Preserving hints and tips

Small batches
Preserves are best made in small batches as large batches take too long to come to the boil and, in the case of jams and jellies, won’t reach a rapid enough boil for a good setting point.

Pectin and acid
Pectin is a natural substance found in fruit and vegetables that, when combined with acid and sugar, results in the setting of jams, jellies and marmalades. Pectin in fruit is at its peak when fruit is slightly under ripe. Fruits that are high in pectin and acid (such as apples, crab apples, quinces, citrus fruit and plums) are usually sour or tart and make for the easiest preserves to set. Fruits that are high in acid and low in pectin (apricots, peaches, pineapples and rhubarb) benefit from the addition of lemon juice to help them set. Fruits that are low in both pectin and acid (berries, pears and cherries) need the addition of other pectin and acid-rich fruit to help make them set.

from
http://www.stuff.co.nz/life-style/food-wine/cuisine/81039468/Preserving-hints-and-tips