if oxygen feeds yeast, why not shake constantly during fermentation?

I'm a little confused about what aerating does. My understanding is that it oxygen helps make it easier for the yeast to digest the sugars.

So given that, why not constantly shake the fermenter during the process to keep it aerated? (or maybe shake every few hours or days or whatever).

My guess is the answer is that you don't want to risk bacteria getting in. But what if you shake the fermenter while the airlock is on, so that nothing gets in to the fermenter, so you're only aerating air that was already in there to begin with?

You're not too far away from fact, but you're missing a few key pieces. Yeast really require oxygen during reproduction. Once the reproductive phase has ended, they switch to eating...i.e., fermenting. During fermentation, little oxygen is required, and any oxygen that is introduced is likely to cause oxidation. Oxidation results in stale off flavors like cardboard. So (under typical circumstances), an initial dose of aeration is all your beer requires.

Also, shaking the fermenter with the airlock in place will not produce any significant aeration. Once fermentation has begun, the head space is likely full of carbon dioxide, so you won't accomplish much other than rousing the yeast.

Aeration is so the yeast can multiply prior to fermentation. Once fermentation has begun aeration isn't as much of an issue. If you aerate the wort you risk oxidation, which can cause off flavors. Aeration before fermentation= good, aeration after fermentation= bad. Once fermentation begins, leave it alone and let the yeasties do their job. Your main concern at this point is temperature.

Edit: Yuri beat me to it. I need to learn to type faster.

Some guys are leaving the air lock off and putting a sanitized piece of aluminum foil over the top of the carboy. I'm doing that on my latest batch so the fermenting beer can breath.

If you leave the air lock on and shake, I dont think it will give the yeast oxygen. The yeast used all the oxygen in the wort already and has replaced it with CO2.

The yeast store the oxygen they need for about four reproductive cycles. So you must pitch enough heat to make 50 million per ml within four generation . And provide enough oxygen at the beginning.
After the yeast reached the target population, it ferments.
It does't keep reproducing since 50 million cells is the most that can live in a ml.
So any oxygen you add here just causes trouble.

Jolly McStanson said:

Some guys are leaving the air lock off and putting a sanitized piece of aluminum foil over the top of the carboy. I'm doing that on my latest batch so the fermenting beer can breath.

Click to expand...

Some people (including me) do this for yeast starters, but I'm not sure what the point of this would be on a primary fermenter. The beer really doesn't need to breath during fermentation. I'm not sure what the problem with an airlock (or blow off tube) is but maybe that's just me.

i wonder why certain breweries do open fermentation, ie Schneider Weiss. what benefits are gained from it? it seems like a lot more work and money to keep a whole room (especially of that size) sanitized.