It's surprising how many threads there are about this topic on these forums. Clearly there are many ways to work this successfully. But, being the analytical guy that I am, I have to ask a couple of questions:
1. If during primary fermentation, the production of CO2 is sufficient to "protect the wine from oxidation", as so many have pointed out, then how is it that keeping the cover loose, or leaving the airlock hole open could allow oxygen to get in at all? Therefore, what harm is there in putting the airlock on, since everything touching the must is CO2 anyway?
2. If you don't use the airlock in the primary, but rather just cover the hole with a towel, is there any harm in just using the 3-piece airlock with the middle piece removed? Wouldn't that accomplish the same thing (namely keeping critters out)? Again, it seems unlikely that oxygen could get in through that hole with all the CO2 coming out of it.
Overall, it just seems that the real answer is that the airlock is 'not necessary' during primary, but the idea that it's letting in 'oxygen needed for the yeast' seems unlikely. But, I'd love to better understand from the experienced guys
1) The CO2 does not seal out 100% of the O2. if present, some O2 will get to the wine. there's a lot more current on top of the wine than you might think.
2) Again, the problem is more that when someone seals the lid and adds an air lock, they will likely not remove the lid at all for extended periods of time. Wine needs stirred during primary to add plenty O2. If you have the wine locked down, it is not likely it is going to even get opened, let along stirred. With the lid not snapped down, some O2 will get in and to the wine. In my opinion, probably not enough will get in; that's why wine, even white wine, needs stirred periodically during primary. Yes, I know even some commercial wineries lock down huge tanks of white wine during primary. I really don't know how they get by with it, but I know they do.
Taking the inside piece out of a three-piece air lock is not the answer. Gnats and other very small critters can get through the small holes in the lid of the air lock. Besides, some of us prefer the "S"-style air lock.
I have written the following several times, but I'll do it again. I guess I need to post it somewhere that I can just post a link to it in the future.
As I have said, there are many different ways to ferment out the wine. Many people don't stir or otherwise allow air to get to their wine during primary and they produce good wine time after time.
Here's what the science says (paraphrased by me):
Alcoholic fermentation by yeast is divided into two main stages - aerobic and anaerobic.
During the first stage - aerobic, which is called primary fermentation, the yeast are acclimating and multiplying at an incredible rate, all the while they are converting sugars to alcohol, CO2 and other minor chemicals. This, as demonstrated by the high activity in primary, is happening at an enormous rate.
At some point in time, wine making instructions will have you lock down the wine by restricting as much as possible the amount of oxygen getting to the wine. This is the beginning of the anaerobic stage of fermentation.
This is the point at which secondary fermentation begins. Anaerobic and secondary fermentation are pretty much the same stage. Some kit makers have us close down the wine at different SG's. That's because they feel that is the correct point to start anaerobic fermentation for that particular wine combined with the particular yeast they have provided.
So, primary fermentation is really aerobic fermentation; secondary is the anaerobic stage.
One can restrict the amount of oxygen getting to the wine during the aerobic stage; the wine will likely turn out fine, if it doesn't get stuck, of course. However, in the end, it is possible the wine is not going to be as "complete" as it might have been. Maybe the yeast were starved of the necessary oxygen and maybe they did their job in the end, but maybe just not as well as they could have. Measuring the SG tells you how much sugar is left, but it doesn't speak of the health of the yeast.
Most stuck fermentations get that way during secondary fermentation. maybe that's because the yeast started that stage in an unhealthy state and they couldn't recover. That is just my own guess, though, but it sounds logical to me.
It's the science!!!
In the end, make the wine however you want. It's your wine.
I hope this helps.
