pressure which is required to force the dispersions medium out of a gel.
Imbibition pressures should not be confused with osmotic pressure, and in
many instances they assume values greatly in excess of values obtainable
by osmotic pressure. If a sheet of dried gelatin is placed in a saturated
solution of sodium chloride, water will be withdrawn by imbibition forces
against the osmotic pressure of the sodium chloride solution, and sodium
chloride will crystallize out in the solution."
The micelles possess either a positive or a negative charge. When the
electrical forces holding the bound water to the micelle are neutralized, as
may happen when electrolytes are added under suitable conditions, the
bound water is released and the viscosity of the system decreases. When
a custard is curdled the bound water is set free. The ability of muscle fibers
to hold water during cooking prevents drying of the meat to a great extent.
Effect o£ added substances upon swelling of gels. The addition
of such substances as acids, alkalies, mineral salts, and sugar may increase
or inhibit the degree of swelling. Many such combinations are made in
cookery. Lemon juice and vinegar may be added for flavor. Soda, salt, and
baking powder may be added to foods. Different proportions of mineral
salts are found in foods, and the proportion may vary in the same food
owing to many causes. In general, the addition of acids or alkalies increases
the swelling of colloidal gels. With acids, this usually continues until a
maximum is reached at pH 3.0 to 2.5, when imbibition is decreased with
greater acidity. With alkalies the maximum swelling is about pH 10.5,
though gluten gels are likely to disintegrate when they become as alkaline
as this. Sometimes the addition of acids or alkalies lessens hydration. This
depends upon the pH of the substance when the acid or alkali is added.
In general, salts lessen the degree of swelling even in the presence of acids
or alkalies.
Syneresis. After gels are allowed to stand protected against evapora-
tion for a number of hours there is a tendency for the gel to separate into
two phases. A liquid may squeeze out of the gel. A typical example is the
separation of the whey from the curd in clabbered milk. It is also noticed
in some jellies, cranberry jelly in particular. This separation into a more
solid and a more liquid part may take a long time in the case of some food
gels, a shorter time in others. In syneresis the liquid part contains a large
proportion of the solvent, a smaller proportion of the solid. The more solid
part has a high concentration of the solid, a lower one of the solvent.
Hydrophilic and hydrophobic colloids. The terms lyophilic and
lyophobic include all dispersing mediums whereas hydrophilic and hydro-
phobic indicate that the dispersing medium is water, Hydrophilic means
"water loving"; hydrophobic means "water hating." The lyophilic colloids
belong to the liquid dispersed in liquid systems, thiough as pointed out by
Gortner and by Fischer this terminology is not strictly accurate, for the
dispersed phase and dispersing medium are more or less soluble in each
other. Thus, with gelatin and water, hydration occurs or the two are
ELECTROLYTES AND HYDROPHOBIC COLLOIDS 9
Page 9
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pressure which is required to force the dispersions medium out of a gel.
Imbibition pressures should not be confused with osmotic pressure, and in
many instances they assume values greatly in excess of values obtainable
by osmotic pressure. If a sheet of dried gelatin is placed in a saturated
solution of sodium chloride, water will be withdrawn by imbibition forces
against the osmotic pressure of the sodium chloride solution, and sodium
chloride will crystallize out in the solution."
The micelles possess either a positive or a negative charge. When the
electrical forces holding the bound water to the micelle are neutralized, as
may happen when electrolytes are added under suitable conditions, the
bound water is released and the viscosity of the system decreases. When
a custard is curdled the bound water is set free. The ability of muscle fibers
to hold water during cooking prevents drying of the meat to a great extent.
Effect o£ added substances upon swelling of gels. The addition
of such substances as acids, alkalies, mineral salts, and sugar may increase
or inhibit the degree of swelling. Many such combinations are made in
cookery. Lemon juice and vinegar may be added for flavor. Soda, salt, and
baking powder may be added to foods. Different proportions of mineral
salts are found in foods, and the proportion may vary in the same food
owing to many causes. In general, the addition of acids or alkalies increases
the swelling of colloidal gels. With acids, this usually continues until a
maximum is reached at pH 3.0 to 2.5, when imbibition is decreased with
greater acidity. With alkalies the maximum swelling is about pH 10.5,
though gluten gels are likely to disintegrate when they become as alkaline
as this. Sometimes the addition of acids or alkalies lessens hydration. This
depends upon the pH of the substance when the acid or alkali is added.
In general, salts lessen the degree of swelling even in the presence of acids
or alkalies.
Syneresis. After gels are allowed to stand protected against evapora-
tion for a number of hours there is a tendency for the gel to separate into
two phases. A liquid may squeeze out of the gel. A typical example is the
separation of the whey from the curd in clabbered milk. It is also noticed
in some jellies, cranberry jelly in particular. This separation into a more
solid and a more liquid part may take a long time in the case of some food
gels, a shorter time in others. In syneresis the liquid part contains a large
proportion of the solvent, a smaller proportion of the solid. The more solid
part has a high concentration of the solid, a lower one of the solvent.
Hydrophilic and hydrophobic colloids. The terms lyophilic and
lyophobic include all dispersing mediums whereas hydrophilic and hydro-
phobic indicate that the dispersing medium is water, Hydrophilic means
"water loving"; hydrophobic means "water hating." The lyophilic colloids
belong to the liquid dispersed in liquid systems, thiough as pointed out by
Gortner and by Fischer this terminology is not strictly accurate, for the
dispersed phase and dispersing medium are more or less soluble in each
other. Thus, with gelatin and water, hydration occurs or the two are
ELECTROLYTES AND HYDROPHOBIC COLLOIDS 9