fermenting and making alcohol.

That is my basic understanding of it. If I am wrong, somebody please correct me.

 

[Ninjutsu PiloT]

in an attempt to build on this thread and ask one of my own questions, how much O2 is bad? in other words if I expose my brew to oxygen moving it from the fermenter to the bottling bucket I have then is 10 minutes ok? is 1 second going to do all the damage it potentially could? At what point can I say I've exposed it for too long?

 

[Bernie Brewer]

in an attempt to build on this thread and ask one of my own questions, how much O2 is bad? in other words if I expose my brew to oxygen moving it from the fermenter to the bottling bucket I have then is 10 minutes ok? is 1 second going to do all the damage it potentially could? At what point can I say I've exposed it for too long?

If you're not splashing your beer around and not making bubbles, then you're not aerating it. Yes you are exposing it to the air, but you are not aerating, or injecting oxygen into it. No worries.

 

[reim0027]

The air is only 21% O2. Therefore, you don't have to worry about over-oxygenating it (by agitation). You will never get the wort's O2 percent higher than the O2 in the atmosphere without injecting pure O2 into it.

I don't inject pure O2 into my wort, so I don't know the specifics to that procedure.

 

[ak47clown]

Thanks for the info. I guess where i was going with this was the question ninjutsu asked-exposing the wort to o2 when transferring it to a secondary. For my first batch, i defintely had some issues with the auto-siphon and sucked in air a number of times which of course bubbled up the wort in the secondary. Overall, i don't think it came out half bad though I guess I can't really compare it against anything else for now...

 

[reim0027]

Shoot, I misunderstood the question. I assumed you were referring to preoxygenating your wort prior to pitching the yeast. Once fermentation has started, you don't want to add any oxygen to it. Disturb it as little as possible. Regarding racking to a secondary or bottling, I don't know how much is too much O2.

Sorry for the misunderstanding.

 

[albannach]

Make sure you use firm fitting hose on the auto-siphon to prevent those air bubbles being sucked in at that joint. If you're transferring to secondary just make sure you don't "splash" or agitate the beer too much, and most importantly keep your siphon hose(s) submerged in the already transferred beer. Other than that, just having the beer exposed to air isn't anything to worry about during racking.

 

[CBBaron]

O2 is needed by the yeast to reproduce in the early stages of fermentation. Once that stage is complete then adding O2 to the beer can lead to oxidation in the beer. This is an oxygen reaction with compounds in the beer leading to off flavors.
Transfering beer using a siphon without splashing minimizes the o2 that dissolves in the beer during bottling. There is CO2 dissolved in the beer from the fermentation and some of that will escape and protect the beer during the bottling process. In addition during the carbonation process the yeast will use much of the little bit of O2 available to them.

The O2 that is left in the beer is the main limit to aging the beer long term. However most of the oxidation processes are slow so the small amount introduced are of little concern.

So just avoid splashing when transfering the beer and bottle efficiently after transfering but don't worry about rushing.

Craig