This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The type of emulsion formed, i.e.: (1) oil-in-water or (2) water-in-oil, depends upon the nature of the emulsifying agent, the nature of the oil, and the effect of electrolytes. With one emulsifier an oil-in-water emulsion may be formed with a specific oil. Sometimes by the addition of the right substance, usually an electrolyte, the emulsion can be reversed and changed to a water-in-oil emulsion. Other emulsifiers with the same oil will form water-in-oil emulsions.
Bancroft states that a hydrophilic colloid tends to make water the dispersing phase, and a hydrophobic colloid tends to make water the dispersed phase.
The potassium and sodium soaps are more soluble in water than in the oil and form oil-in-water emulsions. Magnesium and calcium soaps are more soluble in oil than in water and tend to form water-in-oil emulsions. Aluminum and iron soaps are more soluble in oil than the magnesium and calcium ones and form water-in-oil emulsions.
Bhatnagar emphasizes the influence of the electric charge of the emulsifying agent upon the type of emulsion formed. He makes the following generalization. "All emulsifying agents having an excess of negative ions on them and wetted by water will yield oil-in-water emulsions, while those having an excess of adsorbed positive ions and wetted by oil will give water-in-oil emulsions."
Seifriz, in his work with petroleum oil emulsions stabilized with casein solution, found that the oils with a specific gravity of 0.828 or below form oil-in-water emulsions. When the specific gravity of the oil is greater than 0.857 a water-in-oil emulsion is formed, and oils with specific gravity between 0.828 and 0.857 give coarse, unstable emulsions or cannot be emulsified at all.
Reversal of type of emulsion. No definite rule can be given concerning the reversal of emulsions. The addition of an electrolyte in definite concentrations may stabilize some emulsions and bring about reversal of others. Reversal of some emulsions may occur upon the addition of a definite quantity of a hydroxide. Sometimes the addition of more of the hydroxide will again bring about a reversal of the emulsion into the original type. Shaking after standing may cause reversal of some emulsions.
Some ions are antagonistic to each other. Thus Clowes has shown that, if a rancid oil is dropped from a pipette which has the end immersed under water, drops of a certain size are formed. But if the oil is dropped into a sodium chloride solution, the drops are very much smaller. This is probably due to the formation of sodium soaps with the rancid oil and the lowering of the interfacial tension. If the oil is dropped into a calcium chloride solution the interfacial tension is increased and very large oil drops are formed, owing in this case to the formation of calcium soaps. If both sodium and calcium are in the solution they antagonize each other and the result obtained will depend on the concentration of each present. Sodium and potassium are antagonistic to calcium and magnesium.
Means of determining the type of emulsion. Several ways have been proposed to determine which of the two liquids is the continuous phase.
The drop-dilution method may be used to determine the type of emulsion by the microscope. To a small portion of the emulsion placed on a slide add a drop of water with a pin point and stir slightly. If the water blends with the emulsion, it is an oil-in-water emulsion, but if oil blends with the outside phase it is a water-in-oil emulsion.
Another method of determining the type of emulsion is to use Sudan III or Scharlach R, red dyes soluble in the oil but not in the water. A small portion of the finely powdered dye is dusted over the surface of the emulsion. If oil is the external phase the color gradually spreads throughout the emulsion. But if water is the external phase the color does not spread but is confined to the oil with which it comes in contact on the surface.
The microscope may be used to determine the type of emulsion formed. If the oil is dyed red, a red field with clear globules indicates a water-in-oil emulsion; red globules in a clear field show an oil-in-water emulsion. Sometimes a multiple emulsion is obtained, i.e., a dispersed phase within a dispersed phase. The only means of identifying a multiple emulsion is by the microscope.
Some food emulsifiers. Seifriz has reported that "olive oil stabilized with sodium oleate, sodium stearate, gelatose, gum arabic, albumin, lecithin, saponin, senegin, smilacin, or plant juices (cell sap and protoplasm) forms oil-in-water emulsions, while the same oil stabilized with casein, gliadin, cholesterin, or cephalin forms water-in-oil emulsions." Seifriz has also reported that with casein as the emulsifier the following oils all gave water-in-oil emulsions: olive, castor bean, poppy seed, sperm, and cod-liver oil. Linseed oil forms with casein a dual emulsion with the water-in-oil type predominating. Lecithin favors the formation of oil-in-water emulsions, whereas cholesterol favors water-in-oil emulsions. Saturated casein solutions with common food fats tend to form water-in-oil emulsions.
In food preparation, various fats and oils are used in the formation of emulsions. Oils commonly employed are cottonseed, corn, olive, and peanut. The fats include butter, lard, Crisco, Snowdrift, and others. In a cake batter there are several emulsifiers: the casein of milk, egg yolk, egg white, and the glutenin, gliadin, and starch of the flour. When equal quantities of fat or oil and emulsifying agents were used, the types of emulsions formed varied, depending on the emulsifier used and whether the oil was added to the emulsifier, or vice versa.
With egg yolk all the oils and fats gave a very stable emulsion of the oil-in-water type. With egg white as the emulsifier, oil forms an oil-in-water emulsion. Fig. 26 shows an emulsion of corn oil and egg white. When the oil is added gradually to the egg white, a foam as well as an emulsion is formed. The large white spheres are air bubbles. The oil is colored red and in the photomicrograph shows black. The illustration also shows that the oil is adsorbed in the film or layer at the interface between a liquid and a gas.
When a given weight of butter is added gradually to an equal weight of egg white, an oil-in-water emulsion is formed. With the last additions of the butter the emulsion may break. If beating is continued, a water-in-oil emulsion may form. In melting butter the curdy part containing the casein settles to the bottom of the container and consequently is usually the last portion to be added to the egg white. It may have some effect upon the reversal of the emulsion. Sometimes only an oil-in-water emulsion forms. Whether this is due to the salt or casein content of the butter or to the temperature maintained is not known. The emulsion obtained when egg white is added to the melted butter may be a water-in-oil or an oil-in-water.
Fig. 26. - A coarse emulsion of oil in egg white. Egg white will not foam if a fat like butter or lard is added to it, but with oil both an emulsion and a foam are formed when beaten with an egg beater. The large light spheres are air bubbles. The oil is stained red and appears dark in the photomicrograph. This photomicrograph is interesting for it shows the adsorption or concentration of the oil at the interface between a gas and a liquid (egg white). This adsorption of fat can be seen in later illustrations of cake batter.
Magnification approximately x 135.
Lard and Crisco form rather unstable emulsions with egg white, but both types may be formed.
With a saturated casein solution as the emulsifler, the water-in-oil type of emulsion predominates when butter, lard, Crisco, and oil are used. The emulsions are rather coarse and unstable.
Efficiency of different substances as emulsifiers. Some emulsifying agents are more efficient than others. No study has been made to compare the efficiency of the various emulsifiers under different conditions and for different emulsions. As a class, the hydrophilic colloids seem to be the most efficient emulsifiers. Clark and Mann have reported the efficiency of sucrose, dextrin, starch, gum arabic, and egg albumin in emulsions of benzene and kerosene in water. The oils composed 75 per cent of the total volume in each case. The emulsions were made by shaking in a bottle, the oil being added in 3 parts and a total of 4800 shakes used in making the emulsion. They examined the emulsions after standing 1, 4, and 7 weeks and ranked the emulsifiers for stability on a scale of 10.
With benzene and a 1 per cent solution of each emulsifier the order was as follows: egg albumin, 10; starch, 6; gum arabic, 5; dextrin, 3; sucrose, 0. With kerosene the order was as follows: egg albumin, 10; starch, 9; gum arabic, 3; dextrin, 2; sucrose, 2.
Clark and Mann also determined the efficiency of the above emulsifying agents in the presence of electrolytes. In some cases the electrolytes increased the viscosity; in others they lowered it. They concluded that "no one general rule can be made as to the effect which produces the best emulsions for any one substance nor can any one generalization be made for the effect which produces the best emulsion for all substances under all conditions. Those which seem to be of primary importance are viscosity and film formation."
Brooks reports that egg yolk is four times more efficient in mayonnaise than egg white.