Part 69

1 : K = M : 56100,

56100 Whence M = ------ (2). K

Substituting this value of M in (1) we have

A × 56100 100A F = ------------ = ------- (3). 561 × K K

It is evident that it is not necessary to calculate the acid value (A) of the sample and the saponification value (K) of the free fat acids, the ratio A/K alone being required. It will be sufficient therefore to substitute for A and K the number of cubic centimeters of alkali solutions required for one gram of the fat and one gram of the fat acids, respectively. If a and b represent these numbers the formula may be written

100a F = ------- (4); b

100a and N = 100 - F = 100 - ------ (5). b

To simplify the determinations, it may be assumed that the free fat acids have the same molecular weight as those still in combination with the glycerol in any given sample. On this assumption, the process may be carried on by determining the acid value A and the saponification value K for the total fat acids. The mean molecular weight M, the percentage of free fat acids F, and the proportion of neutral fat N, may then be calculated from the formulas (2), (3), (4), and (5).

Further, let G = the quantity of glycerol and L that of fat acids obtainable from one gram of neutral fat, that is, ¹/₁₀₀ of H the percentage of total fat acids.

The molecular weight of the neutral fat in each case is 3M + 38. Therefore, 3M + 38 parts of neutral fat yield 3M parts of fat acids and ninety-two parts of glycerol (C₃H₈O₃ = 92).

H 3M Then L = ----- = ---------- (6); 100 3M + 38

92 and G = --------- (7). 3M + 38

N per cent of neutral fat yields, therefore, on saponification, the following theoretical quantities of fat acids F, and glycerol G expressed as parts per hundred.

3M F = N × -------- (8); 3M + 38

92 and G = N × --------- (9). 3M + 38

Formula (9) expresses also the total yield of glycerol from any given sample. For a further discussion of this part of the subject a work of a more technical character may be consulted.

=353. Determination of a Free Fat Acid in a Fat.=—The principle of the method rests upon the comparative accuracy with which a free fat acid can be titrated with a set alkali solution when phenolphthalien is used as an indicator. Among the many methods of manipulation which the analyst has at his command there is probably none more simple and accurate than that depending on the solution of the sample in alcohol, ether, chloroform, or carbon tetrachlorid. Any acidity of the solvent is determined by separate titration and the proper correction made. Either an aqueous or alcoholic solution of the alkali may be used, preferably the latter. The alkaline solution may be approximately or exactly decinormal, but it is easier to make it approximately so and to determine its real value before each operation by titration against a standard decinormal solution of acid. About ten grams of the sample and fifty cubic centimeters of the solvent will be found convenient quantities.

Example.—Ten grams of rancid olive oil dissolved in alcohol ether require three and eight-tenths cubic centimeters of a solution of alcoholic potash to saturate the free acid present. When titrated with decinormal acid the potash solution is found to contain 25.7 milligrams of potassium hydroxid in each cubic centimeter. The specific gravity of the oil is 0.917 and the weight used therefore 9.17 grams. Then the total quantity of potassium hydroxid required for the neutralization of the acid is 25.7 × 3.8 = 97.7 milligrams.

The acid value A is therefore:

3.8 × 25.7 A = ----------- = 10.6 9.17

It is customary to regard free acid as oleic, molecular weight 282. On this assumption the percentage of free acids in the above case is found by the formula

3.8 × 25.7 × 282 A (per cent) = ----------------- = 5.35 561 × 9.17

=354. Identification of Oils and Fats.=—Properly, the methods of identifying and isolating the different oils and fats should be looked for in works on food adulteration. There are, however, many characteristics of these glycerids which can be advantageously discussed in a work of this kind. Many cases arise in which the analyst is called upon to determine the nature of a fat and discover whether it be admixed with other glycerids. It is important often to know in a given case whether an oil be of animal or vegetable origin. Many of the methods of analysis already described are found useful in such discriminations. For instance, a large amount of soluble or volatile acids in the sample under examination, would indicate the presence of a fat derived from milk while the form of the crystals in a solid fat would give a clue to whether it were the product of the ox or the swine. In the succeeding paragraphs will be briefly outlined some of the more important additional methods of determining the nature and origin of fats and oils of which the history is unknown.

The data obtained by means of the methods which have been described, both physical and chemical, are all useful in judging the character and nature of a glycerid of unknown origin. The colorations produced by oxidizing agents, in the manner already set forth will be found useful, especially when joined to those obtained with cottonseed and sesame oils yet to be described. For instance, the red coloration produced by nitric acid of 1.37 specific gravity is regarded by some authorities as characteristic of cottonseed oil as well as the reduction by it of silver nitrate. The coloration tests with silver nitrate (paragraph =320=) and with phosphomolydic acid (paragraph =318=) are also helpful in classifying oils in respect of their animal or vegetable origin. The careful consideration of these tests, together with a study of the numbers obtained by treating the samples with iodin, and the heat of bromination and sulfuric saponification, is commended to all who are interested in classifying oils. In addition to these reactions a few specific tests are added for more detailed work.

=355. Consistence.=—It has already been said that oils are mostly of vegetable origin and the solid fats of animal derivation. In the animal economy it would be a source of disturbance to have in the tissues a large body of fat which would remain in a liquid state at the normal temperature of the body. Nearly all the animal fats are found to have a higher melting point than the body containing them. An exception is found in the case of butter fat, but it should be remembered that this fat is an excretion and not intended for tissue building until it has undergone subsequent digestion. Fish oils are another notable exception to the rule, but in this case these oils can hardly be regarded as true glycerids in the ordinary sense of that term.

In general, it may be said that a sample of a glycerid, which in its natural state remains liquid at usual room temperatures, is probably an oil of vegetable origin. Fish oils have also an odor and taste which prevent them from being confounded with vegetable oils. In oils which are manufactured from animal glycerids such as lard oil, the discrimination is more difficult but peculiarities of taste and color are generally perceptible.

=356. Nature of the Fat Acid.=—When it is not possible to discriminate between samples by the sensible physical properties just described, much light can be thrown on their origin by the determination of their other physical properties, such as specific gravity, refractive index, melting point, etc., in the manner already fully described. Further light may be furnished by saponification and separation of the fat acids. The relative quantities of oleic, stearic, palmitic, and other acids will help to a correct judgment in respect of the nature of the sample. The vegetable oils and lard oils, for instance, consist chiefly of olein; lard and tallow contain a large proportion of stearin; palm oil and butter fat contain considerable portions of palmitin, and the latter is distinguished moreover by the presence of soluble and volatile acids combined as butyrin and its associated glycerids.

Oleic acid can be rather readily separated from stearic and palmitic by reason of the solubility of its lead salts in ether. One method of accomplishing this separation has already been described (paragraph =339=).