of separation between sols and gels, nor on the other hand, according to
Jordan Lloyd, between gels and curds. The classification of gels on the
basis of consistency includes many gels that do not have similar proper-
ties. Some gels are formed from sols by coagulation. Custard is an ex-
ample of this type. A starch gel is formed by gelatinization of the starch
during cooking. Gelatin, agar-agar, and soaps form sols above certain
temperatures and gels at lower temperatures. Such gels are called revers-
ible. The sol-gel transformation is brought about gradually, there being
no definite melting or setting temperature. The change from a sol to a gel
by cooling is termed gelation and is a distinct process from that of coagu-
lation. Temperature, time, concentration, and the presence of electrolytes
or non-electrolytes are factors in gel formation. There are various theories
regarding gel structure, but space does not permit considering them here.
It may be added that there is a similarity between crystallization and
gelation. Jordan Lloyd states that "gelation is a process closely parallel to
crystallization and is accompanied in most cases by evolution of heat." One
other factor of similarity is that with rapid cooling a finer structure of the
micelles is obtained.
Swelling of colloidal gels. Freundlich designates the gels that may
imbibe a liquid and give it up as turgescible, and those which do not swell
as non-turgescible. The imbibition of water is known as turgescence; the
giving up of water is designated as deturgescence. The term hydration is
used to indicate imbibition of water by the micelles ; solvation is the general
term used for all liquid dispersion mediums. The lyophilic colloids are
characterized by their affinity for their dispersion mediums. Whether the
micelles of gelatin, agar-agar, etc., actually act as a solvent for the inter-
micellar liquid, or whether the liquid is attracted and bound, probably
by electrical forces, to the surface of the micelle in the form of a shell is
still a disputed question. The amount of water that can be held by many
of the micelles in the form of a shell around the particle is relatively enor-
mous. As the concentration of the micelles is increased, the viscosity of the
solution increases, owing to a larger portion of the dispersion liquid being
bound. A concentration of 0.75 per cent of pectin micelles gives a fruit
jelly of good texture. Increasing their concentration gives a stiffer jelly.
The molecules of the dispersion medium are probably oriented in a definite
manner around the micelle, for the volume of a gelatin gel is less than the
combined volume of the dry gelatin and the water.
The ability of the micelles to take up and hold water is important in
food preparation. The thickening of a cup of milk by an egg in a custard
is due to the ability of the egg proteins to bind the liquid. The thickening
power of starches is due to the swelling of the starch granules during heat-
ing. The ability of proteins, starches, etc., to imbibe water is very great.
A pressure of 2500 atmospheres is required to prevent the swelling of starch
when it is heated in water. Gortner defines imbibition pressure as "the
pressure against which such a colloid will imbibe a liquid, or conversely the
8 RELATION OF COOKERY TO COLLOID CHEMISTRY
Page 8
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of separation between sols and gels, nor on the other hand, according to
Jordan Lloyd, between gels and curds. The classification of gels on the
basis of consistency includes many gels that do not have similar proper-
ties. Some gels are formed from sols by coagulation. Custard is an ex-
ample of this type. A starch gel is formed by gelatinization of the starch
during cooking. Gelatin, agar-agar, and soaps form sols above certain
temperatures and gels at lower temperatures. Such gels are called revers-
ible. The sol-gel transformation is brought about gradually, there being
no definite melting or setting temperature. The change from a sol to a gel
by cooling is termed gelation and is a distinct process from that of coagu-
lation. Temperature, time, concentration, and the presence of electrolytes
or non-electrolytes are factors in gel formation. There are various theories
regarding gel structure, but space does not permit considering them here.
It may be added that there is a similarity between crystallization and
gelation. Jordan Lloyd states that "gelation is a process closely parallel to
crystallization and is accompanied in most cases by evolution of heat." One
other factor of similarity is that with rapid cooling a finer structure of the
micelles is obtained.
Swelling of colloidal gels. Freundlich designates the gels that may
imbibe a liquid and give it up as turgescible, and those which do not swell
as non-turgescible. The imbibition of water is known as turgescence; the
giving up of water is designated as deturgescence. The term hydration is
used to indicate imbibition of water by the micelles ; solvation is the general
term used for all liquid dispersion mediums. The lyophilic colloids are
characterized by their affinity for their dispersion mediums. Whether the
micelles of gelatin, agar-agar, etc., actually act as a solvent for the inter-
micellar liquid, or whether the liquid is attracted and bound, probably
by electrical forces, to the surface of the micelle in the form of a shell is
still a disputed question. The amount of water that can be held by many
of the micelles in the form of a shell around the particle is relatively enor-
mous. As the concentration of the micelles is increased, the viscosity of the
solution increases, owing to a larger portion of the dispersion liquid being
bound. A concentration of 0.75 per cent of pectin micelles gives a fruit
jelly of good texture. Increasing their concentration gives a stiffer jelly.
The molecules of the dispersion medium are probably oriented in a definite
manner around the micelle, for the volume of a gelatin gel is less than the
combined volume of the dry gelatin and the water.
The ability of the micelles to take up and hold water is important in
food preparation. The thickening of a cup of milk by an egg in a custard
is due to the ability of the egg proteins to bind the liquid. The thickening
power of starches is due to the swelling of the starch granules during heat-
ing. The ability of proteins, starches, etc., to imbibe water is very great.
A pressure of 2500 atmospheres is required to prevent the swelling of starch
when it is heated in water. Gortner defines imbibition pressure as "the
pressure against which such a colloid will imbibe a liquid, or conversely the
8 RELATION OF COOKERY TO COLLOID CHEMISTRY