Bancroft states that peptization is always due to adsorption. Since peptiza-
tion occurs frequently in food preparation, additional mention should be
made of it here in connection with adsorption. Bancroft states that theo-
retically there are three possibilities when adsorption occurs at a surface.
(1) If an adsorbed film has a low surface tension on the water side and a
high one on the other side, it will tend to scrunch up and to peptize the solid
as internal phase. (2) If the reverse is true, the solid will tend to form the
external phase. (3) If the two surface tensions are equal, neither will pre-
ORIENTATION OF MOLECULES 17
vail. Many instances of peptization might be mentioned. There is the pre-
vention of coagulation by heat of peptizating action of sugar on egg pro-
tein. There is the peptizing action of soda on flour proteins with increasing
tenderness of the product. This occurs in the chocolate cake known as
devil's food when excess soda is used.
The interfacial tension o£ solids/liquids. Adsorption is very pro-
nounced at the interface between these two phases. Just as small drops of
a liquid will unite through their vapor to form larger drops, thus reducing
the free surface energy and their surface area, so small crystals in a super-
saturated solution will unite to form larger crystals. Small crystals have a
greater solubility than large crystals. They also have a greater surface area
than the large ones per unit mass, thus a greater surface energy. Since
surface energy tends to a minimum, equilibrium can be reached only by
establishment of larger crystals in the solution. A discussion of the growth
of crj'stals in fondant and similar candies during storage is given later.
Formation of surface skins. Bechhold states that one characteristic
of colloidal systems is the forming of a surface skin. This may be similar
to the formation of a boundary layer in a liquid. Staining solutions form
a scum on the surface that must be removed by straining before using.
In food preparation this surface skin may be due to a change, as coagulation
of a portion of the egg by beating or coagulation of proteins by heating.
Whatever the cause, beaten eggs and egg yolks form surface skins after
standing a short time, which is not entirely due to a drying of the surface.
Boiled milk forms a pronounced skin during heating and after cooling.
Orientation of molecules. The molecules at an interface do not
lie at random but are oriented or arranged in a definite manner. The
arrangement assumed depends partly upon the arrangement of the atoms
in the molecule. Molecules of ethane (CH3.CH3), butane, and pentane
are symmetrical, i.e., in each molecule the two ends are identical. In acetic
acid (CH3.COOH) the two ends of the molecule are unsymmetrical. In
ethane the two ends would behave the same at an interface, but this would
not hold for acetic acid. The (CH3) group is called a non-polar group;
the (COOH) group, a polar one. At the air/ water interface polar groups
always orient away from the air and towards the water, for water has
pronounced polar characteristics. "Like attracts like" is a rule that all
students of organic chemistry have learned. In general, the polar liquids are
miscible, but slightly polar liquids are relatively insoluble in polar liquids.
Page 17
Presented as published in 1932. Historical recipes may not meet modern food-safety standards. Cook from the modern interpretation, not the original instructions.
AI-modernized reading of the original text
A modernized reading is not available for this page yet. You are seeing the original text.
Bancroft states that peptization is always due to adsorption. Since peptiza-
tion occurs frequently in food preparation, additional mention should be
made of it here in connection with adsorption. Bancroft states that theo-
retically there are three possibilities when adsorption occurs at a surface.
(1) If an adsorbed film has a low surface tension on the water side and a
high one on the other side, it will tend to scrunch up and to peptize the solid
as internal phase. (2) If the reverse is true, the solid will tend to form the
external phase. (3) If the two surface tensions are equal, neither will pre-
ORIENTATION OF MOLECULES 17
vail. Many instances of peptization might be mentioned. There is the pre-
vention of coagulation by heat of peptizating action of sugar on egg pro-
tein. There is the peptizing action of soda on flour proteins with increasing
tenderness of the product. This occurs in the chocolate cake known as
devil's food when excess soda is used.
The interfacial tension o£ solids/liquids. Adsorption is very pro-
nounced at the interface between these two phases. Just as small drops of
a liquid will unite through their vapor to form larger drops, thus reducing
the free surface energy and their surface area, so small crystals in a super-
saturated solution will unite to form larger crystals. Small crystals have a
greater solubility than large crystals. They also have a greater surface area
than the large ones per unit mass, thus a greater surface energy. Since
surface energy tends to a minimum, equilibrium can be reached only by
establishment of larger crystals in the solution. A discussion of the growth
of crj'stals in fondant and similar candies during storage is given later.
Formation of surface skins. Bechhold states that one characteristic
of colloidal systems is the forming of a surface skin. This may be similar
to the formation of a boundary layer in a liquid. Staining solutions form
a scum on the surface that must be removed by straining before using.
In food preparation this surface skin may be due to a change, as coagulation
of a portion of the egg by beating or coagulation of proteins by heating.
Whatever the cause, beaten eggs and egg yolks form surface skins after
standing a short time, which is not entirely due to a drying of the surface.
Boiled milk forms a pronounced skin during heating and after cooling.
Orientation of molecules. The molecules at an interface do not
lie at random but are oriented or arranged in a definite manner. The
arrangement assumed depends partly upon the arrangement of the atoms
in the molecule. Molecules of ethane (CH3.CH3), butane, and pentane
are symmetrical, i.e., in each molecule the two ends are identical. In acetic
acid (CH3.COOH) the two ends of the molecule are unsymmetrical. In
ethane the two ends would behave the same at an interface, but this would
not hold for acetic acid. The (CH3) group is called a non-polar group;
the (COOH) group, a polar one. At the air/ water interface polar groups
always orient away from the air and towards the water, for water has
pronounced polar characteristics. "Like attracts like" is a rule that all
students of organic chemistry have learned. In general, the polar liquids are
miscible, but slightly polar liquids are relatively insoluble in polar liquids.