I too just racked five gallons from primary to a six gallon carboy and was wondering the same thing. There was a lot of trub in the primary which meant it was closer to only four gallons that I recovered. I did notice that it was still emitting a random bubble here and there while siphoning it so I'm pretty sure the Co2 it produces will protect it. I have also learned, while searching this blog, that I should probably just leave it in primary for 2 -3 weeks, rack to the bottling bucket and bottle it. This is only my second batch (kit) but that's my plan for the next batch. Thanks for being such a great resource!