Remove the orange peels, and cut in half. (They weigh 1 kg at this point.)
Put the oranges into a blender, and blend until all the inner membranes are chopped into small pieces. It's a little bit long, but for about 20 seconds.
Transfer the blended orange in several batches to a pot, and turn on the heat to high.
When it starts to boil, skim off the scum, then turn down the heat to medium.
Once at a boil, add the sugar all at once, then simmer for about 25 ~ 30 minutes while stirring occasionally.
When the mixture boils down to half the original amount, add the starch based sweet syrup, and mix, then simmer some more.
When the mixture is boiled down to 1/3 of Step 5 and it becomes thick, turn off the heat.
Pour into bottles, which were sanitized by boiling, while the mixture is still hot, and put the bottles upside down.
It's finished. Please keep them in the fridge.
Story Behind this Recipe
I was given a lot of mandarin oranges and wanted to use them up.
Adding starch based sweet syrup makes the jam lustrous and beautiful, but you don't have to use it. Please adjust the amount of sugar depending on the sweetness of mandarin orange. Turn off the heat while the mixture is still runny. When it's cooled, it thickens.