The term "lime" here is perhaps something of a misnomer, as the fruit is widely accepted as a lemon/mandarine cross. It is as sharp, however, as any lime, and should probably be treated as such for the purposes of making marmalade. (It also does very nicely with a g&t).
Rangpur are not very common in New Zealand. Despite a widespread search by our local treeman at Kaipara Coast Plants we found none for sale when he looked about six years ago, and I settled for saving the seed from a fruit that was given me and starting from scratch. First fruit last year, about three in all, and a good crop this year. Later on I discovered the rangpur has a fairly wide use among nursery people as a citrus graft parent, so you may be able to get hold of a tree by checking closer to point of origin.
It resembles a mandarine in appearance, though with a small nipple similar to the end of a lemon, and in flavour a very sharp lemon with a hint of orange. Like the damson, I suspect, the secret is in the skin, and it delivers a full-flavoured sharp and bitter marmalade that rivals any I've had from Seville oranges. For those with a taste that is less austere, a 50-50 mix of rangpur and clementine mandarine adds a wonderful sweetness and fruit to the mix.
Start with 1kg of rangpur, including at least one lemon, preferably not Meyer, in the total. Slice them finely into a large bowl, non-metallic, and non-earthenware, and add 2 litres of cold water. Allow to sit overnight or for 24 hours or so. (Note: Regular lime peel remains fairly tough and chewy even with soaking. Rangpur more resembles ordinary mandarine or lemon in this respect and softens easily. You might even get away without an overnight soak)
If you are a pedant you will carefully remove all the free-floating seeds that you can see before you start boiling. This will reduce the final bitterness somewhat, if that is important.
Bring the fruit and water to the boil, and simmer for, say, 30 minutes, or until the peel is soft. At this point I use a stick blender to reduce the bit size. Otherwise I get a raft of peel at the top of every jar. (Purists throw up their hands in dismay at this.) I also mix in about 1/2 teaspoon of baking soda to drop the acidity level slightly and ensure a better set. It froths like hell and then stops frothing.
At this point I clean and dry about 8 jam jars of assorted sizes and pop them all in the microwave, with a timer set to about 7 minutes. I put the kettle on to boil and place the lids in a bowl ready to cover with boiling water. Do not start the microwave yet.
Add about 2.5kg of sugar, or if you want to be super-accurate, add 1 cup of sugar to every cup of mix you have. Stir until dissolved, and bring to a rolling boil. Maintain this for anything from 20-40 minutes until the marmalade passes the set test.
Switch the microwave on about 12 minutes into the rolling boil.
Start testing at about 20 minutes. Testing consists of dribbling a small patch of mixture from your stirring paddle onto a china dinner plate. Leave for a minute until it has cooled and then push against one end of the patch with your finger. If the marmalade merely moves out of your way, keep boiling. If it forms a skin that wrinkles ahead of your finger, it is done.
Pour into the hot jars, remove lids from boiling water and dry with a clean teatowel, Screw lids on and wait for the lids to pop as they cool down. If the vacuum seal does not happen, use that jar first.
Can't find a definitive on this but general impression seems to be that Meyer lemons lack the ingredient in their makeup that assists in setting - which is why we add lemons in the first place. My marmalade seems to be easily bluffed, so perhaps not as important as some think.
