This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Shortening power has been defined and tested in various ways. The ordinary household way is to test the product by feel either by breaking or by crushing. Davis has defined shortening as follows: "That material has the greatest shortening power, which when baked in a dough under standard conditions gives to the product a minimum crushing or a minimum breaking strength." Shortening power or the ability of fats and oils to make a product more tender so that it breaks or crushes more readily is due to several factors. These factors for discussion may be classified as follows: (1) differences in structure of the dough, (2) differences in adsorption of different fats and oils at interfaces and orientation, and (3) the surface area covered by the fat or oil. The author is aware that this classification is somewhat superficial, for plasticity, the melting point of the fats used, and other factors might be listed. But differences in plasticity of the fats used will influence the structure of the dough and thus may be considered under the classification given.
Differences in structure of the dough. In the chapter on batters and doughs it was found that differences in structure of batters and doughs were due to several causes, i.e., the method of mixing or combining of ingredients, the temperature of the ingredients when mixed, the extent of mixing, the plasticity of the fat used, the ingredients used, and the proportions of these ingredients. It is rather difficult to exactly duplicate the structure of a dough, but it is particularly difficult when the material is thin like pastry. The fat or oil is seldom distributed evenly throughout the dough. This gives structural weakness in certain areas. For this reason breaking tests of individual pastries may vary considerably.
Theory concerning differences of shortening power of different fats and oils. The theory for differences in shortening power of different fats and oils is based upon the concepts of differences in cohesion, adhesion, adsorption at interfaces, and orientation of molecules. These are in turn based on the concept of polarity. Polar groups or polar molecules, owing to an electric charge, are very reactive and strongly attracted to each other. Some substances like fatty acids have polar groups, while a portion of the molecule is non-polar. This theory for the shortening power of fats and oils has been developed from the work of Lang-muir, Harkins, and their co-workers, upon cohesion, adhesion, interfacial tension, and molecular attraction, between water and organic liquids.
Langmuir has reported: "When a small quantity of an oil, such as olive oil, is placed upon a large clean surface of water, the oil spreads rapidly upon the water surface until a definite area has been covered, and then the oil shows little or no tendency to spread further." The following is a brief summary of his article. Some portions of the molecule have greater affinity for water than other parts. In determining the cause of this spreading of the oil upon the water he found that the film was one molecule thick. If the cause of spreading is an attraction between the oil and water this attraction may be the molecule as a whole or certain portions of the molecule may have greater attraction than other parts for the water. If the whole molecule is attracted there would be solubility of oil in water. If a portion of the oil molecule is attracted by the water while another portion is more attracted by the oil molecules we have a ready explanation of the spreading of the oil on the water.
If oleic acid is considered, as an example, the carboxyl group has a strong affinity for water, and when placed on water this group is probably absorbed in it. The hydrocarbon chain of oleic acid has no attraction for water but it is attracted by other hydrocarbon chains. From this, one can picture the hydrocarbon chains as standing in the air side by side and the carboxyl group dissolved in the water. Since the hydrocarbon chains have an attraction for each other when the amount of oleic acid upon the surface of the water is small, there is a tendency for it to remain in a globule. This attraction of parts of the molecule for each other is in the nature of a chemical force. With very small amounts of oleic acid the hydrocarbon chain may lie along the surface of the water. With enough for a layer the chains may stand upright.
The hydrocarbon chain of oleic acid is not a saturated one. It contains a double bond. With a limited amount of oil on the surface of the water the double bond as well as the carboxyl group may be drawn down onto the surface of the water. In this way it covers a larger surface area of the water.
Unsaturated fatty acids cover a larger surface area than saturated ones. Langmuir has measured the length of the molecules and the area of the water covered by them. He reports that the area covered by the three saturated acids, palmitic, stearic, and creotic, is practically the same, yet the number of carbons in palmitic is 16 and in creotic 26, the latter having the longer molecule. The glyceride tristearin covers an area three times that of one molecule of stearic acid. From this it is seen that the area covered is the same whether as an acid or as an ester of a glyceride. In measuring the height above the surface of the water he found it to be the same as the length of the hydrocarbon chains, showing that they are packed vertically side by side. The results are probably best told in his words. "The unsaturated fatty acids all cover much greater areas per molecule than the saturated. The double bond in oleic acid is thus apparently drawn down onto the water surface. It is interesting to note, however, that linoleic acid with its two double bonds, does not cover any greater area per molecule than oleic acid. It may be that the double bonds attract one another to some extent in place of the water. Linolenic acid, however, with three double bonds, covers a considerably greater area than oleic or linoleic. As the number of double bonds increases, the energy consumed in compressing the film decreases, probably because the carboxyl groups tend to be attracted by the double bonds in adjacent molecules and are thus more easily separated from the water surface."