This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
A machine to test the shortening power of cooked foods needs to be very sensitive. Davis has devised such a machine, called a shortometer. From tests upon cookies he reports lard to be the best shortening agent. He gives no explanation of the results obtained.
Platt and Fleming have stained fats and oils with Sudan III, a red dye soluble in fats but not soluble in water, and examined the baked products and the batter, in this case cookies, under the microscope. They find the fat to be in thin layers between the starch granules when the material is mounted dry. In some instances the fat completely surrounds the starch granules; in others the granules touch with no fat layer between them. These layers of fat, which physically separate different parts of the dough and prevent them from coming together in a solid mass, vary in thickness in different sections of the dough. To have the best shortening effect a fat must adhere to the flour particles and not be squeezed out by pressure. They give the following order for the shortening power of different fats, lard having the greatest shortening power.
Purified petroleum oils, as Nujol
For these differences in shortening power, Platt and Fleming have suggested that the following factors have minor or major effect: viscosity, surface tension of oil, the melting point of the fat, and orientation of the molecule due to double bonds of the fatty acid chains of the glycerides. The following is a short summary of their article.
Viscosity gives a fat the power of spreading easily in the dough; a more viscous fat will run out of the dough less readily in baking, but it is of relatively little importance in shortening power, as shown by the fact that some of the petroleum products were as viscid as lard but gave less shortening power.
A melting point close to that of lard seems desirable, yet this factor alone is not a satisfactory explanation in all cases, as coconut oils may be prepared having the same melting point as butter but less shortening power.
Plasticity gives a fat the ability to spread easily, which is beneficial; but the fact that the oils having no plasticity give greater shortening power than butter shows that plasticity is only one of the important factors.
Owing to orientation of molecules, fats containing unsaturated glycerides cover a greater surface area than those with saturated glycerides. The shortening power of fats tested, with the exception of lard and lard compound, was in the same order as the percentage of unsaturated glycerides contained by the fats.
The author has found that an oil is partially or wholly emulsified as an oil-in-water emulsion in uncooked cake batter. However, the cake batter contained milk and egg, whereas the cooky batter of Platt and Fleming did not contain milk. In cake batters an unmelted fat is distributed in sheets, layers, or films of different thicknesses between the starch granules, and around the air bubbles and sugar crystals. Whether the oil remains emulsified during baking of a cake batter might have some effect on the finished product, but the distribution of the oil or fat when the batter or dough is being mixed undoubtedly affects the tenderness, for this is the period when the gluten is being developed.
Sudan III was first used in oils and fats. In trying to take photomicrographs it was found that the Sudan III dye was not always deep enough in value to give a clear picture. Dr. Benbrook and Miss Sloss of the Veterinary Department suggested using Scarlet R or Scharlach R, which stained the fat or oil a deeper color than Sudan III. It was a surprise to find that Scarlet R aided in locating some distributions not found with Sudan III. Often the dyed fat can be seen through the microscope when the color is not intense enough to show clearly in a photomicrograph.
Fisher found lard to have the greatest shortening power of the plastic fats tested in pastry. Using 41 and 44 per cent of fat (based on weight of the flour) she found the shortening value of 5 lards, 2 hydrogenated cottonseed oils, 1 hydrogenated lard, an animal stearin, and 1 all-vegetable oil compound as determined by breaking strength with the Bailey Shortometer approximately paralleled their congealing points, those fats having the lowest congealing point having the greatest shortening power.
Noble, McLaughlin, and Halliday investigated several factors influencing the apparent shortening value of a fat, using a modified shortbread (sugar added to a pastry formula). They found that creaming the fat and sugar to a maximum volume produced wafers with smaller breaking strength, as determined by a Bailey Shortometer, than when the same fat was creamed to a minimum volume. Also after adding the flour and liquid to the creamed mass, thorough mixing so that no spots of creamed fat and sugar showed produced markedly more tender wafers than when the dough was incompletely mixed. If no flour was sprinkled on the board for rolling, wafers markedly more tender resulted than when the minimum amount of flour to prevent sticking was used. Beating the extra flour into the dough gave a more tender product than using it in rolling but a less tender one than omitting it entirely.
Shortening experiments with pastry. Denton and Lowe have determined the shortening power of different fats in pastry. "Pastry was chosen for the baked product in their experiments, as it is commonly used, contains a large proportion of fat and is made with a small number of ingredients. In addition to fat it contains water, flour, and salt." The pastry was mixed in a KitchenAid and the materials were incubated before mixing, so that the temperature of the ingredients when mixed was 80°F.
Denton and Lowe used the shortometer as designed by Davis for the shortening tests. Weight is applied at the upper end of a vertical rod. At the lower end of the rod a bar is attached which rests across the material to be broken. The crushing strength was determined by substituting for the bar a rod which punctured a hole in the pastry. Sometimes the weight rested an appreciable length of time on the pastry before breaking or crushing occurred. Thus they found in the breaking and crushing tests that the rate at which the load is applied and the time through which the load acts are both factors in determining the breaking and the crushing strength of pastry, which is quite fragile. In order to obtain a more uniform rate of loading than with shot they used mercury, run from a burette into a receptacle resting on a cap at the top of the shortometer. Thus the weight or load for crushing the pastry gradually increased.