Adeps, B.P. Lard.
Lard is the purified abdominal fat of the hog, Sus scrofa, Linn. (Order Ungulata), prepared from the "flare" or omentum, by washing the flat leafy masses, if necessary, until free from any salt which may have been used as a preservative, removing external membranes as far as possible, then exposing the fat to the air for some hours, so as to remove all traces of water, and thus render the lard less liable to become rancid and mouldy; the purified fat is then cut into small pieces, and reduced by pounding to a uniform mass, the whole put into a vessel surrounded by warm water, heated to a temperature not above 57° until the fat has melted, and strained. Lard is also official in the U.S.P. It occurs as a soft, white, fatty solid, of uniform consistence, neutral to litmus, having a faint but not rancid odour, and a bland taste. Melting-point, 38° to 40°, at which temperature, or a little higher, it forms a clear, almost colourless, liquid, without any separation of water. Specific gravity, 0.934 to 0.938 (about 0.917 at 25°, and about 0.904 at 40°), varying considerably according to age. Iodine value, 50 to 64. Saponification value, 195 to 203. The presence of cotton seed oil as an adulterant may be detected by heating 3 grammes of the lard in a salt-bath with 1 mil of amyl alcohol and 1 mil of a 1 per cent. solution of sulphur in carbon bisulphide, when no reddish colour should develop in ten to fifteen minutes if the lard is pure. Lard should be free from sodium chloride and starch. The limit of acidity is determined by dissolving 10 grammes in a mixture of equal volumes of chloroform and alcohol, adding 1 decimil (0.1 milliliter) of phenolphthalein solution and 2 decimils (0.2 milliliters) of the volumetric solution of sodium hydroxide, which should produce a permanent red colour. Lard that will keep good for a long period without becoming rancid may be obtained by repeatedly washing lard with boiling water, removing all traces of water, and finally filtering through paper by means of a hot water funnel. Exposed to air and moisture, lard rapidly becomes rancid, the rancidity being partly due to hydrolysis and partly to oxidation induced by the presence of fermentative organisms. Commercial lard often consists of hog fat other than that obtained from the omentum, and should not then be used for making ointments, as the excess of olein present tends to separate, and the lard usually begins to melt below 36°. Lard deprived of a portion of its oil by pressure is sanctioned for use in India and the Colonies, where prevailing high temperatures render the B.P. lard too soft for use in ointments and plasters; it is known as indurated lard (Adeps Induratus).
Insoluble in water; soluble in ether (1 in 22), oil of turpentine (1 in 16); readily in chloroform, carbon bisulphide, or petroleum ether, almost insoluble in alcohol. The chloroformic solution is clear if the lard be free from water; the solution in carbon bisulphide is slightly turbid.
Constituents.—Lard consists of olein associated with variable amounts of stearin and palmitin, approximately 40 per cent. of stearin and palmitin, and 60 per cent. of olein. The olein, or lard oil, is separated on a commercial scale from the stearin, or solid portion, by pressure at about 0°.
Use.—Lard is employed as a basis for the preparation of ointments which are intended to be absorbed. On account of its instability it is, however, less used than formerly; a mixture of hydrous wool fat with soft paraffin has all the advantages of lard without its tendency to become rancid. In order to diminish its tendency to become rancid, lard should be kept in the dark. It is most important that fresh neutral lard should be used for pharmaceutical purposes, and especially for such preparations as Unguentum Potassii Iodidi.
- Adeps Benzoatus, B.P.—BENZOATED LARD.
- Lard, 100; benzoin, in powder, 3. The benzoic acid present in the benzoin acts as an antiseptic and prevents the lard from becoming rancid. The acid, however, renders the lard slightly irritant and unsuitable for application to such sensitive parts as the conjunctiva.
- Adeps Benzoinatus, U.S.P.—BENZOINATED LARD.
- Lard, 100; benzoin, in coarse powder, 2.
- Adeps Induratus, I.C.A.—INDURATED LARD.
- This preparation is lard deprived, by pressure, of a portion of its oil.