What does it do

Being still new to the craft I've been watching lots of youtube vids on brewing. One thing I see a lot is people adding air/O2 to the beer when its in the fermenter. So, what exactly does this do? Cause I thought oxygen was bad for the beer. Can anyone explain this to me?
Thx

 

I think you got that wrong. You are probably referring to the oxygenation step of the wort, before pitching yeast. Oxygen helps the yeast to multiply and ferment the wort. After you pitched the yeast and the yeast has began fermenting, you will have to avoid introducing oxygen in your beer.

 

You want a good burst of oxygen in the beginning because yeast needs oxygen to multiply effectively. Yeast is going to eat up this oxygen and multiply first. When it runs out of oxygen, it starts to ferment (=turn sugars to alcohol). When the fermentation takes place (and especially when it is done), you need to avoid introducing any additional oxygen to the beer.

You can agitate the vessel vigorously or splash the wort to introduce some oxygen from air. Or inject air into the beer with a pump. This kind of aeration will be enough for a decent fermentation, but higher levels can sometimes be beneficial. Those can be reach by using pure (bottled) oxygen instead of air, which only contains 21% oxygen. Lack of initial oxygen in fermentation vessel can lead to off tastes and even stalled fermentations because of poor yeast health.

 

Hello and welcome. My understanding is that the yeast require oxygen to do their job, make alcohol and co2. Boiling the mash significantly reduces the amount of dissolved oxygen in the liquid so you must replace it after cooling your wort and before pitching your yeast. If there isn't enough dissolved oxygen the yeast will essentially suffocate before they can finish fermenting and you will be left with a higher FG, sweeter beer, and lower ABV. Once fermentation has started, oxygen will create off flavors in your beer. I have produced some decent beers and been able to hit FG numbers by splashing wort onto the fermenter an swirling before pitching, but there is equipment to really add pure o2. Enjoy the hobby!

Chris

 

Oxygen is used by the yeast in the early stages to reproduce. So before (or at the same time) you add yeast, you should aerate the wort. A good shaking, or a whisk works well. Once you think you have aerated enough, you should repeat it. Just pouring into the fermenter adds some O2.

Dry yeast suppliers say you don't need to aerate, and the yeast itself has everything it needs, but I would still aerate.

You do NOT need to use Oxygen. For a beginner, aerating the wort is sufficient. On high gravity beers 10%+, pure O2 will help as you need to maximize the yeast, but for most beers the difference between a decent aeration of the wort and pure O2 is minimal (maybe .001 on FG).

And yes, after the first 24 hours aeration/O2 is bad.

 

Yeast have an aerobic (with oxygen) and anaerobic (without oxygen) phase of growth. As such, some small amounts of additional free oxygen can be beneficial to early phases of yeast growth.

Is it necessary? No. It's a step you can add once you've totally nailed your process. Think of it as 800 level brewing and you're just taking the 101 course, Intro to Brewing.

https://www.morebeer.com/articles/how_yeast_use_oxygen