Addition of solidified gelatin to a gel solution. Lloyd states that
gelation is similar to crystallization, in that it takes place from a saturated
solution. There are various theories to account for gel formation. It has
been suggested that nuclei, strands, rods, or micelles form in the solution
and solidification takes place upon cooling, with the formation of a gel
structure. If some gelatin that has solidified is added to freshly made
gelatin solution solidification takes place more rapidly. Alexander suggests
that this is due to the addition of nuclei, and that other particles aggregate
around these nuclei. This is similar to "seeding" or adding of crystals to
crystalloidal solutions to hasten crystallization. Dahlberg, Carpenter, and
Hening find that this property of gelatin has a practical application in
ice-cream making. The gelatin is added to ice cream to improve the texture
184 GELATIN
and body of the ice cream. If the unfrozen ice-cream mix is aged at a
temperature just above the freezing point a smoother ice cream is obtained.
One reason attributed for this is the low viscosity of the freshly made
gelatin solutions compared with the viscosity of the aged solutions, and
another is the speed at which the gelatin gel previously aged reforms
after whipping during freezing.
Hydrogen-ion concentration. The acidity of the gelatin affects its
stififening power. According to the kind and amount of ash and its buffer
action the gelatin may have a hydrogen-ion concentration greater or less
than its isoelectric point, which is />H 4.7. The initial pH of different
brands and samples tried in the laboratory has been above the isoelectric
point. Probably the greater the acidity or alkalinity, unless it is beyond the
maximum or minimum pH for the greatest water absorption, the less firm
the resulting jelly. However, the pH of a gelatin solution does not entirely
control the time for setting or the stiffness of the resulting gelatin. Two
gelatin solutions may have the same pH and the same proportions of ingredi-
ents added, but one may require much longer than the other to set and never
produce as firm a jelly.
Acids and alkalies. The addition of fruit juices or acid salts to the
gelatin solution increases its acidity, the increase depending on the acidity
of the fruit juice added, its buffer capacity, and the quantity added. Lemon
juice has a lower pH than most fruit juices. Its pH varies slightly but
is usually between 2.0 and 2.5. The addition of ^ cup of lemon juice
to ^ ounce of gelatin with an initial pH 6 to 6.5, 3^^ cups of water and
y2 cup of sugar, lowers the pH of the mixture around 3. This is lower
than the isoelectric point of the gelatin. Many recipes use larger quantities
of lemon juice than the proportion given above. This would increase the
acidity still more. With a definite concentration of gelatin a less firm jelly
is obtained as the amount of lemon juice is increased. A longer time is
also required for the gelatin to set with the increased lemon juice or
acidity.
Patten and Johnson found that a 3 per cent gelatin solution liquefied
on standing when the pH was sufficiently low. Liquefaction also varied
with the temperature. It occurred at a temperature of 18°C. when the
gelatin solution was at pH 3.6 or lower.
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Addition of solidified gelatin to a gel solution. Lloyd states that
gelation is similar to crystallization, in that it takes place from a saturated
solution. There are various theories to account for gel formation. It has
been suggested that nuclei, strands, rods, or micelles form in the solution
and solidification takes place upon cooling, with the formation of a gel
structure. If some gelatin that has solidified is added to freshly made
gelatin solution solidification takes place more rapidly. Alexander suggests
that this is due to the addition of nuclei, and that other particles aggregate
around these nuclei. This is similar to "seeding" or adding of crystals to
crystalloidal solutions to hasten crystallization. Dahlberg, Carpenter, and
Hening find that this property of gelatin has a practical application in
ice-cream making. The gelatin is added to ice cream to improve the texture
184 GELATIN
and body of the ice cream. If the unfrozen ice-cream mix is aged at a
temperature just above the freezing point a smoother ice cream is obtained.
One reason attributed for this is the low viscosity of the freshly made
gelatin solutions compared with the viscosity of the aged solutions, and
another is the speed at which the gelatin gel previously aged reforms
after whipping during freezing.
Hydrogen-ion concentration. The acidity of the gelatin affects its
stififening power. According to the kind and amount of ash and its buffer
action the gelatin may have a hydrogen-ion concentration greater or less
than its isoelectric point, which is />H 4.7. The initial pH of different
brands and samples tried in the laboratory has been above the isoelectric
point. Probably the greater the acidity or alkalinity, unless it is beyond the
maximum or minimum pH for the greatest water absorption, the less firm
the resulting jelly. However, the pH of a gelatin solution does not entirely
control the time for setting or the stiffness of the resulting gelatin. Two
gelatin solutions may have the same pH and the same proportions of ingredi-
ents added, but one may require much longer than the other to set and never
produce as firm a jelly.
Acids and alkalies. The addition of fruit juices or acid salts to the
gelatin solution increases its acidity, the increase depending on the acidity
of the fruit juice added, its buffer capacity, and the quantity added. Lemon
juice has a lower pH than most fruit juices. Its pH varies slightly but
is usually between 2.0 and 2.5. The addition of ^ cup of lemon juice
to ^ ounce of gelatin with an initial pH 6 to 6.5, 3^^ cups of water and
y2 cup of sugar, lowers the pH of the mixture around 3. This is lower
than the isoelectric point of the gelatin. Many recipes use larger quantities
of lemon juice than the proportion given above. This would increase the
acidity still more. With a definite concentration of gelatin a less firm jelly
is obtained as the amount of lemon juice is increased. A longer time is
also required for the gelatin to set with the increased lemon juice or
acidity.
Patten and Johnson found that a 3 per cent gelatin solution liquefied
on standing when the pH was sufficiently low. Liquefaction also varied
with the temperature. It occurred at a temperature of 18°C. when the
gelatin solution was at pH 3.6 or lower.