The principal uses of fats and oils in cookery are as follows: (1) to give richness and flavor, as the addition of fat to vegetables, and in mayonnaise and French dressings; (2) for sauteing and for cooking foods in deep fat; (3) for shortening, as in biscuits, muffins, cakes, and pastries. For the first use, flavor is important and may be the deciding factor, unless cost needs to be considered. Examples of this are butter or bacon fat for vegetables in preference to lard or Crisco, and olive oil for salad dressings in preference to other oils. This is a matter of personal choice. For the latter two uses, not only the flavor but also the chemical and physical characteristics may to some extent be deciding factors as to the preferable fat to use.

True fats and oils are composed of hydrogen, carbon, and oxygen, though certain fat derivatives, the lipins, contain either nitrogen or nitrogen and phosphorus.

Properties of fats and oils. One characteristic of fats and oils that may be an advantage or disadvantage in food preparation is that they are excellent solvents for many aromatic compounds. They are solvents for odors of onions, fish, perfumes, and many of the aromatic flavors used for seasoning food. Onions, celery, parsley, etc., may be cooked in fat, and this fat stored in the refrigerator to use in seasoning sauces, meats, etc. This can save many preparations of small amounts of onion or other vegetables used for seasoning. Perhaps the enhanced flavor when fat is added to the fruit filling of pies is partially due to the fat retaining more of the aromatic flavor of the fruit as well as to the enriching of the filling. Because the fat is a good solvent for vanilla and other flavor, it is often advised that it be added to the fat during creaming in mixing cake.

The fats, the fatty acids, and the soaps formed from the fatty acids and alkalies all lower the surface tension of water.

Fats and oils are only sparingly soluble in alcohol, but are readily soluble in ether, chloroform, carbon tetrachloride, etc. Fats and oils are insoluble in water. They have a greasy feel and produce characteristic transparent spots on paper.

Specific gravity of fats and oils. All the naturally occurring fats and oils have a specific gravity of less than 1.0. The term specific gravity is used to denote the ratio between the weight of a substance and the weight of an equal volume of water. Usually the weights are compared at the same temperature. Thus if the specific gravity of an oil is given as 0.919 at 25°/25°C. it means that the weight of the oil at 25° is compared with the weight of an equal volume of water at 25°C. Fats and oils are lighter than water, and if they are poured into water they float on its surface. Cream has a higher fat content than milk and is lighter than the milk. The average specific gravity of some edible fats and oils is given below.

Butter not less than 0.905 at 40°/40°C.

Cottonseed oil about 0.917-0.918 at 25°/25°.

Corn oil about 0.919-0.921 at 25°/25°.

Lard about 0.931-0.932 at 15°/15°.

Olive oil about 0.916-0.918.

Distinction between fats and oils. One distinction between fats and oils is that at ordinary temperatures, 18° to 24°C, the fats are solid and the oils are liquid. Chemically the fats contain higher percentages of the saturated glycerides than the oils, and the oils contain more of the unsaturated glycerides than the fats.

Glycerides. Fats and oils are esters of glycerol and fatty acids, and thus are often spoken of as glycerides. Glycerol is a trihydric or triatomic alcohol and as such combines with three fatty acids to form a molecule of fat or oil. Of the three fatty acids which combine with glycerol to form the molecule of fat or oil, all may be the same acid. If, for example, the three are stearic acid, the name of the glyceride is tristearin. Two of the acids may be the same and the third different. If the molecule contains two stearic and one palmitic acid, it is known as distearo-palmitin. Each of the three may be a different acid. This gives many possible combinations in fats and oils.

The approximate percentage of the unsaturated glycerides in some of the fats and oils used in cooking is given in Table 59.