In all fruits, when ripe or nearly so, there is found pectin, a
carbohydrate somewhat similar in its properties to starch. It is because
of this substance in the fruit juice that we are able to make jelly.
When equal quantities of sugar and fruit juice are combined and the
mixture is heated to the boiling point for a short time, the pectin in
the fruit gelatinizes the mass.
It is important that the jelly maker should understand when this
gelatinizing agent is at its best. Pectose and pectase always exist in
the unripe fruit. As the fruit ripens the pectase acts upon the pectose,
which is insoluble in water, converting it into pectin, which is
soluble. Pectin is at its best when the fruit is just ripe or a little
before. If the juice ferments, or the cooking of the jelly is continued
too long, the pectin undergoes a change and loses its power of
gelatinizing. It is, therefore, of the greatest importance that the
fruit should be fresh, just ripe or a little underripe, and that the
boiling of the sugar and juice should not be continued too long.
Fruits vary as to the quantities of sugar, acid, pectin, and gums in
their composition. Some of the sour fruits contain more sugar than do
some of the milder-flavored fruits. Currants, for example, often contain
four or five times as much sugar as the peach. The peach does not
contain so much free acid and it does contain a great deal of pectin
bodies, which mask the acid; hence, the comparative sweetness of the
ripe fruit.