FruitThis is my non-scientific explanation of how to chose your fruit, and why some don't work on their own.
For jam to set it needs to have both acid and pectin. Jam works best with fresh fruit, because the pectin lessens as fruit is stored. Pectin is the thing in fruit that makes jam set, so when choosing fruit, you need to make sure that it is a type with plenty of pectin. It also needs to have some acid. If the type of fruit that you are using is lacking in either acid or pectin, you can add some lemon juice, because it is high in both, or you can use something like "jamsetta" (which I think is made by Fowler's Vacola) or some other jam setting additive, in which case, you should follow the instructions on the packet.
Fruit with both pectin and acidThis fruit can be used without adding any lemon juice.
Fruit that is low in pectinAdd two tablespoons* (40 ml - Australian tablespoons are 20 ml each) lemon juice to 1 kg fruit.
Fruit that is low in acidAdd two tablespoons* (40 ml) lemon juice to 1 kg fruit.
(These fruit go brown when exposed to air)
Fruit that is low in both acid and pectinIt is generally best to make jams that have this fruit mixed with other fruit, use added jamsetta or a similar product or lots of juice.
You might notice that lots of our favourite jam flavours come into this category of low acid and low pectin. That is why many of the recipes for these jams use a jam setting additive.
*In Australia, our tablespoons are 20 ml. In the US, a tablespoon is 15 ml. To complicate things, many kitchenware shops in Australia seem to sell US tablespoons (and perhaps don't even realise that there is a difference), and of course, Australians often cook from US recipes. I like to give a measurement in millilitres where possible to avoid confusion.