For theoretical study of pectin jelly Spencer recommends mixing the
constituents of the jelly while cold instead of the usual boiling of the
sugar and juice, which she calls the hot evaporation method. The reasons
for this are four in number. Decomposition of both sugar and pectin occurs
during cooking. The proportions of the various constituents change by
evaporation, hence it is difficult always to obtain the concentration desired.
The precipitating agent sugar has a limited solubility. There is danger
of premature precipitation of the pectin if all the cold sugar is added at
one time to the hot pectin mixture. For this experimental work dried pectin
or a concentrated pectin solution is necessary.
Olsen summarizes the theory of jelly formation as follows: Granted that
pectin is a negatively charged hydrophilic colloid, the following may be
assumed: (1) sugar is the dehydrating agent; (2) the hydrogen-ion con-
centration reduces the negative charge on the pectin, thereby permitting the
pectin to precipitate and coalesce in the form of a network of insoluble
fibers, provided the concentration of sugar is sufficient; (3) the dehydration
of the pectin micelles by sugar is not instantaneous but requires time to
come to an equilibrium; (4) the rate of hydration and precipitation in-
creases in direct ratio as the hydrogen-ion concentration; (5) the maximum
jelly strength is reached when the system reaches equilibrium, and depends
upon the position of that equilibrium; (6) any component added to a pectin
jelly system, including salts, which causes a change in the ultimate jelly
152
SOURCES OF PECTIN IN PLANTS 153
strength of that system may function either (a) by changing the rate of
gelation, or (b) by affecting the ultimate jelly structure, or (c) a com-
bination of these two.
Fruits Used for Jelly
For a fruit juice to make a good jelly by boiling with the addition of
sugar only, the juice must contain a sufficient amount of a substance called
pectin and also sufficient acid. Some fruit juices will not make acceptable
jelly by this method because they are deficient in pectin, whereas others
do not contain enough acid. In the middle states west of the Mississippi
River the fruits most commonly used to make jelly without addition of
pectin are apples, crab-apples, blackberries, sour plums, grapes, currants,
and gooseberries. During the winter months cranberries are used exten-
sively.
Cruess and McNair have reported that the following fruit juices con-
tain sufficient acid and pectin to produce good jelly: blackberries, logan-
berries, Isabella grapes, Tokay grapes, cranberries, currants, whole lemons,
and pomelos. They state that oranges have enough pectin but not always
enough acid. Apricots and cherries sometimes make jelly but are usually
deficient in pectin. Pomegranates and strawberries have enough acid but
lack pectin, and pears, peaches, and huckleberries lack both pectin and acid.
Citron melons and mission figs have enough pectin but lack acid. They state
that lemon is too high in acid to produce a good jelly, but that two whole
oranges and one whole lemon make a good combination, the acidity of the
lemon allowing for the deficiency in the orange.
By the addition of pectin and sugar, all fruit juices, so far as the author
knows, can be used for jelly.
The Pectic Substances
Page 123
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For theoretical study of pectin jelly Spencer recommends mixing the
constituents of the jelly while cold instead of the usual boiling of the
sugar and juice, which she calls the hot evaporation method. The reasons
for this are four in number. Decomposition of both sugar and pectin occurs
during cooking. The proportions of the various constituents change by
evaporation, hence it is difficult always to obtain the concentration desired.
The precipitating agent sugar has a limited solubility. There is danger
of premature precipitation of the pectin if all the cold sugar is added at
one time to the hot pectin mixture. For this experimental work dried pectin
or a concentrated pectin solution is necessary.
Olsen summarizes the theory of jelly formation as follows: Granted that
pectin is a negatively charged hydrophilic colloid, the following may be
assumed: (1) sugar is the dehydrating agent; (2) the hydrogen-ion con-
centration reduces the negative charge on the pectin, thereby permitting the
pectin to precipitate and coalesce in the form of a network of insoluble
fibers, provided the concentration of sugar is sufficient; (3) the dehydration
of the pectin micelles by sugar is not instantaneous but requires time to
come to an equilibrium; (4) the rate of hydration and precipitation in-
creases in direct ratio as the hydrogen-ion concentration; (5) the maximum
jelly strength is reached when the system reaches equilibrium, and depends
upon the position of that equilibrium; (6) any component added to a pectin
jelly system, including salts, which causes a change in the ultimate jelly
152
SOURCES OF PECTIN IN PLANTS 153
strength of that system may function either (a) by changing the rate of
gelation, or (b) by affecting the ultimate jelly structure, or (c) a com-
bination of these two.
Fruits Used for Jelly
For a fruit juice to make a good jelly by boiling with the addition of
sugar only, the juice must contain a sufficient amount of a substance called
pectin and also sufficient acid. Some fruit juices will not make acceptable
jelly by this method because they are deficient in pectin, whereas others
do not contain enough acid. In the middle states west of the Mississippi
River the fruits most commonly used to make jelly without addition of
pectin are apples, crab-apples, blackberries, sour plums, grapes, currants,
and gooseberries. During the winter months cranberries are used exten-
sively.
Cruess and McNair have reported that the following fruit juices con-
tain sufficient acid and pectin to produce good jelly: blackberries, logan-
berries, Isabella grapes, Tokay grapes, cranberries, currants, whole lemons,
and pomelos. They state that oranges have enough pectin but not always
enough acid. Apricots and cherries sometimes make jelly but are usually
deficient in pectin. Pomegranates and strawberries have enough acid but
lack pectin, and pears, peaches, and huckleberries lack both pectin and acid.
Citron melons and mission figs have enough pectin but lack acid. They state
that lemon is too high in acid to produce a good jelly, but that two whole
oranges and one whole lemon make a good combination, the acidity of the
lemon allowing for the deficiency in the orange.
By the addition of pectin and sugar, all fruit juices, so far as the author
knows, can be used for jelly.
The Pectic Substances