Making jam is rich in kitchen science. You can, for example, persuade jam to 'set' in different ways:
- if you add a tea-spoon of orange juice to a kilo of sugar et voila! it's set;
- you can buy pectin as a powder or, as I say, as apple-cores;
- pectin sets better at low (acidic) pH, which shouldn't be an issue with oranges but lemon juice is more tart and so has more bang per pip.