Chapter II: Introduction

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Fat as Food—Physiological Considerations—Constitution of Fats and Oils.

The food of man may be divided broadly into two classes, _nitrogenous_, or flesh-forming, which is almost entirely of animal origin, and _carbonaceous_, or energy-producing, derived both from the animal and vegetable kingdoms. Besides nitrogen and carbon, many other elements, of course, such as phosphorus, calcium, iron, etc., normally enter into the composition of human food, but the nitrogen and carbon constitute the chief ingredients thereof, and are absolutely necessary to maintain the body in a healthy and efficient state.

There is an almost infinite variety of forms in which carbon may be taken into the stomach, but the vast majority of carbonaceous foods may be classified in two great chemical families: (1) the _carbohydrates_, comprising starch, sugar, and similar substances, which consist of carbon, hydrogen, and oxygen, the two latter in quantities having the same ratio as in water; and (2) the _oils and fats_, with which in the present volume it is proposed to deal.

_Physiological Considerations._—The primary function of carbon-containing food is, by its combustion, to produce heat or other form of energy. The combustion of 1 gram of carbon to carbon dioxide produces 8080 calories; of 1 gram of hydrogen to water, 34,462 calories; whilst the presence of oxygen actually reduces the calorific value of the substance. Hence the higher the proportion of carbon, and lower the amount of oxygen, the greater will be the heat-producing power of a food; and since fats are much richer in carbon than starch or sugar, containing about 2½ times as much, they constitute the most concentrated form in which fuel can be supplied to the body. In the case of animal fats, carbon in the form of carbohydrate is converted into fat by the animal organism, and is thus rendered more suitable for the food of man; as although man in his internal economy, principally by means of his liver, is quite capable of himself transforming starchy matter into fat, in so doing he expends a certain amount of energy. Moreover, the human stomach is relatively smaller than that of an animal, and therefore a more highly concentrated form of carbon-containing food is desirable. Dripping is especially rich in carbon, containing over 10 per cent. more than does butter or suet, and it is regrettable that the use of dripping, formerly so popular among the working classes, has now become so largely a thing of the past.

The average relative proportions of fat and carbohydrate in the food of man vary with the climate, and are also governed to a considerable extent by their availability. Thus in very hot regions carbohydrates predominate, whilst in colder countries chiefly fat is consumed. The Eskimo takes almost all his carbon in the form of oil and fat, whereas the Indian or Chinese subsists mainly on carbohydrates. In this country the proportions recommended by physiologists, though varying slightly, are about 1 part of fat to 10 parts of carbohydrates, the amount of fat desirable being slightly higher in winter than in summer.

Besides its value as a heat or energy producer, the presence of a proportion of fat in human food is important in other ways, for the “food value” of any substance depends not only upon its composition, but also on its digestibility and palatability; and whilst fats are much more readily assimilated than carbohydrates, they also render more palatable, and assist in the digestion of, other articles of food.

Butter is the most easily digested of all fatty foods, and in cases where a fat diet is necessary, up to ¼ lb. of butter can be absorbed per diem. Margarine, which is usually made to approximate fairly closely to butter in composition, except in so far as the butter contains butyric and other volatile fatty acids, should be as digestible as butter, and is almost universally agreed to be so.

Yet another useful purpose served by a proportion of fatty food consists in facilitating the passage of masticated food to the stomach, and of the refuse matter through the bowel.

_Constitution of Fats and Oils._—The difference between a fat and an oil is entirely dependent upon temperature, a fat becoming an oil when it is melted, and an oil a fat when solidified. The term _oil_ is used for substances differing widely both in composition and properties, but all the fats and oils used for edible purposes are of the same general type of constitution, viz. esters or salts of glycerin with one or more fatty acids, which are termed “glycerides.” Their composition was first placed on a scientific basis by Chevreul, who in the early part of the last century showed that when a fat, such as tallow or lard, was converted into soap by the action of sodium or potassium hydroxide, the fat was decomposed into glycerin and fatty acids, the latter combining with the alkali to form the soap, while the glycerin, remaining free, was separated in the lyes. The three most commonly occurring glycerides are stearin and palmitin (of which tallow chiefly consists) and olein (the principal constituent of olive oil), and the action of sodium hydroxide on these may be represented by the following equations:—

CH₂OOC₁₈H₃₅ CH₂OH
/ /
CHOOC₁₈H₃₅ + 3NaOH = 3NaOOC₁₈H₃₅ + CHOH
\ \
CH₂OOC₁₈H₃₅ CH₂OH

(Stearin) (Sodium (Sodium (Glycerin)
hydroxide) stearate)
--------------------------------------------------------------------

CH₂OOC₁₆H₃₁ CH₂OH
/ /
CHOOC₁₆H₃₁ + 3NaOH = 3NaOOC₁₆H₃₁ + CHOH
\ \
CH₂OOC₁₆H₃₁ CH₂OH

(Palmitin) (Sodium (Sodium (Glycerin)
hydroxide) palmitate)
--------------------------------------------------------------------

CH₂OOC₁₈H₃₃ CH₂OH
/ /
CHOOC₁₈H₃₃ + 3NaOH = 3NaOOC₁₈H₃₃ + CHOH
\ \
CH₂OOC₁₈H₃₃ CH₂OH

(Olein) (Sodium (Sodium (Glycerin)
hydroxide) oleate)

The conclusions of Chevreul as to the composition of fats were subsequently confirmed by Berthelot, who succeeded in producing the glycerides synthetically by heating the fatty acids with glycerin under pressure in sealed tubes. Heating together, for example, stearic acid and glycerin, he obtained stearin, according to the equation:—

3 C₁₈H₃₅O₂H + C₃H₅(OH)₃ = C₃H₅(C₁₈H₃₅O₂)₂

In view of the fact that glycerin contains three hydroxyl (OH) groups in which the H is displaceable by an acid radicle, it follows that compounds may be formed, in which only one, or two, or all three of the hydrogen atoms are replaced by an acid, compounds of the following types resulting; where R represents a fatty acid radicle.

Monoglyceride:—
CH₂OR CH₂OH
| |
(Alpha) CHOH and (Beta) CHOR
| |
CH₂OH CH₂OH

Diglyceride:—
CH₂OR CH₂OR
| |
(Alpha) CHOH and (Beta) CHOR
| |
CH₂OR CH₂OH

Triglyceride:—
CH₂OR
|
CHOR
|
CH₂OR

Intermediate products, corresponding to the above formulæ for the mono- and di-glycerides, were obtained by Berthelot in his syntheses, but in natural oils and fats glycerides are only met with in which all the hydrogen atoms in the hydroxyl groups are displaced by an acid.

Formerly it was believed that in nature the acid radicles combining with the same molecule of glycerin were all identical, but during the last few years a large number of so-called “mixed glycerides” have been discovered in various oils and fats, which may be represented by the above formula for the triglyceride, if the radicles denoted by R are assumed to be not all alike. Among these mixed glycerides may be mentioned oleodipalmitin, C₃H₅(OC₁₈H₃₃O)(OC₁₆H₃₁O)₂; stearodipalmitin, C₃H₅(OC₁₈H₃₅O)(OC₁₆H₃₁O)₂; oleopalmitostearin, C₃H₅(OC₁₈H₃₃O)(OC₁₆H₃₁O)(OC₁₈H₃₅O); and palmitodistearin, C₃H₅ (OC₁₆H₃₁O) (OC₁₈H₃₅O)₂ obtained by Hansen, and by Bömer from tallow; stearodipalmitin being also found in goose and turkey fat by Klimont and Meisels, and palmitodistearin in lard by Kreis and Hafner.

Oleodidaturin C₃H₅ (OC₁₈H₃₃O) (OC₁₇H₃₃O)₂ has been found in olive oil by Holde and Stange to the extent of one to two per cent., and it is probable that the butyric acid present in butter fat exists as a mixed glyceride, and not as butyrin; indeed, mixed glycerides are claimed to have been found in butter fat by Bell, and Blyth, and Harrison respectively.

The following are the chief pure triglycerides, together with their source, formulæ, and more important constants:—

It will be observed that butyrin and olein are both liquid at ordinary temperatures, while tallow and palmitin have comparatively high melting points. Fats such as tallow or palm oil, therefore, in which the proportion of these latter is high, are firm and hard, the degree of hardness increasing with the percentage of these glycerides.

_Butyrin_ (Tributyrin) may be obtained by heating together butyric acid and glycerin under pressure. According to Scheij its specific gravity is

20° 60°
_d_ ———— = 1·0324, and _d_ ———— = 0·9963.
4 4

It is almost insoluble in water, and has an intensely bitter taste.

_Laurin_ (Trilaurin) may be produced by heating together lauric acid and glycerin. It is readily soluble in ether, but only slightly so in cold absolute alcohol, and crystallises in needles, melting at 45-46° C., and having, according to Scheij, the specific gravity

60°
_d_ ———— = 0·8944.
4

_Myristin_ (Trimyristin) may be isolated from nutmeg butter by fractional distillation in vacuo, or can be prepared by heating together myristic acid and glycerin. It crystallises in laminæ, which on heating first melt at 56°·5, but again solidify as the temperature is further raised, at 57-58°. The product then has a melting-point of 45-55°. Its boiling point in vacuo is 290-300°, and its density

60°
_d_ ———— = 0·8848.
4

_Palmitin_ (Tripalmitin) may be prepared artificially by heating together palmitic acid and glycerin, repeatedly boiling the product with alcohol, and allowing it to crystallise, when greasy scales are obtained, having a peculiar pearly appearance. The effect of heat on palmitin is somewhat curious, indicating the existence of distinct modifications. Thus when heated to 46° C. it liquefies, but again becomes solid on further raising the temperature, melting once more at 61°·7, and becoming cloudy, with separation of crystalline particles. Further increase of temperature to 63° C. renders the liquid clear, and this temperature is regarded as the true melting-point. After melting and re-solidifying, palmitin possesses no crystalline fracture.

_Stearin_ (Tristearin) may be separated from tallow by dissolving it in ether and allowing it to crystallise, when small crystals separate, having a bright pearly lustre. Stearin when heated also shows the existence of two modifications. Thus, on raising the temperature to 55° C., stearin liquefies, but again becomes solid on further increasing the temperature until 71°·6 is reached, when it again melts. If this liquid is further heated to 76°, and then allowed to cool, solidification does not take place until the temperature has fallen to 55°, but if, after attaining 71°·6, it is immediately cooled, it will solidify at 70° C.

_Olein_ (Triolein) is one of the most widely distributed natural glycerides, and may be prepared in an impure form from olive oil by separating the solid glycerides by cooling. After maintaining the oil at a low temperature for several days, and separating the liquid portion, the latter may be freed from traces of stearin and palmitin by solution in alcohol. Olein may also be produced artificially by heating together oleic acid and glycerin. It is an odourless, colourless, and tasteless oil, which may be distilled _in vacuo_, without decomposition, but which rapidly absorbs oxygen from the air, and becomes rancid.

As already stated, the natural glycerides of which edible fats and oils are composed, consist of combinations of glycerin with various fatty acids. These may be separated by saponifying the fat or oil with sodium or potassium hydroxide, dissolving the resulting soap in hot water, and adding sufficient dilute sulphuric acid to decompose the soap, when an oily layer gradually rises to the surface. This when melted by gentle heat and washed free from mineral acid, is soluble in alcohol and reddens blue litmus paper. It consists of the insoluble fatty acids of the fat, those soluble in water, such as acetic, propionic, butyric, caproic, caprylic, and capric, remaining for the most part dissolved in the aqueous portion underneath.

All the acids naturally present in fats and oils are mono-basic, _i.e._ contain only one carboxyl (COOH) group, but they may be arranged in five classes or homologous series, based on their chemical constitution, these series having the following general formulæ:—

I. Stearic Acid Series CₙH₂ₙ₊₁COOH.
II. Oleic Acid Series CₙH₂ₙ₋₁COOH.
III. Linolic Acid Series CₙH₂ₙ₋₃COOH.
IV. Linolenic Acid Series CₙH₂ₙ₋₅COOH.
V. Ricinoleic Acid Series CₙH₂ₙ₋₇COOH.

The more important members of these series, together with their formulæ, melting-points, and principal occurrence, are given in the following tables:—

_I. Stearic Series_

The acids of this series are all what is termed saturated compounds, _i.e._ they do not form addition compounds when brought in contact with bromine, iodine, or ozone. The two first are liquid at ordinary temperatures, distil unchanged under atmospheric pressure, and are miscible with water in all proportions. The next four are more or less soluble in water, and readily distil with steam, as does also lauric acid, though the latter is practically insoluble in cold water, and only dissolves very slightly in boiling water. These first seven acids are termed _Volatile Fatty Acids_, and on their volatility are based the Reichert process and its modifications and the Polenske method for the examination of butter fat for adulteration, vide pp. 111-114. The higher acids of the group are solid, and are completely insoluble in water. The whole series is readily soluble in warm alcohol, and undergoes no change when heated with solid caustic alkali.

_II. Oleic Acid Series_

These acids differ essentially from those of Series I. in being unsaturated, and combine directly with bromine, iodine, and ozone. The earlier members are readily reduced, by the action of sodium amalgam in alkaline solution, to the corresponding acids of Series I. Thus:—

C₄H₇COOH + H₂ = C₄H₉COOH
(Tiglic acid) (Hydrogen) (Valeric acid)

Unfortunately, however, from the candlemaker’s point of view, this reduction does not take place in the case of the higher acids of the series, and for the reduction of oleic acid to stearic acid other methods have to be adopted.

Acids of this group may also be converted into those of the Stearic Acid Series by heating them to 300° C. with solid potassium hydroxide, when hydrogen is also liberated, the reaction, with oleic acid, for example, being generally represented by the equation:—

C₁₈H₃₄O₂ + 2KOH = KC₂H₃O₂ + KC₁₆H₃₁O₂ + H₂

though since, as Edmed has shown, a considerable quantity of oxalic acid is also formed, the action must strictly be more complex than this indicates.

One of the most important properties of this group of acids, and one which is of great value in judging the purity of olive oil, is the elaidin reaction, which is based on the formation of isomeric acids of higher melting-point by these acids when treated with nitrous acid. Oleic acid, for example, when acted upon by nitrous acid, yields elaidic acid, melting at 45° C., and erucic acid gives brassic acid, melting at 60° C. A similar reaction also takes place with the neutral glycerides of these acids, olein being converted into elaidin, which melts at 32°.

The lead salts of the acids of this series are much more soluble in ether, and the lithium salts more soluble in alcohol, than those of the stearic series, upon both of which properties processes have been based for the separation of the solid from the liquid fatty acids.

_III. Linolic Acid Series_

These acids are also unsaturated, and readily combine with bromine, iodine, oxygen, or ozone. They do not give an elaidin reaction when treated with nitrous acid, and their lead salts are soluble in ether.

_IV. Linolenic Acid Series_

These acids are very similar in properties to those of the preceding series, but combine with six atoms of bromine or iodine, whereas the latter only combine with four atoms.

_V. Ricinoleic Acid Series_

This acid combines with two atoms of bromine or iodine, and when treated with nitrous acid is converted into the isomeric ricinelaidic acid, which melts at 52-53° C. It differs from most fatty acids in possessing optical activity, its specific rotation being

[_a_] = +6° 25′.
ᵈ

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Edible fats and oilsChapter II: Introduction

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