Can someone help me understand something?

So I understand you reaaaaaally don't want O2 while you're trying to ferment. But how bad is a quick whiff of it, as opposed to prolonged exposure? For example, let's say I'm in the middle of fermenting, and I take the top off the carboy for some reason, and O2 probably rushes in, and then I put the top back on. My guess is that the yeast now uses the available O2 to do aerobic respiration until the O2 runs out. But how bad will that be? How much will it affect the flavor?

I'm guessing it can't be that bad, because people have to take gravity measurements, right? Or do they only do that at the beginning and end?

Thanks!

P.S. Could someone also suggest a cheapish but decent hydrometer? Thanks!

The beer will be under a protective blanket of CO2 since it is heavier than air. So, as long as all you have done is pop the top have a looksie and covered it back upo you are safe. No harm. Nothing fouled.

Now, if you lifted the lid and then carried it up or down stairs, that is different.

Edit: Just realized this is cider forum. Same applies.

Generally, you won't get much oxygen opening the fermenter, because you have built up quite a bit of CO2 in the headspace. It does not rush out when you open the fermenter. There's nothing to worry about unless you're constantly mucking around in your cider or beer.

Thanks! Ok, let's assume that you opened it up, and blew into it really quickly to displace all the CO2 with O2. I'm just curious as to how much a small finite amount of O2 will mess things up.

After the O2 is used, it will go back to fermentation if there is still sugar left, right? So is the main worry with letting O2 get to it that it just produces less alcohol and more water, or something more sinister?

Thanks for the helpful responses!

+1 just don't muck around excessively. anyways, you do want O2 when you have active fermentation. furthermore, some cider makers and many wine makers rack each batch multiple times, each time (carefully) exposing the cider to a little bit of O2, and it almost always comes out good. if you have a homebrew or cider to hand i'd say pop that open and drink it fairly rapidly, then re-evaluate, you'll see that everything will be fine

i'd be a lot more worried about what kind of snotty crap and bacteria i blew into it with my mouth than a bit of oxygen getting in. are you going to rack to a secondary? unless you fill it with another gas that's got about 20% O2 in there...
be cautious where you can but don't sweat it!

I always give my cider a little shake once I put the airlock back on so it gives it a rush of co2 and pushes any air out. Idk if it works but it makes since to me

The risk of O2 is for oxidation of fatty acids thus cause the product to taste stale and flat. At the deeper end, in beer at least, it can become so oxidized that it tastes like "wet cardboard".

Now, I have never tasted wet cardboard but I have tasted oxidized beer. the flavor matches the smell of wet cardboard and that is enough for me.

Basilisk said:

Ok, let's assume that you opened it up, and blew into it really quickly to displace all the CO2 with O2.

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I would be more concerned about sanitation. Keep the oral experiments to your S.O.

Basilisk said:

I'm just curious as to how much a small finite amount of O2 will mess things up.

After the O2 is used, it will go back to fermentation if there is still sugar left, right? So is the main worry with letting O2 get to it that it just produces less alcohol and more water, or something more sinister?

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AFAIK, the issue is not less alcohol, it's staling. In an active fermentation, the oxygen will get kicked out right away by the CO2 being produced. In any event, it's not a good idea to introduce oxygen post-fermentation. What tiny effect a little O2 might have when added during fermentation, I can't say with any certainty. My best answer is that there would be some staling effect, but that you would not detect it.