Foods and Their Adulteration / Origin, Manufacture, and Composition of Food Products; Description of Common Adulterations, Food Standards, and National Food Laws and Regulations

by Harvey Washington Wiley

VEGETABLE OILS AND FATS.

The production of a substance known as fat or oil, composed of oxygen, hydrogen, and carbon in the form of a fatty acid and combined with glycerine, is a function of almost every plant. The fat acids are usually in combination with glycerine, which plays the part of a base and in so far as its proportion by weight is concerned is much less important than the fatty acid itself. In round numbers it may be said that nine-tenths of all glycerids or fats are composed of a fatty acid and one-tenth of glycerine. When at ordinary temperature this combination is in a liquid form it is called an oil, and when at ordinary temperature it is in a solid or semi-solid condition it is known as a fat. The term “ordinary temperature” means in this connection that of an ordinary living room and not the extremes of outside temperature. In general terms it may be said that the temperatures referred to are included between the minimum of 50 degrees and the maximum of 85 degrees F. In so far as chemical composition and dietetic properties are concerned, there is no distinction between the oils and the fats. The names are simply a means of ordinary discrimination which has assumed importance by reason of common usage.

There are three of the fatty acids which are particularly important from a dietetic point of view which go to make up the greater part of these fatty and edible vegetable oils and fats. These three acids are oleic, stearic, and palmitic. Of the three, oleic acid is by far the most important, as it constitutes the greater part of nearly all these bodies, especially of oils. In fact the term “olein” and oil are of common origin. Palmitic acid exists chiefly in certain forms of vegetable oil and fats, while stearic acid is a very important constituent of animal oils and fats.

These three acids uniting with glycerine form the glycerids which make up the great body of edible and animal oils and fats, and these principal glycerids are known as olein, palmitin, and stearin, respectively.

Chemical Characteristics.

—The chemical composition of these bodies has been pointed out above. There is, however, in almost all cases, some free acid present in the compound, that is, an acid which is present uncombined with the glycerine. This free acid is usually present in small quantities and is more abundant in the overripe and older plants than in the freshly matured parts. The natural oil also contains certain other ingredients which may be regarded as impurities, and which it is necessary to remove from the oils by a process of purification or refining before they are ready for the table. These impurities may be of a mechanical nature, that is, consisting of parts of the material itself from which the oil is expressed or of certain juices not oils which are found in the animal tissue, portions of protein and other forms of nitrogenous matter, and traces of carbohydrates and gums. The oils have certain definite chemical reactions which are common to them as a class. Among these may be cited, principally, the faculty of absorbing, under certain conditions, the halogens, namely iodin, bromin, and chlorin.

Without entering into any technical description of this process it is sufficient to say here that the degree of absorption of iodin is in a measure the test for the varieties of oil. The different vegetable oils have, as a rule, certain definite relations to the absorption of iodin by means of which they may be to a certain extent identified or separated from similar bodies. The degree of absorption is expressed in the percentage by weight of the oil itself and is known as the iodin number. If, for instance, a gram of any particular oil absorbs one gram of iodin, it is said to have an iodin number of 100. Many oils absorb more than their own weight of iodin, while many others absorb very much less. Another characteristic of oil is found in the fact that with certain reagents, such as an acid either in a dilute state or in a concentrated state, definite colors are produced which are characteristic of the variety of oil in question. As an example of this may be cited the faculty which cottonseed oil has of reducing nitrate of silver to the metallic state, leaving the silver in that finely divided form which has a black color. This is the only oil in common use which has this faculty, and hence it may be regarded as a characteristic test.

Another characteristic chemical property of cottonseed oil is the color which is produced in the Halphen reaction, which has already been described.

One of the most valuable chemical properties of oil is the amount of heat which is produced when it is burned. Inasmuch as oils in relation to their food value are useful chiefly for the production of animal heat, this chemical property becomes of great hygienic and dietetic significance. Of all classes of food products the oils and fats have the highest calorific power. If, for instance, it is said in general that one gram of carbohydrates, such as sugar or starch, on complete combustion will yield 4,000 calories, one gram of protein 5,500 calories, then one gram of oil or fat will yield 9,300 calories. The fats and oils vary among themselves in respect of the number of calories yielded, but all of them give, approximately, the number last mentioned. It therefore follows that oils and fats are the most valuable constituents of food in respect of the production of heat and energy.

Crystalline Characteristics.

—The forms of crystals which the fats assume on solidifying are valuable indicators of the nature of the oil. While these crystal forms are not in all cases distinct, yet they are influenced to a greater or less extent by the nature of the oil itself. Thus the presence of any particular oil may very often be ascertained by the examination of the crystals produced by lowering the temperature very slowly or by dissolving the oil in a volatile solvent and gradually evaporating the solvent. Tests of even greater delicacy may be obtained by first saponifying the fat or oil, separating the fatty acid, and subjecting it to crystallization.

Distribution of Oils in Plants.

—In nearly all cases the part of the plant which contains the most oil is the seeds. In fact all of the vegetable oils which are used for edible purposes are extracted from the seed of the plant. In the case of olives the meaty portion around the seed yields the edible oil of highest value, but in all other cases of edible oils they are derived from the seeds themselves. It is a mistake to suppose that the seeds are the only parts of the plant that contain oil. It is found in all parts of vegetable substances, but is usually concentrated in the seed. It is rather an interesting fact to know that in the seeds of plants both the protein and fats or oils are found, as a rule, in a highly concentrated state, while the carbohydrates are not found chiefly in the seed itself, that is the germ, but distributed in the fleshy envelope surrounding it or in roots or tubers.

The oils and fats are almost all soluble in ether and petroleum ether, though there are some exceptions to this, as in the case of castor oil, which is also insoluble in petroleum ether or gasoline. On the contrary, oils and fats, as a rule, are not soluble in alcohol, but the fatty acids derived from them are. Castor oil is also an exception to this rule, since it is quite soluble in pure alcohol.

Drying and Non-drying Vegetable Oils.

—It might be supposed that if one vegetable oil be edible they all would be. This would probably be the case if vegetable oils were all composed almost exclusively of the three classes of glycerids, which have been mentioned above, but such is not the case. There are other fatty acids in combination with the glycerids which exist in vegetable oils, and chief among these may be mentioned linoleic acid, which exists in considerable quantities in the oil of flax seed, and gives to it its valuable property of a drying oil which makes it so useful in the manufacture of paints. Whenever vegetable oils and fats contain any especial quantity of linoleic acid, or any other fatty acid which has drying properties, they are rendered more or less unfit for human consumption. The number of drying oils is very great, but the most important are linseed oil, hempseed oil, and poppyseed oil. Other vegetable oils have, to a certain degree, drying properties, and among those which are most marked in this particular may be mentioned cottonseed oil, sesamé oil, maize or corn oil, and rapeseed oil. Types of the oils which have the least drying properties and which are regarded as types of non-drying oils are olive oil and peanut oil. The castor oil group is distinguished partially from the other vegetable oils because it contains, or is likely to contain, more or less of a somewhat poisonous substance, namely, ricinolein, which is peculiar to castor oil and to which its purgative value as a medicine is due. The castor bean also contains a very poisonous nitrogenous base, ricin, very small quantities of which may be incorporated in the oil itself.

Melting Point and Solidifying Point.

—The oils and fats differ greatly among themselves in the temperature at which they become solid or liquid. If a solid fat or oil is subjected to a gradual rise of temperature it does not pass at once or suddenly from a solid to a liquid state, but there is a gradual liquefying,—thus olein first becomes liquid and the stearin and palmitin become liquid at a higher degree of temperature. The same phenomenon in its inverse order occurs when a liquid fat is cooled until it solidifies. The moment at which the fats become semi-liquid, liquid, or semi-solid, therefore, is not to be determined with absolute precision, but only approximately, and that temperature is designated as the melting or solidifying point respectively. When the process is carefully conducted under standard conditions the different fats and oils have very definite melting or solidifying points, as determined in the manner described above, and these temperatures should be sufficient to make the melting and solidifying points valuable indications of the character or kind of oil.

Physical Characteristics.

—The difference in the physical characteristics of vegetable fats and oils is even greater than in their chemical composition. Unfortunately for the chemist, the vegetable fats and oils naturally have about the same color or at least very slight variations therefrom, namely, an amber tint, so that, as a rule, it is impossible to discriminate between these oils by their mere color alone. The edible oils also have very much the same taste, so that this physical property is not of any very great diagnostic value. Some of the more important physical properties by which the oils are distinguished are the following:

Refractive Index.

—The well-known phenomenon which is shown by water of bending sharply a ray of light falling upon it in a direction oblique to its surface is known as refraction, and the degree of deflection of the ray is a measure of the refractive index. This is easily illustrated by putting a straight stick or rod into still water at an angle to its surface. The stick or rod will appear to be broken or bent at the surface. Oils have a higher faculty of deflecting the ray of light than water. For instance, if in round numbers the refractive index of water is represented by 1.33, the refractive 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 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.