Simmer gently in water till the fruit is tender and thoroughly broken down (usually about ¾–1 hour). If in any doubt about its setting properties, test for pectin at this stage, as a good set depends upon the amount of acid, pectin and sugar present.
Test for pectin: After the fruit has cooked till tender, squeeze from it 1teasp. of juice. Place to cool in a cup or glass. Then add 3teasp.methylated spirits. Shake gently and leave 1 min. If there is plenty of pectin in the fruit, a transparent jelly-like lump will form. If there is only a moderate amount of pectin there may be two or three lumps, not very firm. If there is insufficient pectin, the lump will break into many small pieces and the fruit should be simmered for a little longer before another pectin test is made. It is a waste of effort to strain the juice and attempt to make jelly if there is only a poor amount of pectin. It is preferable to mix with another fruit which is known to be a good setter (e.g. apple—see Blackberry and Apple Jelly, below).
After cooking, strain the fruit through a jelly bag, first scalding the bag by pouring boiling water through it. Hang the bag on a special frame, or suspend it from the legs of an upturned stool or chair with a basin below to catch the drips.
Never hurry the straining of the juice by squeezing the bag —this might make the jelly cloudy, but do not leave the juice too long before completing the jelly—certainly not more than 24 hours.
Measure the juice into a preserving-pan, bring to the boil. Add the sugar. Strained juice rich in pectin needs 1lb.sugar to each pint of juice. Juice with only a fair pectin content needs only ¾lb. sugar to each pint.
After dissolving the sugar, boil rapidly till setting point is reached (about 10 min.—test by any of the methods for Jam on).
Skim, removing the last traces of scum from the surface with the torn edge of apiece of kitchen paper. Pour into warm jars at once, before it has time to begin setting in the pan. Put on waxed circles (waxed side down) immediately. Cover when hot or cold. Do not tilt the jars until the jelly has set. Store in a cool, dry, dark place.