just finished , , , , I have head space in the carboy. Can I put a vacuum on this or what do I need to add to bring it up to the neck?
* if you have just finished a primary you are still outgassing. ie the vacuum you pump out will be filled with CO
2 . CO
2 in the headspace is a good strategy to prevent oxidation, basically this is our goal when we have an airlock or burper silicone cork or the traditional put a balloon over the neck of the carboy. As a new wine in secondary you are safe only as long as gas is being pushed out. You need a longer term solution within a few weeks.
* Bryan has pointed out glass marbles. I have seen similar ideas as Amazon had plastic balls to sink into a wine, or I bought LDPE one inch rod, weighted it with SS and filled the space up to about 3 or 5 cm. Your key in this approach is what can you find that is food grade? won’t react like brass and iron? and fits through the opening? I have tried plastic like Nomacork but it floats and eventually could be a raft for mold to grow. ,, I wish there were one inch wide balloons to fill with water and sink into a carboy.
* this is a new wine. When I have racked from primary to glass with airlock i will routinely top off to 4cm with the juice that I was fermenting. There are live yeast to eat the added sugar. ,,, I won’t do this at second racking/ comming off gross lees.
* you have vacuum, good! One person in the wine club has check valves on all his carboys and will repump the vacuum as it is consumed. He in the process is degassing the wine, ,,, the method works as long as you keep negative pressure. Once the wine is fully degassed / six month this is more viable / less work.
* on my wines I try to go from about 5 liters into a 4 liter glass into a 3.78 liter / gallon,, into a 3 liter. OR 7 gallon into a 6.5 gallon into a 6 gallon. I have a variety of containers. I am still looking for more 10 liter/ 2.5 gallon size/ have just one from Europe. ,,, You didn’t mention the volume carboy you are working on.
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You are asking a good question. Oxidation is a major fault that shows up in contest wines.