Everything I read/hear says to top up carboys once off the secondary. Of course I understand that, but how sensitive are the wines to oxidation time-wise? I.E. 5 minutes or 5 days?
The reason I ask is I'm finding it difficult to manage the changing volumes in the process. When you rack off sediment you now have ullage that needs to be managed. So you top up but later you'd like to add something else (juice for example) but you have to remove wine to do so. This oxidation thing is not convenient.
There is not a definite answer to your question. 5 min. probably will not hurt a thing. 5 days ... That depends. If unsulfited you are in trouble. If it is properly sulfited, ... that depends as well. What is the surface area over which the air is acting? If it is a carboy filled up into the neck so the surface area is small, probably it will not ruin the wine though it is not good for it. If it is a carboy that is filled below the neck so there is a large surface area, you are probably in trouble.
The solution: Keep them toped up. The method: There are several. My choice is to have a variety of carboy sizes on hand so I can go to a carboy that is about the proper size so I do not have to add much top up fluid. From a 6 go to a 5, from a 5 go to four
1's and a 1/2. Then to do the little topping up necessary, use a similar wine or water. Do not use fruit juice unless you want a sweet wine. If you do, this is fine but use sorbate and sulfite to prevent a refermentation or you are right back where you started.
Another method is to purge the air space with a more or less inert gas. You can use CO2 or N2 effectively.
Others who are more knowledgeable can probably tell us how sensitive it is. I think this is one of the areas where you need to pay attention and try to do more not less. If I remember correctly, Lum once responded to a posting of mine that a 4 hour drip filtering (because I had added a second pad) was getting dicey.
Another tip that I use after racking, is to give the carboy a little swirl to force out some CO2 and displace the O2 in the neck.
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