Chapter V: Lard

=Rendering of Lard.=—Lard may be defined as the fat separated for use as food from the adipose tissue of the pig.

In the process of rendering lard the tissue is first finely divided or minced in a machine, or is crushed between rollers, and is then heated either by dry heat or by means of steam to expel the fat from the ruptured fat-cells.

A product of excellent quality is obtained by subjecting the mass to dry heat at a temperature just sufficient to melt the fat; and a process of this kind is frequently used in the preparation of the fat for margarine.

At higher temperatures some decomposition of nitrogenous compounds in the adipose tissue takes place, and special means must be provided for carrying off the obnoxious vapours thus produced.

The use of steam in a steam-jacketed pan or digester enables the temperature to be regulated with greater nicety, and prevents the fat being overheated and acquiring a burnt flavour.

A more rapid process of rendering lard is to heat the finely divided material with water in a suitable vessel and subsequently to skim off the fat that rises to the surface. At first the temperature is kept as low as possible in order to separate the portions of the fat of lowest melting-point. Then the temperature is raised, with the result that a fat of greater consistency rises to the surface; and finally the water is brought to the boiling-point to separate, as far as is possible by this means, the residual fat. In this way a fractionation of the lard takes place.

A process more frequently employed than the preceding one is that in which the adipose tissue is rendered, in a closed vessel or digester, capable of resisting the action of steam introduced under pressure.

Numerous patents for apparatus embodying this principle have been taken out, especially in the United States, various modifications of the mode of heating, separation of the fat, and filtration being claimed.

In some of the largest works in Chicago and other centres of the American lard industry a great number of such digesters is employed, some of them having a capacity of upwards of 20,000 gallons. Each of these vessels is reserved for its particular kind of fat, so as to facilitate the speed of working.

In a typical digester the finely minced fatty matter, which has been separated from the recently killed animal, is introduced from above through a man-hole, which is then screwed down hermetically. Within the digester is a perforated false bottom upon which the material rests, and beneath this is a perforated steam coil, connected by means of a pipe with an outside boiler, from which steam is admitted until a pressure of about four atmospheres is shown upon the gauge. Condensed water is drawn off through a cock near the bottom of the vessel, while a similar cock is provided near the top for testing whether the fat is being liberated. After a digestion of about twelve hours the steam supply is shut off, and the pressure in the digester reduced by opening the safety valve. Then, after standing for some time for separation to take place, the aqueous layer is drawn off through taps arranged at different levels near the bottom. The rendered fat, which has separated on the surface as a melted layer, is next withdrawn, and finally the residual mass is expelled through an outlet in the bottom, which can be controlled from above by a rod passing through the body of the apparatus.

=Commercial Grades.=—In the United States (which probably produces more lard than the rest of the world put together) various grades of lard are recognised, depending mainly upon the part of the animal whence they were derived. Thus, according to Wiley (U.S. Dept. Agriculture, Bull. 13), the following descriptions of lard are known in the trade:—

(1) _Neutral lard_, which is the product of the absolutely fresh leaf of the hog, rendered at a temperature between 105° and 120° F. It is chiefly used in the manufacture of margarine.

(2) _Leaf lard_, prepared from the residue left after removal of neutral lard at a lower temperature.

(3) _Choice steam lard_, or _Choice lard_, obtained from the residual tissue from neutral lard and from adipose tissue from the back of the animal.

(4) _Prime steam lard_, which may be the product of any part of the animal, and especially of the fatty tissue from the head, heart, and intestines.

(5) _Guts_, a low grade of lard rendered from scraps from any part of the animal, with the exception of the heart and lungs.

In addition to these, still lower grades of pigs’ fat, which are used for soap and other technical purposes, are separated from hogs that have died on their way to the slaughter-houses, or from refuse material. These include _white_, _brown_, and _yellow grease_, and _pigs’ foot grease_.

The product known in Germany as _pure lard_ is obtained by a process of steam-rendering, followed by mechanical agitation of the separated fat in a closed vessel until it begins to solidify. An addition of a small proportion of a more solid lard is then made, or, in the best grades, a small proportion of lard stearin is added. According to Voigtländer (_Zeit. angew. Chem._, 1898, 857), this treatment, which is known as the Hungarian process, prevents the lard becoming fluid on keeping.

American lard, even when free from any addition of vegetable oil, is often of a more fluid character than European lard, this being due partly to differences inherent in the hogs and partly to the influence of the food given to the animals.

=Composition.=—Lard, like most natural products, shows wide variations in its chemical and physical characteristics, and this increases the difficulty of basing definite conclusions as to purity on the results of analyses. Even in the case of the same animal wide variations are observed in the composition of fat taken from different parts of the body, as is shown below.

Chemically considered, lard may be said to consist of glycerides of stearic, palmitic, lauric, and myristic acids, and of the liquid fatty acids, oleic and linolic acids; and the different physical properties of different samples of lard are due, in the main, to variations in the proportions of these different constituents. For instance, the firm fat from the leaf or kidneys of the animal contains fatty acids with a low percentage (about 15 per cent.) of stearic acid, and a high percentage (about 58 per cent.) of oleic acid; whereas the fat from the neck and back, which is very soft, contains fatty acids with about 9 per cent. of stearic acid, and about 75 per cent. of oleic acid.

=Lard Crystals.=—A curious property of lard, upon which reliance has often been placed as a test of purity, is that when it is dissolved in ether and the solution is gradually allowed to evaporate, crystals of characteristic form are often produced.

The fat crystals from an ordinary soft lard have flat edges and chisel-shaped ends, whereas the similar crystals from beef fat are in the form of bunches of needle-shaped crystals. A small proportion of beef fat added to a soft lard influences the crystallisation sufficiently to produce bunches of needle-shaped crystals, though chisel-ended crystals may also be detected.

Unfortunately for the purpose of this test, flare or leaf lard may also produce crystals which very closely resemble those derived from beef fat, and this resemblance is rendered closer by re-crystallisation, so much so that in some cases the two kinds of crystals are practically indistinguishable.

Hehner and the writer (Mitchell) found that the needle-shaped crystals contained a higher proportion of stearic acid than the chisel-ended crystals, and attributed the characteristic differences to this fact. It has been shown, however, by Kreis and Hafner that the difference is due to the characteristic flat crystals of lard consisting of a mixed glyceride heptadecyldistearin, whereas the crystals from beef and mutton fat consist of another mixed glyceride, palmitodistearin.

According to Dunlop, it is possible, by continued re-crystallisation of beef fat crystals, to obtain eventually a deposit containing crystals which are practically indistinguishable from those given by an ordinary lard.

From all this it will be seen that there is considerable risk of making a mistake, if judgment of the purity of lard is based solely upon the form of the crystals, as has sometimes been done. The fact that the crystals are chisel-shaped may be regarded as presumptive evidence of the purity of a lard, but the occurrence of bunches of needle-shaped crystals does not necessarily indicate an addition of beef fat.

=Influence of Food.=—A point of considerable importance to the manufacturer of lard is that the food given to the animal may have a pronounced influence upon the composition and chemical reactions of the lard.

The fat of the wild boar differs from that of the domestic hog in being of a much more fluid nature. There is also some difference between the fat of wild and domestic hogs, the former containing a greater amount of unsaturated glycerides.

The more fluid character of American lards as compared with European products is to be attributed, in part at all events, to the animals being fed upon cotton-seed cake.

It has been shown by Dunlop, however (_J. Soc. Chem. Ind._, 1906, xxv. 459), that hogs fed upon cotton-seed cake give lard yielding pronounced colour reactions for cotton-seed oil, but not, judging by the iodine value, containing an abnormal amount of fluid fatty acids.

Animals fed upon cocoanut oil cake (copra) also show the influence of the food in their lard, which, without special knowledge of this fact, would certainly be condemned as having been adulterated with that vegetable fat.

This is shown in the following analytical values of lards from Philippine hogs to which a daily supply of cocoanut oil cake had been given (Gibbs and Agcaoili, _Philippine J. Science_, 1910, v. 33):—

+---------------------+---------+--------+------+--------+----------+
| | Refract-|Saponif-| | Iodine | Melting- |
| | ometer |ication |Iodine| Value | point |
| Lard. | reading | Value. |Value.|of Fatty| of Fatty |
| |at 40° C.| | | Acids. |Acids. °C.|
+---------------------+---------+--------+------+--------+----------+
|From maize-fed hogs:—| | | | | |
| Maximum | 46·0 | 199·0 | 52·7 | 53·8 | 43·8 |
| Minimum | 42·5 | 196·0 | 46·7 | 50·0 | 41·6 |
| Mean | 44·7 | 196·9 | 49·4 | 51·7 | 42·7 |
|From copra-fed hogs:—| | | | | |
| Maximum | 47·0 | 213·7 | 42·5 | 46·2 | 42·3 |
| Minimum | 44·0 | 204·6 | 32·5 | 36·2 | 39·4 |
| Mean | 45·3 | 208·9 | 37·7 | 41·3 | 40·4 |
+---------------------+---------+--------+------+--------+----------+

=Acidity of Lard.=—In order to obtain a practically neutral product, the fat intended for lard should be rendered as soon as possible after the animal has been killed. After standing for some time, the fat in the adipose tissue gradually undergoes decomposition, with the liberation of free fatty acids, and the acidity steadily rises. In refined lard the process of hydrolysis is greatly checked through the removal of nitrogenous substances, but the development of acidity will continue when the fat is exposed to the influence of light and air.

As a rule, a freshly prepared lard does not contain more than 0·5 per cent. of free fatty acids (as oleic acid), and frequently the proportion is very much lower. Thus the acidity of fats from the leaf and kidneys of six Philippine hogs examined by Gibbs and Agcaoili (_loc. cit._) ranged from 0·28 to 0·38 per cent. On the other hand, the intestinal fat contained considerably more free fatty acids (0·86 per cent.). Samples of American lards examined by Wiley (_loc. cit._) had an acidity ranging from 0·35 to 1·0 per cent. An acidity in excess of 1 per cent. probably indicates either that the lard was not prepared from fresh material or that it has been exposed to the air for some time.

=Water.=—Lard should not contain more than a small proportion (0·75 per cent. at most) of water, and as a rule most commercial samples contain considerably less than 0·5 per cent.

Polenske (_Arb. a. d. Kaiser. Gesundheitsamte_, 1907, xxv. 505) found that there was a relationship between the temperature at which the melted fat became turbid on cooling and the proportion of water present, and on this fact based a rapid method of estimating the moisture. Thus the following figures were obtained by Fischer and Schellens (_Z. Unters. Nahr. Genussm._, 1908, xvi. 161):—

+--------------------------+----+----+----+----+----+----+----+
| Water, per cent. |0·45|0·40|0·35|0·30|0·25|0·20|0·15|
| Turbidity Temperature °C.|95·2|90·8|85·0|75·8|64·6|53·2|41·2|
+--------------------------+----+----+----+----+----+----+----+

In their opinion, based upon the examination of a large number of German lards, the proportion of water should be less than 0·3 per cent., corresponding to a turbidity temperature not exceeding 75° C.

Gross adulteration of lard with water is no longer a common practice.

=The Iodine Value.=—Important information as to the purity of lard is sometimes afforded by a determination of the percentage of iodine with which it will combine.

As a rule, European lards have iodine values not exceeding 61, and an iodine value considerably in excess of that figure, say 66, suggests the addition of cotton-seed oil or of other vegetable oils with high iodine values.

From what has been stated above, however, it will be seen that an abnormal iodine value is not in itself sufficient proof of adulteration, since American lards of genuine character may show that characteristic.

The differences in this respect between American and German lards are attributed by Voigtländer (_loc. cit._) to the fact that American lards contain a large proportion of lard oil of a more unsaturated character. Thus, while German lard contains only about 50 per cent. of lard oil with an iodine value of 70-75, American lard may contain about 60 per cent. of lard oil with an iodine value of 88.

Some of the semi-fluid Russian lards sold in Germany contain as much as 90 per cent. of lard oil.

Of 100 samples of American lard imported into Germany, 88·5 per cent. had an iodine value between 61 and 66, and 41 per cent. a value exceeding 64.

The fixing of a standard for the iodine value of lard is unlikely to result in the sale of a purer product, judging by the experience of places where such a standard has been fixed. Thus at one time 62 was fixed as the limit for the iodine value in Bavaria, and the result was a widespread addition of beef stearin to make the lards answer to this requirement.

=Lard Oil.=—The fluid portion expressed from lard in the separation of lard stearin is known as _lard oil_.

It has a soft, pleasant taste, and, being almost free from odour, forms a good edible oil.

It consists in the main of olein, with a small proportion of glycerides of solid fatty acids, chiefly palmitic acid. A typical sample will have values similar to the following:—

+--------+--------------+------+------+--------------+-------------+
| Sp. gr.| | | | |Melting-point|
| at |Saponification|Hehner|Iodine|Solidification| of |
|15·5° C.| Value. |Value.|Value.| point. | Fatty Acids.|
+--------+--------------+------+------+--------------+-------------+
| 0·914 | 194 | 97 | 76 | -4° C. to | 35° C. |
| | | | | +10° C. | |
+--------+--------------+------+------+--------------+-------------+

It is thus evident that a large amount of lard oil can be added to olive oil without making the values of the mixture abnormal.

It also gives a similar product in the elaidin test, and the phytosterol test (p. 101) is probably the best means of detecting it.

In turn it is liable to be adulterated with cheaper vegetable oils, such as arachis, cotton-seed, and sesame oils, the presence of each of which would be shown by their special tests. The oleo-refractometer is also a means of determining the purity, since the ordinary reading of lard oil, which ranges from about -1° to +5°, would be increased by most of the oils employed as adulterants (with the exception of other animal oils and arachis oil).