Put the apples in a preserving pan and pour enough water to just cover the fruit. Cover; bring to a boil, and then simmer for 20 minutes or until soft.
Strain the pulp through a large sieve lined with muslin, or a jelly bag set over a bowl, and allow the juice to drip through into a bowl positioned below.
Pour the strained juice into a clean pan and simmer gently until thick and reduced by half its volume. Then test it's pectic strength.
Pour in to small frezer-safe containers (generally 2/3 cup of strong pectin stock will be sufficient to set 4 1/2 pounds of medium or low pectin fruit as jam).
Leave to cool then seal, label and refirgerate or freeze until required.
SIMPLE PECTIN TEST
This test enables you to judge if your stock contains enough pectin to achieve a set. Put 1 tsp of stock in a small container. When cold and 1 tablespoon of denatured alcohol, swirl together, leave for 1 minute and check for a set:
High Pectin: the mixture formes a jelly-like clot.
Medium Pectin: 2-3 soft clots form. The stock may or may not set satisfactorily, return to the heat to reduce a little further and test again.
Low Pectin: Numerous small globules form. Return to the heat and reduce the volume of the stock and test again.
NOTE: discard the tested stock- denatured alcohol is poisionous.