Jelly needs to be clear, and firm, firm enough to hold its shape when cut, without being so firm that it won’t wobble.
Good jelly can be made from high pectin fruits such as apples, blackberries, crabapples, cranberries, gooseberries, grapes, plums, red currants, and quinces.
Jelly is also a good use for problematic fruits such as mulberries, whose stems are very hard to remove: juice the berries, and strain out the stems and fruit. Mulberries, however, are very low in pectin and will need a lot of added pectin. The problem with adding a lot of pectin to a jelly is that it can dilute the fruit taste.
To make a Jelly, fresh fruit is juiced, strained, then the juice is cooked with sugar.
It is very tricky to make double or triple batches of jelly at one time. It sets best, and comes out with the freshest flavour, if each batch uses no more than 4 to 6 cups (32 to 48oz / 1 to 1 1/2 litres) of juice at a time. To allow for frothing and prevent boiling over, the pot or kettle you use should be able to hold at least four times that. The juice and sugar should be boiled rapidly as a slow boil will destroy more pectin.
Literature & Lore
Michel de Nostradumus (1503 to 1566) is now known for his book of prophecies. His first book, however was on making jellies, published in 1555, “Excellent & Useful Treatise to all Needed who want Knowledge of Several Exquisite Recipes.”