Rahn has shown analytically the results of the application of "the law of Gibbs and Thomson which states that substances which cause a depression of surface tension will accumulate in the surface. In a gelatin solution, there is more gelatin in the very surface layer than in the center of the solution." In a milk foam the concentration of protein is greater in the foam than in the solution. The analytical results of Rahn are given in Table 2.

Rahn states that "If protein is concentrated on the surface it has a tendency to become solid, but all proteins do not behave alike. Some solidify rapidly, others slowly, and some do not solidify at all. Quite often, this solidification is irreversible, and the protein, when put back into the solution, will not dissolve again." When milk foams, solid walls form around the air bubbles and, when the foam settles, these walls of protein can be seen with the aid of a microscope.

Interfacial tension of liquids/liquids. When two non-miscible liquids are poured together, one liquid forms a layer on top of the other, thus making a liquid/liquid boundary. The less the solubility of the two liquids in each other, the greater their interfacial tension; but most liquids are not completely insoluble in each other. Just as a substance may concentrate at a liquid/gaseous interface, so likewise those substances which decrease interfacial tension tend to concentrate at a liquid/liquid or a liquid/solid interface. With the addition of a third substance soluble in water, Traube's rule holds, but if it is more soluble in the second liquid then the lowering of the interfacial tension is small and Traube's rule scarcely applies.

Table 2 Composition of Skimmed Milk and Its Foam (Rahn)

Constituent of the milk

Average of 9 experiments with intensely foaming skimmed milk

Average of 6 experiments with slightly foaming skimmed milk

Liquid part

Foam

Liquid

Foam

%

%

%

%

Protein.............

3.09

3.51

3.01

3.24

Fat and lactose......

4.85

4.73

4.92

4.92

Ash................

0.75

0.78

0.74

0.76

Total solids.........

8.69

9.02

8.67

8.92

Protein increase in the foam...............

13.6

7.6

Adsorption. Adsorption has been defined as the concentration of a substance at an interface. The increased concentration at the interface is designated as positive adsorption; a decrease in the boundary layer is called negative adsorption. The amount adsorbed depends on the concentration of the material being adsorbed and the extent of the surface at which it can be adsorbed. The importance of surface reactions or phenomena cannot be over-emphasized in food preparation. Lowering of interfacial tension aids in the forming of emulsions. Fat is strongly adsorbed by sugar crystals, and when the two are mixed this aids in distributing the fat throughout the batter.

Bancroft states that peptization is always due to adsorption. Since peptization occurs frequently in food preparation, additional mention should be made of it here in connection with adsorption. Bancroft states that theoretically 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 prevail. Many instances of peptization might be mentioned. There is the prevention of coagulation by heat of peptizating action of sugar on egg protein. 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 of solids/liquids. Adsorption is very pronounced 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 crystals 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. A substance like acetic acid, containing a short hydrocarbon chain and a polar group, is completely miscible with water, for the (COOH) group is attracted to the water so strongly that the hydrocarbon chain is also dragged into the water. When two (CH3) groups are attached to the (COOH) group, the solubility is lessened, and with increasing length of the hydrocarbon chain solubility in water progressively decreases. Thus the possession of a polar group confers upon the substance a certain solubility in water, and the possession of a non-polar group may confer upon it a greater solubility in some other solvent. Orientation occurs at the interface of two liquids, such as emulsions. Polar groups are those containing oxygen, nitrogen, sulfur, or iodine. Thus substances containing (COOH), (CHO), or (NH1) groups, or double bonds, contain polar groups. Polarity is based upon the concept that some parts of a molecule may have gained while other parts may have lost one or more electrons. This results in differences in charges at a small distance apart. The polar groups or polar substances are reactive; the non-polar groups or substances are more inert.

Cohesion. Cohesion refers to the property of a substance whereby the particles of a body are united throughout the mass. The particles as a unit resist being torn apart.

Adhesion. Adhesion refers to the attraction whereby the surface of a substance sticks to the adjacent surface of another substance. Drops of water adhere to glass, and the dough adheres to the sides of the container ' in which it is placed. Pans for sponge and angel cakes are not greased, so that the adhesion of the cake mixture to the sides of the pan will aid expansion of the cake.