Lime marmalade requires a little extra attention, or it just won't set for you. I still reckon Rose's make the best. It's an exception to the general rule about commercial marmalade.
I make an OK lime marmalade, but it's somewhat more robust than the Rose's.
In my researches, I did discover some interesting facts, however.
Bought limes are often wax coated to improve appearance and improve shelf life. They may also contain traces of pesticide residue. For both reasons, a brisk scrub before use is a good idea. Better still, if you can, grow your own. Locally, the Bearss variety of Tahitian lime seems to be the tree of choice for new or replacement planting.
Lime marmalade is notoriously reluctant to set. The reason, I am told, is that limes are quite a bit more acid than other citrus, and often the acidity is outside the normal gelling range. (Same applies to a lemon marmalade if you want to try that.) Commercial manufacturers use a variety of hi-tech methods to counter this, but one that will definitely work for the home marmaladier is bicarbonate of soda in SMALL quantities.
I used a little under a level teaspoon for a kilogram of pure lime fruit, and a half teaspoon for a 50/50 mix with orange or mandarine. I add it to the citrus and water mix immediately before I begin cooking it. Lots of froth and bubble for 20 seconds and then all is quiet. The marmalade sets perfectly.
The other tip is that lime peel can be notoriously chewy in home-made marmalade. This is why we soak the citrus and water mix overnight, or even 24 hours, and then simmer gently for an hour or so before adding the sugar and getting down to business. You should be able to take a piece of peel and squash it easily between finger and thumb. Once the sugar is added, not a lot of extra softening seems to happen.
There are two schools of thought when it comes to stirring. One says, stir until the sugar is dissolved and then rolling boil for twenty minutes and don't come near it in between. This is alleged to give you a clearer marmalade, the idea being that the stirring breaks the fruit into millions of tiny particles = cloudiness. This school of thought is also committed to quartering the limes and finely slicing the quarters.
Not stirring, however, leaves you open to the marmalade sticking/burning on the bottom of the pot, especially as the mix starts to evaporate some. I prefer cloudy marmalade to burnt marmalade. I stir throughout the cooking process, using a 75mm wide flat bottomed paddle.
I also on occasions use a blender before I've simmered the fruit and added the sugar. I do this with other marmalade as well.
It seems to me to result in a more intensely flavoured marmalade, and improves the setting quality, though it's not going to win clarity tests. It does, however solve the problem of how to stop all the finely sliced bits of peel collecting at the top of the jar while it sets.
So, with all the above under your belt:
Take 1kg of limes, and 2 litres of water.
Slice the limes according to your preference and soak in a non-metallic bowl overnight.
Organise your jars. You will need about 6-12 clean jars depending on size. I sterilise these by placing them in the microwave on high for 6 minutes, about 12 minutes into the rolling boil. The jars should be hot when you pour the jam in, otherwise they will crack. Sterilise the lids by placing them in a saucepan of boiling water for 5 minues.
Place the mix in a large stainless steel saucepan. (When it starts to boil it can double in effective volume in seconds. I have an 8 litre heavy base saucepan, stainless steel, not aluminium or cast iron, which I use. (If you want to use the blender to adjust particle size, do it now. Now, before you start to boil it, is also the time to add bicarb if that is what you want to do.
Bring the mix to the boil and simmer for half an hour to an hour until the bits are easily squashable. Add an equal volume of sugar, stir to dissolve and continue stirring for about twenty minutes after rolling boil is reached. Begin testing around 18 minutes. It can take as long as 35 minutes but that's unusual. 20-25 is more the norm.
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.
