Pectin

Testing the pectin content
If making a jam, jelly or marmalade with a medium- or low- pectin fruit, it’s worth using the following test to be sure of a good result.  Once the fruit is cooked, but before you add the sugar, put 1tsp of the juice into a small dish.  When cool, add 1tbsp methylated spirit, shake the dish and leave for 1min then pour the mixture on to a plate. The consistency of the juice is a clear indication of how firm the resulting jam will be.  If the fruit contains plenty of pectin it will have an almost jelly-like consistency.  If very loose or not holding together at all, you will need to add more pectin.

Pectin content of fruit
High
Cooking apples, crab apples, cranberries, citrus fruit, damsons, gooseberries, redcurrants, plums (some varieties), quinces

Medium              
Apricots, blackberries, dessert apples, greengages, loganberries, mulberries, raspberries, plums

Low                     
Bananas, cherries, figs, grapes, japonicas, marrows, medlars, melons, nectarines, peaches, pears, pineapples, rhubarb, strawberries

From Good Housekeeping Complete Book of Preserves

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