Page:Foods and their adulteration; origin, manufacture, and composition of food products; description of common adulterations, food standards, and national food laws and regulations (IA foodstheiradulte02wile).pdf/430

 of water is assumed to be unity or 100 or 1000—usually unity or 1000. If the relative weight of water is unity, then the relative weight or specific gravity of oil is expressed as a decimal fraction. For instance, if water is taken as unity the specific gravity of oil equals .912; if the relative weight of water is assumed to be one thousand then the specific gravity expressed above is 912. Unless it is stated otherwise, in all references to specific gravity of these oils it is assumed that the comparison is between the unit weight of water and oil at the same temperature. This is the most convenient form for comparison for general use, though for strictly scientific purposes it is customary to refer all specific gravity numbers to water at the temperature of its maximum density, namely 4 degrees C. (39 degrees F.). At this temperature a given weight of water has its smallest volume, in other words its greatest density. When water is raised to a temperature above that mentioned, it expands and its volume becomes larger. When it is cooled to a temperature below four degrees C., its volume also expands.

The variations in the specific gravity of the common oils is not very great, and therefore the specific gravity is not the most valuable indication in discriminating between these oils.

While there is very little chemical difference between the fats of animals and the oils of plants, the difference is sufficiently distinguished to secure a proper degree of identification and classification. Both classes of bodies are composed of the fatty acids combined with glycerine. The three fatty acids which are most important from the edible point of view and also from the chemical are oleic, stearic, and palmitic. When these acids are united with glycerine as the basic element, they form three classes of oils or fats to which the names olein, stearin, and palmitin are respectively given. A distinction may also be made between a fat and an oil by observing its physical consistence at ordinary room temperature of approximately from 70 to 80 degrees F. It is usual to speak of the bodies which are liquid at such temperature as oils, while those that are solid under like conditions are known as fats. A compound of this description does not pass suddenly from one state to another. In the case of a fat, for instance, which is solid at ordinary temperature, it passes by gradual stages from that condition to a slowly softening mass and then to a complete liquid as the temperature is raised. On the other hand, an oil passes gradually through the same stages to the condition of a solid body as the temperature is lowered. Of the different constituents the olein has the lowest melting point, pure olein being still liquid at quite a low temperature, approaching even the freezing point of water. Stearin and palmitin on the contrary, if in a pure state, are solid at a temperature even above that of the room and above that of blood heat.