Head space required in secondary?

Ingredients
1

I just finished reading Josh Weikert’s Divide and Conquer in the latest BYO issue which is about splitting home brew batches, and the idea appeals to me.

I’ve got a 5-gal batch of English mild fermenting in the primary, so anything I do with splitting this beer would have to be done in a secondary or at bottling. I’ve never done secondary fermentation before.

I’ve ordered two 1-gal fermenters to use to split part of the batch in the secondary, and thought of  different situations:

A: I would add a coffee extract, cold brewed with water.  This would require no additional fermentation. But if I wanted to cold-crash it in a fridge, how much head space should I allow in a 1-gallon fermenter?

B: I thought about adding about 2 ozs. of unsweetened grapefruit juice (yes, I know it contains natural sugars and yes I know most brewers don’t add it to mild or brown styles) for the second experimental gallon in the secondary.  I’m assuming this would go back to an active primary fermentation because of the natural sugar content (4 grams sugar/2 ozs. juice) so I should allow at least two weeks for it to end primary fermentation, check gravity and bottle.  Again, how much head space should I allow in this fermenter?

C. Also, if I racked 4 gallons of wort to a 5-gal fermenter as a secondary fermentation, will I have too much head space and risk oxidation?

Thanks in advance for your advice.

2

Unless expecting a secondary fermentation, you will want to limit the head space as much as possible. With a secondary fermentation, head space is not important. Just aging on some extract, oak, etc, limit head space. Not sure what is appropriate, but as little as possible. I would say putting 1 gallon into your new 1 gallon jugs with a coffee extract should require no extra head space. I would put in the extract, and rack beer onto it to essentially fill the jug. Just my thoughts, no experience.