cooking, and they may become mushy and disintegrate. This is probably
due to the greater dispersion of the cellulose and pectic substances. In dried
legumes, alkalies may also increase the disintegration of some of the protein.
Milk is prevented from curdling or coagulating by the addition of alkali.
Curdling is a lessened dispersion of the milk protein, casein. The addition of
alkalies to eggs elevates the temperature for coagulation. Alkalies added
to doughs cause a greater degree of dispersion of the gluten, which results
in a dough that is runny and sticky to handle. In larger quantities the bak-
ing quality of the flour is partially destroyed. Alkalies added to gelatin
tend to prevent its setting, and they may cause greater dispersion in emul-
sions.
Dispersion by enzymes. Enzymes may also cause an increased or
lessened degree of dispersion in foods. The clotting of milk upon the ad-
dition of rennin is an example of lessened dispersion, but the proteinase
enzyme in flour increases the dispersion of the gluten.
Classification and Properties of Colloids Based upon
Physico-Chemical Relationships in Liquids
Each colloidal solution as w^ell as each true solution has its own peculiar
properties. These depend upon the nature of the particles in solution and
the dispersing medium. But a large group of colloidal systems may have
similar properties, and for convenience they are classified in a group or
subdivision. The classification of colloidal systems into groups is not
always satisfactory, for there is no distinct line of demarcation between
the different subdivisions. Ostwald, Freundlich, Gortner, and Buchanan
and Fulmer give excellent discussions of the properties of colloidal systems
which are of interest to those concerned with food preparation.
Suspensoids and emulsoids. One basis for classification of colloidal
systems is the nature of the dispersed phase. In a suspensoid the dispersed
particles are in a solid state. In an emulsoid the dispersed particles are in a
liquid state. Many authorities classify suspensoids and lyophobes, emulsoids
and lyophiles, as being coextensive, but Freundlich states that this is in-
correct, for there are many emulsoids with lyophobic properties.
Reversible and irreversible colloids. If after a colloidal solution is
evaporated, a sol is reformed upon the addition of water, the colloid is
classified as a reversible colloid. Gelatin and dried egg white are examples
of this type of colloid. An irreversible colloid does not spontaneously form
a sol with the addition of water, after water has been evaporated. Re-
versible and hydrophilic colloids are coextensive; irreversible and hydro-
phobic colloids belong to similar groups.
Sols and gels. Colloidal solutions are also classified upon the basis
of their consistency. Those which are apparently solutions are called sols.
Those with a jelly-like consistency are called gels. The consistency of fruit
jelly or a gelatin dessert is that of a typical gel. There is no distinct line
SWELLING OF COLLOIDAL GELS 7
Page 7
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cooking, and they may become mushy and disintegrate. This is probably
due to the greater dispersion of the cellulose and pectic substances. In dried
legumes, alkalies may also increase the disintegration of some of the protein.
Milk is prevented from curdling or coagulating by the addition of alkali.
Curdling is a lessened dispersion of the milk protein, casein. The addition of
alkalies to eggs elevates the temperature for coagulation. Alkalies added
to doughs cause a greater degree of dispersion of the gluten, which results
in a dough that is runny and sticky to handle. In larger quantities the bak-
ing quality of the flour is partially destroyed. Alkalies added to gelatin
tend to prevent its setting, and they may cause greater dispersion in emul-
sions.
Dispersion by enzymes. Enzymes may also cause an increased or
lessened degree of dispersion in foods. The clotting of milk upon the ad-
dition of rennin is an example of lessened dispersion, but the proteinase
enzyme in flour increases the dispersion of the gluten.
Classification and Properties of Colloids Based upon
Physico-Chemical Relationships in Liquids
Each colloidal solution as w^ell as each true solution has its own peculiar
properties. These depend upon the nature of the particles in solution and
the dispersing medium. But a large group of colloidal systems may have
similar properties, and for convenience they are classified in a group or
subdivision. The classification of colloidal systems into groups is not
always satisfactory, for there is no distinct line of demarcation between
the different subdivisions. Ostwald, Freundlich, Gortner, and Buchanan
and Fulmer give excellent discussions of the properties of colloidal systems
which are of interest to those concerned with food preparation.
Suspensoids and emulsoids. One basis for classification of colloidal
systems is the nature of the dispersed phase. In a suspensoid the dispersed
particles are in a solid state. In an emulsoid the dispersed particles are in a
liquid state. Many authorities classify suspensoids and lyophobes, emulsoids
and lyophiles, as being coextensive, but Freundlich states that this is in-
correct, for there are many emulsoids with lyophobic properties.
Reversible and irreversible colloids. If after a colloidal solution is
evaporated, a sol is reformed upon the addition of water, the colloid is
classified as a reversible colloid. Gelatin and dried egg white are examples
of this type of colloid. An irreversible colloid does not spontaneously form
a sol with the addition of water, after water has been evaporated. Re-
versible and hydrophilic colloids are coextensive; irreversible and hydro-
phobic colloids belong to similar groups.
Sols and gels. Colloidal solutions are also classified upon the basis
of their consistency. Those which are apparently solutions are called sols.
Those with a jelly-like consistency are called gels. The consistency of fruit
jelly or a gelatin dessert is that of a typical gel. There is no distinct line
SWELLING OF COLLOIDAL GELS 7