I've had a go at making jam just a couple of times but have never known if what I'm boiling has reached setting point. Resulting preserves have been too liquid. Is there an easy way to tell? Thank you.
Jam is set by a combination of pectin and sugar. Pectin occurs naturally in fruit but in differing levels, if you are having trouble getting your jam to set then it could be due to the pectin content in the fruit being too low. Some popular fruits such as strawberries have a very low pectin content. Adding lemon juice to the fruit can help as lemons are high in pectin. Alternatively try using jam sugar (which has added pectin and should not be confused with preserving sugar), or adding powdered or liquid pectin to the fruit to help the jam to set. If you are not sure about the pectin content then drop half a teaspoonful of the fruit juice into 2 tablespoons of methylated spirits (for the US use rubbing alcohol with 70% ethyl alcohol), if the fruit is high in pectin the juice will form into a slightly jellied blob, if it remains liquid then the fruit is low in pectin.
There should also be enough sugar in the mixture, roughly equal weights of fruit and sugar. Setting point for jam is 105c (220F) so a good way to test for setting point is to have a sugar thermometer clipped to the side of your saucepan, with the end dipped in the boiling jam mixture. Once the boiling mixture has reached the correct temperature then your jam should set. You can also check for setting point using the "wrinkle" test. Before cooking the jam put 3 or 4 small heatproof plates in the freezer. Once your jam has boiled for several minutes, take the pan off the heat and carefully spoon a little jam onto one of the cold plates. Let it stand for a minute then push the blob of jam with your finger, if the surface of the jam wrinkles then it has set, if it is still quite liquid then put the pan back on the heat and boil the jam for another 3 to 5 minutes before testing again. Pectin can also be destroyed by extended boiling so make jam in a large saucepan (the fruit and sugar mixture should not come more than one third up the side of the pan) so that the mixture boil rapidly, and start testing for setting point fairly early.