This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Polar groups of molecules and water are attracted to each other. Harkins and co-workers have reported that, if an organic liquid contains polar groups in its molecules, these groups turn towards the water, and the amount of work required to separate it from the surface of the water is greater than when the polar groups are absent. These polar groups are substances containing oxygen, nitrogen, sulfur, iodine, bromine, chlorine, or double or triple bonds.
In a later paper Harkins and Cheng have reported that a paraffin with a triple bond increases the adhesional work very decidedly; they state: "This and other studies in this paper should prove of value in study of lubrication and other adhesional phenomena."
Wilson and Barnard have reported that a good lubricant is one that has the property of "oiliness." They define oiliness as "The property by virtue of which one fluid gives lower coefficients of friction (generally at slow speeds or high loads) than another fluid of the same viscosity."
Orientation in lubricants. Wilson and Barnard, with other investigators, find the "animal and vegetable oils to be superior in oiliness to straight mineral oils." They conclude that the property of oiliness is due to selective adsorption of parts of the oil by the metal surface so that they adhere closely and are not squeezed out from between the metals, or in other words the polar groups of the animal and vegetable oils are attracted by the polar metal, and the fat or oil is less readily squeezed out from between the metal surfaces than oils or fats containing no polar groups. Thus they make better lubricants.
Theory of shortening power applied to cookery. If the results of Langmuir and Harkins can be applied to lubricants they should be equally applicable to the use of fats and oils in cookery. The glycerol part of the fat is attracted by water. If it adheres closely to the water, one can conceive that the water might cement the flour particles less firmly together. Fats and oils with double bonds in their fatty acid chains, or those containing unsaturated glycerides, can be spread over a greater surface area than the saturated fats and would thus adhere to a larger water surface, or compete with the gluten for the water. Also, since the double bond of the fat adheres closely to the metal surface, it may be that it will adhere to the amino groups of the flour proteins, as well as to the water. This would make it more difficult to squeeze out from between the flour particles. Fats and oils containing unsaturated glycerides would have more shortening power because they cover a greater surface area per molecule and because they adhere more firmly to the water surface. If this is true, the fats and oils having the highest percentage of unsaturated acids and covering the greatest surface area should give the tenderest baked products. However, the area covered by a given amount of fat or oil in a dough or batter may vary. A sphere gives the least surface area for a definite weight or volume of a substance. If the oil or fat is emulsified and distributed in the batter in small spheres the total area covered may be less than if the same weight were distributed in very thin films or sheets. Even in oil-in-water emulsions the surface area covered for a definite weight of oil will vary with the degree of dispersion of the oil, for the greater the dispersion the greater the surface area covered.
The surface area covered determines the shortening power of a fat or oil in baked products. The degree of shortening produced by a fat or oil in a given product depends primarily upon the surface area of the flour particles covered by the fat or oil. To a lesser degree shortening power may also depend upon the extent of separation of the egg and milk proteins and the sugar from the flour particles by the fat. The fats and oils are insoluble in the other ingredients of baked products. Thus they may to a greater or less degree mechanically separate the ingredients of the batter but they never entirely lose their identity or change their characteristics. If flour and water are mixed together and baked, the resulting product is very hard, firm, and tough. When the water is added to the flour the protein particles become hydrated and form a continuous phase, somewhat similar to a gel, throughout the mass. When coagulated by heat they form a cement-like structure throughout the dough. If fat or oil is mixed with the flour particles, and water is then added, the fat coats a portion of the flour particles and forms layers, sheets, or films between them, giving the baked product tenderness. The factors that affect the area of the flour covered are (1) manipulation, (2) temperature, (3) the ingredients used and their concentration, (4) the consistency of the fat or oil when added and its degree of unsaturation, and (5) the concentration of the fat.
Manipulation. The manipulative factors include creaming, the thoroughness with which the fat is mixed with the flour, and the manner of rolling, kneading, etc. In products containing sugar, if the fat and sugar are creamed so that air is incorporated, the product may be shorter or more tender. This will depend somewhat on the proportion of fat and sugar, and whether the maximum amount of air is incorporated. With certain proportions of fat to sugar more air can be incorporated. The increased shortness of the product may be attributed to the increased porosity. But creamed fat is also more plastic and thus more easily blended with flour. In addition the sugar crystals absorb fat which may aid in distributing fat in a batter but will probably reduce the area of the flour covered by the fat and thus tend to reduce the shortening power of the fat. This may at least partially explain why wafers, made by adding sugar to a standard pastry dough, are shorter with creamed fats, and least short with oils, the order of shortening power being nearly reversed from the order in pastry.
In pastry the fat is mixed with the flour before the water is added. If this mixing is extensive so that the flour particles are well coated with the fat or oil, the product is shorter than when made from the same fat only slightly mixed with the flour. In the last instance the distribution of the fat increases and in the first decreases the heterogeneity of the final product. An oil or melted fat coats the flour particles more thoroughly with short mixing than a plastic fat. Noble et al. found flour added to the board when the dough was rolled lessened the shortening value of a modified shortbread.
Temperature. Temperature affects the plasticity of a fat and the mobility of a melted fat or of an oil. At lower temperatures the plasticity of fats such as butter is not great. They spread less readily and the area of flour and other particles covered is less with the same extent of mixing than at higher temperatures. But at higher temperatures the surface area covered by a melted fat may decrease because of the increased tendency to form an oil-in-water emulsion, particularly in thin batters. In addition, at temperatures at which the fat is melted or very soft the gluten absorbs water rapidly and becomes developed rapidly. This development of the gluten may offset increased shortening power owing to the soft fat coating a greater flour surface.
The ingredients and their concentration. When the proportion of water in the product is rather high, as in griddle cakes, waffles, and muffins, or in the so-called "pour" batters, the consistency of the batter permits the fat to form spheres easily because the batter is not stiff enough to deform the spheres entirely. Melted fat or oil is generally used in the "pour" batters. In addition the lecithio-protein of the egg yolk used in these products has strong emulsifying properties and increases the tendency to disperse the melted fat or oil as an oil-in-water emulsion. Observations have shown that the major portion of the fat, unless the fat solidifies and with the exception of fat adsorbed by the sugar crystals, is distributed as an oil-in-water emulsion in thin batters.
In biscuits and pastry, in which the proportion of water is relatively smaller, and because the method of adding the fat to the flour is different from that in the "pour" batters, the major portion of the fat is adsorbed by or coats the flour particles. In cake a plastic fat like butter, when creamed with the sugar, always produces a more tender product than when the butter is melted. And, in general, all plastic fats produce more tender cakes than oils or melted fats.
Consistency of fat and degree of saturation. Both these factors have been considered. Obviously unsaturated glycerides have an opportunity to cover a greater surface area of the flour particles in biscuits and pastry because there are no emulsifying agents such as egg to tend to prevent their spreading.
Concentration of fat. Other things being standardized, the surface area covered is greater as the concentration of the fat or oil is increased.
Shortening power of a fat or oil varies in different products. Because of differences in methods of combining, in the concentration and kinds of ingredients, in concentration of the fat, in temperature of the fat, and in manipulative factors the shortening power of the same fat may vary in relation to that of other fats in different products. For example, butter generally produces tougher pastry than any of the plastic fats or the oils commonly used in food preparation, but butter generally produces more tender cakes than oils. The more tests one makes to determine the shortening power of fats and oils, the more one hesitates to make dogmatic statements about the comparative shortening powers of various fats. However, in considering the comparative shortening power of fats and oils, the results published in the literature are usually based on the performance of the fat in pastry only.
Summary of Experimental Work by Different Investigators on