Although acid is not essential for jelly formation its presence in fruit
jellies is very important. Singh has reported that "between certain limits
the greater the acidity of the juice the lower the amount of sugar re-
quired." He adds that it has long been known that juices of high acidity
yield firmer jellies than juices deficient in acid but with as high a pectin
content.
Spencer has published data showing the increase of rigidity of jellies
with increase of acid when the pectin and sugar concentrations are con-
stant. Spencer explains the action of acid in this way. The strength of a
jell netw^ork depends upon the continuity and the rigidity of the structure.
"Continuity of structure, by hypothesis, depends upon the number and
proximity of pectin particles at the time of precipitation," which in turn
is determined by the degree of dispersion and concentration of the pectin.
"DifFerepces in rigidity are due to the amount of water retained by the
pectin at the equilibrium established during precipitation." Hydrogen (or
hydroxyl) ions lessen the stability of the pectin sol by decreasing the hydra-
tion capacity of the pectin. Hence in an acid medium less sugar is required
to bring about precipitation. Nearly neutral fruit juices will not form
jelly with sugar because the sugar is not soluble enough to allow precipita-
tion of the more stabile pectin. Hence with a definite concentration of
pectin the rigidity of the jelly is determined by the sugar and acid con-
centrations.
Hydrogen-ion concentration and jelly. Tarr has determined the
minimum amounts of several acids required to produce a jelly and also
the amount of the acids to produce an optimum jelly by the hot evaporation
method with pectin, sugar, acid, and water. Optimum jelly is defined as
ACIDS IN FRUITS 161
the jelly which in his judgment has the best texture. For jelly formation,
when other conditions were standardized, the minimum amounts of the
acids were as follows: 8.5 cc. of 0.1 A^ sulfuric acid; 27.5 cc. of 0.1 A^
phosphoric acid; 22.7 cc. of 0.1 N tartaric acid; 52.9 cc. of 0.1 N citric
acid; and 583.3 cc. of 0.1 A^ acetic acid. The total acidity of the minimum
amounts required for forming a jelly varied, but the acids were all at the
same pH, 3.40. For optimum jelly the acids were all at pH 3.1.
But although hydrogen-ion concentration controls the formation and
character of the jelly to a certain extent, the salt content of pectin or of
fruit juices, the temperature to which the pectin is heated, and the rate of
pouring may extend this pH range, as will be seen later. In Tarr's jelly a
pH of 3.46 gave a very tender jelly, but increasing the hydrogen-ion con-
centration below />H 3.1 gave syneresis or weeping. The best jellies were
obtained with pH 3.3 to 3.1. Olsen and others have suggested that with pH
lower than 3.1, jelly failure and increased syneresis may result because of
the increased rate of setting. Olsen has shown that an increase of hydrogen-
ion concentration increases the rate at which the jelly sets.
The hydrogen-ion concentration of ordinary fruit juices depends upon
the particular acid present, upon the quantity of acid present, and upon
the "buffer" action exerted by the particular juice.
Page 130
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Although acid is not essential for jelly formation its presence in fruit
jellies is very important. Singh has reported that "between certain limits
the greater the acidity of the juice the lower the amount of sugar re-
quired." He adds that it has long been known that juices of high acidity
yield firmer jellies than juices deficient in acid but with as high a pectin
content.
Spencer has published data showing the increase of rigidity of jellies
with increase of acid when the pectin and sugar concentrations are con-
stant. Spencer explains the action of acid in this way. The strength of a
jell netw^ork depends upon the continuity and the rigidity of the structure.
"Continuity of structure, by hypothesis, depends upon the number and
proximity of pectin particles at the time of precipitation," which in turn
is determined by the degree of dispersion and concentration of the pectin.
"DifFerepces in rigidity are due to the amount of water retained by the
pectin at the equilibrium established during precipitation." Hydrogen (or
hydroxyl) ions lessen the stability of the pectin sol by decreasing the hydra-
tion capacity of the pectin. Hence in an acid medium less sugar is required
to bring about precipitation. Nearly neutral fruit juices will not form
jelly with sugar because the sugar is not soluble enough to allow precipita-
tion of the more stabile pectin. Hence with a definite concentration of
pectin the rigidity of the jelly is determined by the sugar and acid con-
centrations.
Hydrogen-ion concentration and jelly. Tarr has determined the
minimum amounts of several acids required to produce a jelly and also
the amount of the acids to produce an optimum jelly by the hot evaporation
method with pectin, sugar, acid, and water. Optimum jelly is defined as
ACIDS IN FRUITS 161
the jelly which in his judgment has the best texture. For jelly formation,
when other conditions were standardized, the minimum amounts of the
acids were as follows: 8.5 cc. of 0.1 A^ sulfuric acid; 27.5 cc. of 0.1 A^
phosphoric acid; 22.7 cc. of 0.1 N tartaric acid; 52.9 cc. of 0.1 N citric
acid; and 583.3 cc. of 0.1 A^ acetic acid. The total acidity of the minimum
amounts required for forming a jelly varied, but the acids were all at the
same pH, 3.40. For optimum jelly the acids were all at pH 3.1.
But although hydrogen-ion concentration controls the formation and
character of the jelly to a certain extent, the salt content of pectin or of
fruit juices, the temperature to which the pectin is heated, and the rate of
pouring may extend this pH range, as will be seen later. In Tarr's jelly a
pH of 3.46 gave a very tender jelly, but increasing the hydrogen-ion con-
centration below />H 3.1 gave syneresis or weeping. The best jellies were
obtained with pH 3.3 to 3.1. Olsen and others have suggested that with pH
lower than 3.1, jelly failure and increased syneresis may result because of
the increased rate of setting. Olsen has shown that an increase of hydrogen-
ion concentration increases the rate at which the jelly sets.
The hydrogen-ion concentration of ordinary fruit juices depends upon
the particular acid present, upon the quantity of acid present, and upon
the "buffer" action exerted by the particular juice.