Can topping up be a good idea?

[Rounding_error]

I have a 5 gl batch of wild/sour fermenting beer/cider* in a glass carboy. I am curious if it would be advisable to top the batch up to get the liquid as close to the air lock as possible to resist possible acetobacter. I've transferred the ingredients a fair amount and have not flushed air out of the secondary's head space.

After ~two months of primary and a month into the secondary, I tasted to make sure things hadn't gone wrong early on and found good results. The combo had an improving flavor and increasing sourness relative to the start of the secondary. The sourness is funky but does not seem vinegary. Out of caution, I plan to limit tasting in the future.

My inclination is to leave it alone and let the pellicle do its job. Still, if it's going to take numerous months more, would it be advisable to top up as a precaution?

This is one of my first sour batches, so any thoughts are greatly appreciated.

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*I primaried 4 gl of fairly generic ale with a saison yeast. Parallel to that, I primaried a gl of cider using champagne yeast. A month into the cider's primary, I added 2 lbs of unwashed cherries. The cider/cherries then fermented a second time with little head space and no formation of a pellicle. After a further month, the cherries dropped, the champagne yeast-looking cake tightened up, and visible fermentation ended. The gl was quite sour with a good hint of cherry character. I then combined the two batches -- cakes, cherries, and all -- in the ale's primary.

I came back after two weeks and the batch had thrown off a thin white pellicle, fermentation had started again, and the cherries were back at the surface. I racked the combo off the cake and left the cherries and pellicle behind. In a 5 gl secondary, barely visible fermentation went on for about another week or so and the pellicle reformed -- again thin and white.

 

[dantheman13]

Interesting sounding beer! Topping up isn't necessary, but I do it when it is convenient. Tonsmeire (you should buy his book, "American Sour Beers" if you haven't yet), says that he doesn't bother topping up, where as Jamil Zainasheff says to ferment in a 6 gallon fermenter, then move it to a 5 so as to get the beer as close to the neck as possible. I prefer Jamil's recommendation because less empty space = less possible air getting in during sampling, but the same amount of air probably gets let in either way.

I usually top up with wort rather than beer because it tends to be more convenient for me and because there's nothing wrong with giving the Brett/bacterias some more simple sugars early on in the aging process.

 

[MileHighBrewer]

I top up occasionally if I have extra wort. Nothing wrong with it, but it likely will change the flavor as you're feeding the bugs, which will put them to work again. I do it more for finished volume then headspace volume, and I hate wasting wort.