Solution. According to Bogue, soaked gelatin goes Into solution more
rapidly than unsoaked gelatin owing to solution occurring from the outer
and inner surfaces, whereas If gelatin Is put In hot water before soaking,
swelling Is Inhibited to a certain degree and solution takes place from the
outer surface only. The process of solution may be analogous to hydrolysis
of collagen. I.e., just as gelatin is formed more readily from the swollen
collagen, so solution may take place more readily from the swollen gelatin.
Very little of the gelatin goes Into solution while It Is soaking In cold
water. When the temperature is Increased to about 35 °C. the gelatin goes
into solution rapidly. For food preparation this is often accomplished by
adding hot water to the hydrated gelatin. It can also be accomplished by
melting the hydrated gelatin over hot water and then adding cold water
or fruit juices.
GELATION AND STIFFENING POWER OF GELATIN 181
Initial pH. Bogue states that ordinary gelatin usually has a pH about
7. The pH of different commercial brands used in the laboratory for food
preparation has been found to vary from pH 5 to 7. The isoelectric point
of gelatin is given as pH 4.7. At this point it is least soluble, least ionized
and swells the least. Bogue has found turbidity and foaming to be at a
maximum at the isoelectric point. Loeb states that 1 gram of dry gelatin
at the isoelectric point absorbs 7 grams of vi^ater. As the />H is decreased
below 4.7 or increased above 4.7, greater absorption of water occurs until
a maximum is reached. Bogue states that it is desirable from the standpoint
of solubility for the gelatin to have a pH of 3.0 to 3.5, or 8 to 9. The
variation of the pH. of commercial gelatins is due to differences in salt
content, which may increase the acidity or alkalinity. This acidity due to
electrolytes, or to the treatment in acids and alkalies during manufacture,
is often referred to as the residual acidity or alkalinity of the gelatin.
Acids. At a pH below 4.7 the amount of water absorbed increases with
increasing acidity until a maximum is reached at a pH of about 3.5. As
the pH is lowered more than 3.5, the amount of water absorbed decreases.
Loeb states that at a pH of 3.2 to 3.5 a gram of dry gelatin absorbs about
35 grams of water. In food preparation the acid fruit juices are added
after the gelatin is in solution, so that the extent of swelling during hydra-
tion is affected by the initial acidity of the gelatin. In gelatin mixtures that
contain acid salts, as acid phosphates and citric acid added for flavoring,
the amount of swelling would be affected by the acidity.
Alkalies, Alkalies have the same effect as acids upon the swelling of
gelatin, except that it is not so pronounced. Bogue states that the maxi-
mum swelling on the alkaline side is at pH 9.
Salts. Salts also affect the water absorption of gelatin. Some salts in-
hibit the swelling and others favor it. Ostwald states that when salts and
acids are combined the salt usually decreases swelling below the amount
which w^ould result in the presence of acid alone, even though the salt
alone may increase swelling. Sugar decreases swelling. Loeb states that, if
the pH of the solution is kept constant, all ions of the same valency and
charge have nearly the same depressing effect upon swelling. The depress-
ing effect increases with increasing valency.
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Solution. According to Bogue, soaked gelatin goes Into solution more
rapidly than unsoaked gelatin owing to solution occurring from the outer
and inner surfaces, whereas If gelatin Is put In hot water before soaking,
swelling Is Inhibited to a certain degree and solution takes place from the
outer surface only. The process of solution may be analogous to hydrolysis
of collagen. I.e., just as gelatin is formed more readily from the swollen
collagen, so solution may take place more readily from the swollen gelatin.
Very little of the gelatin goes Into solution while It Is soaking In cold
water. When the temperature is Increased to about 35 °C. the gelatin goes
into solution rapidly. For food preparation this is often accomplished by
adding hot water to the hydrated gelatin. It can also be accomplished by
melting the hydrated gelatin over hot water and then adding cold water
or fruit juices.
GELATION AND STIFFENING POWER OF GELATIN 181
Initial pH. Bogue states that ordinary gelatin usually has a pH about
7. The pH of different commercial brands used in the laboratory for food
preparation has been found to vary from pH 5 to 7. The isoelectric point
of gelatin is given as pH 4.7. At this point it is least soluble, least ionized
and swells the least. Bogue has found turbidity and foaming to be at a
maximum at the isoelectric point. Loeb states that 1 gram of dry gelatin
at the isoelectric point absorbs 7 grams of vi^ater. As the />H is decreased
below 4.7 or increased above 4.7, greater absorption of water occurs until
a maximum is reached. Bogue states that it is desirable from the standpoint
of solubility for the gelatin to have a pH of 3.0 to 3.5, or 8 to 9. The
variation of the pH. of commercial gelatins is due to differences in salt
content, which may increase the acidity or alkalinity. This acidity due to
electrolytes, or to the treatment in acids and alkalies during manufacture,
is often referred to as the residual acidity or alkalinity of the gelatin.
Acids. At a pH below 4.7 the amount of water absorbed increases with
increasing acidity until a maximum is reached at a pH of about 3.5. As
the pH is lowered more than 3.5, the amount of water absorbed decreases.
Loeb states that at a pH of 3.2 to 3.5 a gram of dry gelatin absorbs about
35 grams of water. In food preparation the acid fruit juices are added
after the gelatin is in solution, so that the extent of swelling during hydra-
tion is affected by the initial acidity of the gelatin. In gelatin mixtures that
contain acid salts, as acid phosphates and citric acid added for flavoring,
the amount of swelling would be affected by the acidity.
Alkalies, Alkalies have the same effect as acids upon the swelling of
gelatin, except that it is not so pronounced. Bogue states that the maxi-
mum swelling on the alkaline side is at pH 9.
Salts. Salts also affect the water absorption of gelatin. Some salts in-
hibit the swelling and others favor it. Ostwald states that when salts and
acids are combined the salt usually decreases swelling below the amount
which w^ould result in the presence of acid alone, even though the salt
alone may increase swelling. Sugar decreases swelling. Loeb states that, if
the pH of the solution is kept constant, all ions of the same valency and
charge have nearly the same depressing effect upon swelling. The depress-
ing effect increases with increasing valency.