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/429

 index of oil may be represented by 1.44. The oils differ greatly among themselves in the magnitude of the refractive index, but these indexes are all approximately of the magnitude last mentioned. Hence a determination of the refractive index is a valuable means of helping to discriminate between oils of different kinds.

Reichert-Meissl Number.—Attention was called above to the fact that in addition to three special forms of fatty acids there were many others present in oils in small quantities. Among these are found acids which are volatile in a current of steam, which is not the case with the oleic, palmitic, and stearic acids. Among the most important of the volatile acids is the one which exists in large quantities in butter, namely butyric acid. The quantity of volatile acid is determined arbitrarily by the amount of a standard alkali solution which will be neutralized by the volatile acid from five grams of fat. In the case of butter, for instance, it may be said that in round numbers it requires 28 cubic centimeters of standard alkali to neutralize the volatile acid produced according to the above method of procedure. In cottonseed oil the amount of standard solution required to neutralize the volatile acid obtained in the same way is extremely minute, amounting to less than one-half cubic centimeter.

I have given above a brief description of some of the physical and chemical characteristics of oils and fats in order that the reader not specially trained in chemistry may understand thoroughly the references made to these properties in the general description given of vegetable fats and oils. It is not necessary to be a skilled chemist in order to have a general knowledge of some of the points which are of most interest in this respect.

Saponification Value.—As is well known, one of the most common uses of oils and fats is in soap making. Soap consists of the products of chemical reactions by means of which the glycerine contained in an oil or fat are set free and a mineral or other base substituted therefor. For instance, lye consists of the hydrate or carbonate of potash and soda. When an oil is heated with a lye the fatty acid leaves the glycerine in the oil and combines with the potash or soda of the lye. The number of milligrams of potash or soda required to saponify one gram of fat or oil is called its saponification value. For instance, in the case of cottonseed oil it requires, in round numbers, 190 milligrams of potash or hydrate of potash (KOH) to replace the glycerine in one gram of oil. The quantity of potash required for an edible oil to make a complete saponification varies, and hence this number becomes one of the means of distinguishing between them.

Specific Gravity.—The relative weight of a given volume of oil compared with the weight of the same volume of water at the same temperature or at some standard temperature is known as its specific gravity. The oils and fats are universally lighter than water, and in the comparison the unit weight