4979 The Cooking, Bottling and Covering of Jams

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The amount of time required for cooking any kind of jam, marmalade or fruit jelly can only be roughly determined. It is a mistake to try to fix the exact time, since the duration of the cooking time depends solely on the degree of heat applied and consequently the rate of evaporation of the water contained in the fruit. As a general rule, the quicker the jam is made, the better the result, because the fruit will keep its colour better.
However, unless constant care and attention is given to a jam in which there is a high proportion of fruit, it should not be cooked over a fierce heat because of the risk of it burning. On the other hand, when making a jelly in which only the juice of the fruit is used, it should be cooked on a very hot stove so as to reach as quickly as possible, the stage which indicates that the cooking has reached its correct degree.