Quick question - If I add a little bit of fresh apple juice/cider to a keg of bone dry cider and keep it cool (40s) will the yeast remain dormant indefinitely or will it start to ferment out?
Need more info! I find that fresh cider, non-pasteurized and non-stabilized, will ferment on its own in the fridge. If what you are planning to use is stabilized or pasteurized, I would expect it be a long time before there is fermentation.
I have back sweetened with frozen concentrate with good results.
It will ferment unless you stabilize with sorbate and sulfite… and even then it might ferment but slowly.
+1
If you use a yeast that isn’t very cold tolerant and keep it very cold (like 35F) it can work - at least short term.