Foods, their selection and preparation

Melting Temperatures. Some fats are solid and some liquid at room temperature (65° to 75° F.). While chemically all are fats, those which are liquid at ordinary tempera¬ ture and can be poured are called oils. An oil is a liquid fat. The point of temperature at which a fat changes from a solid to a liquid form is called the melting point. The fats containing large amounts of the saturated fatty acids and the acids with the larger numbers of carbon atoms have the higher melting points and form solid fats. Fats containing large amounts of the un¬ saturated fatty acids have low melting points and many of these are liquid at room tem¬ peratures. Table XXVIII gives the approxi¬ mate melting points of some of the common fats. By holding fats at different temperatures and separating any portion which liquefies, [137] fats can be obtained with different melting points and correspondingly different fatty-acid content and properties. The oleo oil separated in this way from the harder fats in beef suet was formerly used in the manu¬ facture of oleomargarine and contributed in part to the naming of this important table fat. Table XXVIII • Melting Points of Food Fats Fat Approximate Melting Temperature 0 Fahrenheit Bacon fat . Beef suet . 104 to 113 Butter . 77 to 99 Hydrogenated lard . . . Hydrogenated vegetable oil Lard . 104 or below Mutton tallow . 120 to 124 Vegetable oils . Liquid at ordinary tem¬ peratures Since a solid fat is more easily packaged than an oil and is more satisfactory for many uses, a chemical process has been developed by which hydrogen can be added to the unsaturated components of fats, thus chang¬ ing them to saturated forms with a corre¬ sponding raising of the melting point. This process is known as hydrogenation. Smoking Temperature. When fats and oils are heated, they do not vaporize or boil like water; but if heated to sufficiently high temperatures, they decompose and give off fumes. The temperature at which this takes place is an important characteristic of a fat, known as the smoke point or smoking tem¬ perature. Table XXIX gives the smoke points of some of the most used fats and also indicates the amount of free fatty acid present in each. The smoking temperature is impor¬ tant for fats used in frying. The smoke point of each fat may vary within fairly wide limits depending upon (1) the amount of free fatty acids present, (2) the relative amount of surface exposed to the volume of fat during heating, and (3) the presence of foreign particles in the fat. Table XXIX shows that, in general, the smoke point varies inversely with the amount of free fatty acid present. With the same amount of fat, the greater the diameter of the cooking utensil, the lower the temperature at which the fat will smoke. For frying, therefore, a utensil with a narrow diameter is best. If there are crumbs, flour, and other food particles in the fat, it will smoke at a lower temperature than if these are strained out. Table XXIX • Smoke Points of Fats1 Fat Smoke Point Free Fatty Acid °Fahrenheit Per cent All- hydrogenated fats Brand 1 . .17 Brand 2 . .06 Brand 3 . .03 Brand 4 . .03 Compounds Animal and vegetable .12 All vegetable . .08 Vegetable and animal . . .07 Lards Steam-rendered, Brand 1 .49 Steam-rendered, Brand 2 .45