Excess headspace in secondary...but what if there is more fermentation?

[TripleC223]

Trying to confirm a hypothesis here...

So I've read that excess headspace in a secondary fermenter (think 2.5 gallons in 5-gallon carboy) is a no-no because all the oxygen contained in that headspace can spoil the beer. I know purging with co2 can combat that, but like many brewers, I don't have any co2 on hand.

So with that premise, if you were racking into a secondary on top of fruit, wouldn't the secondary fermentation triggered by the fruit sugars create a protective blanket of co2 just like in primary fermentation? So if you're racking onto something sugary that you know will cause more fermentation, is it safe to leave excess headspace in a secondary?

And taking it a step further, would adding a small amount of fermentables after transferring to secondary be a reasonable way to create a protective blanket of co2?

 

[CJ-3]

For the most part I do believe you are correct. I just am not 100% positive with the specific example you have where the secondary is 2x the volume of the beer.

 

[BobBailey]

You are correct about racking onto fruit or other fermentables. The ensuing fermentation would create plenty of CO2. The only other reason I would use a secondary would be for bulk aging, and then I would want almost no head space.