This recipe can also be used for other variants ("cultivars") of oranges. This is in fact no difference between making marmelade of blood oranges and other types of oranges. But it was blood oranges that I made marmelade of last time...
There are three types of blood oranges:
The photos on this page is of the variant moro.
More information on blood oranges: Blodappelsin (Danish), Blood orange (English).
2 days. Yes, 2 days. You have to let the sliced oranges marinate overnight.
A funnel with a wide hole can be useful.
Use the potato peeling knife to make thin strips of the peel from 3-4 oranges
Cut the strips into thin strips (circa 2-3mm)
Peel the oranges. remove as much as possible of the white stuff. That usually goes well with the first oranges but after 2kg of them it starts getting boring. Don't skimp - the white stuff is what makes the marmelade bitter, so you want as little of it as possible in the marmelade.
Separate the oranges into halves and remove the white stuff from the center.
Slice the oranges (circa 3-6 mm). Don't worry about removing the seeds now.
Put them into the pot together with the peel strips, and add 2-4dl water. They shouldn't be covered by water, but there has to be enough juice and water so most of the slices are kept moist (se why in the footnotes).
Put the pot on the stove and get it boiling weakly. You should be able to see the steam.
Reduce oranges&slices by simmering for several hours. How much is difficult to say, but a reduction of 20-40% is common.
The way I do it and know when it is done is by letting it simmer on step 4 (our of 12) on the large plate on my stove. I let it stay there until it starts boiling more heavily and starts making a foam. Then I turn down the heat a little, and leave it until it starts boiling heavily and foaming again. I repeat this 2-3 time
Don't stir too much in the pot (otherwise you might as well just have blended the whole thing)).
Add the sugar and turn up the heat.
When the sugar has been dissolved the seeds will start to float to the top when stirred. Remove them.
Is it reduced enough and have you added sugar enough? You test that by taking half a teaspoon of the mixture, dropping on a plate and letting it cool (eg. in the fridge). When you cut it when a spoon and it doesn't immediately flow together again then marmelade is done.
If the marmelade seems too watery add more sugar.
Reduce the marmelade a bit more until it is done.
When the marmelade is ready, turn off the heat and remove the foam with a spoon.
Add sodium benzoate
Put into jars.