Modern cookery for private families by Eliza Acton

6. To preserve both the true flavour and the colour of fruit in jams and

jellies, boil them rapidly until they are well reduced, _before_ the sugar is added, and quickly afterwards, but do not allow them to become so much thickened that the sugar will not dissolve in them easily, and throw up its scum. In some seasons, the juice is so much richer than in others, that this effect takes place almost before one is aware of it; but the drop which adheres to the skimmer when it is held up, will show the state it has reached.