After an amazing success a couple years ago making lemoncello and candied citron from some unloved citrons that wound up on the distressed produce rack at my local grocer, I've done some experimenting with candied citrus rinds on several occasions when old citrus was going for cheap. A couple of successes and quite a bit of recipe refinement resulted, but I also had a couple of spectacular failures while learning that if citrus fruit has been held forever on the store shelves until the peel starts to dehydrate, the candying process won't work; it may taste fine but it will always have the unchewable texture of old shoe leather. On the other hand, I discovered that "old shoe leather" lemons can be pressure cooked with sugar to yield an amazing "bitter lemon" syrup that's excellent in mixed cocktails, summery seltzer drinks, and for making quite passable shandies (traditionally a beer/lemonade mixture) out of cheap/bad beer.
A digression about bitter lemon -- this used to be sold as a beverage (I think it was a Schwepps product) but I haven't seen it in years. In that case the bittering agent was quinine, as with tonic water, and the carbonated lemon/quinine/sugar/sparkling-water mixture was extremely tasty. It was sold for mixing with gin, I believe, but I used to just buy it for a beverage before it was (apparently) discontinued.
Well, virtually every recipe that involves cooking or making beverages and syrups from citrus calls for the juice and the zest, painfully separated from the bitter pith. (The bitterness of citrus pith varies a lot, but it's especially pronounced in lemons.) My happy discovery when I "failed" at making candied lemon slices from very old leathery lemons was that -- after I gave up on simmering them and put them in their syrup in the pressure cooker for half an hour to see if the rind would ever soften -- the resulting syrup was extremely lemony, golden brown from carmelization of the sugars, and quite pleasantly bitter from the prolonged cooking of the pith. It reminded me a lot of my long-lost bitter lemon beverage, and a bit of experimentation determined that my "bitter lemon syrup" was an extremely useful mixer. I ended up with a couple of quarts and I've gone through that, mostly, a couple tablespoons at a time.
So today I found five pounds of really disreputable key limes on the distressed produce shelf, going for next to nothing. So I thought I'd make a big batch of syrup and pressure can all but one bottle that I'll be using first in my liquor cabinet. (It probably doesn't need to be canned; being all sugar and acid it seems to keep just fine without refrigeration. But canning doesn't hurt it, and makes the difference between "I should probably use this in the next few months" and "this will keep until the lids rust through.")
Making the syrup was simplicity itself. I rinsed and roughly quartered all the tiny little limes, and threw them into the pressure vessel of my big Instant Pot. A pinch of salt, a tablespoon of cooking oil, about ten cups of granulated sugar, and water to the fill line. Half an hour on the high pressure setting. Half an hour draining in a colander over my stock pot. Proportions of sugar to liquid were a guess, aimed at a 1:1 ratio similar to simple syrup. In the event, I think the limes had more juice in them than I expected; the syrup was not quite as thick as I wanted, nor as thick as my "bitter lemon" batch from my earlier discovery.
Visually the syrup is quite pretty; a deep golden-brown caramel color. Taste-wise, it's got more sourness and less bitterness than the lemon syrup did. But it's extremely good if you like that sweet/sharp/bitter combination, which I obviously do.
My yield was about five quarts, or just a bit less; 3 quarts and a pint pressure canned, plus two .75 screw top vodka bottles for the liquor cabinet, one of which was not full. Probably a two year supply at the rate I used up the much smaller lemon batch.
I am now really anxious for my Poncirus Trifoliata (trifoliate orange, mock orange) trees to start bearing. Their skins and juice have good flavor components, but the bitter flavors are quite overwhelming. People do make marmalade and dispute whether it's worth the making, but I think now that I'd like to try making bitter syrup. The result would be somewhere between syrup, tonic, and bitters, I imagine.