I just got around to doing some serious cider work. The last few years have been hectic and I've been pressing and doing a little racking but the back log has been overwhelming. I finally got around to kegging several carboys that had been in secondary. I usually have a taste (or two) while I'm doing this work and while one of the ciders tasted great, dry, tart but rounded and well balanced with the suggestion of sweetness and apple after flavor (perhaps it underwent MLF); the other two were very sour and almost seemed to be off. I kegged them anyway, determined to make the most of it and sure that even bad cider could be easily pawned off on drunk friends late at night. Before sealing them up I added a quart of Santa Cruz Organics sour cherry juice to one and a teaspoon or so of both tannin and acid blend to both. I crossed my fingers and began drinking the tasty cider. Well, last night I lifted up the keg of tasty cider and realized that it was so tasty, it was almost gone. I switched the keg tap over and poured. The cider I had added the sour cherry juice to was delicious! The flavors had blended well and the sour cherry covered up the gross sour cider flavor. The tannins and the acid blend gave it a distinct tartness and filled out the flavor profile. The other keg is still sitting there waiting to be tapped. I'll lift the sour cherry cider keg soon and it will feel light. Then I'll tell you how the other one is.
The moral of the story is: Don't toss a cider that is not immediately appealing. Age it and if it still tastes like crap, add some stuff to it. We are scientists. Drunk scientists.