This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
The abdominal fat of the hog is obtained in the form of flat, leafy masses known as 'flare.' These should be first washed to free them from any salt that may have been used to preserve them, then stripped as far as possible of external membrane, and hung in a current of air for a few hours to dry. They must then be crushed or comminuted in any suitable manner, such as by beating in a stone mortar or passing through a mincing machine, in order to break the membranous vesicles and liberate the fat contained in them. If this were not done, either the fat would be retained in the vesicles or so high a temperature would have to be applied that it would acquire an unpleasant taste and odour. The crushed fat is exposed to a temperature which should not exceed 57° (in order to avoid the injurious effect of too great a heat), and when completely melted strained through fine muslin and gently stirred till cool, avoiding any form of beating which would introduce air into the melted fat and favour the development of rancidity. If not stirred the lard is liable to assume a granular condition, from the crystallisation of the constituents of higher melting-point (stearin and palmitin).
Lard is a uniform, soft, white, homogeneous, fatty substance melting at about 38° and having at 15° a specific gravity of about 0.934 to 0.938. Odour slight, fatty, but not rancid or otherwise disagreeable; entirely soluble in ether. Acid value not over 1.2; saponification value 192 to 198; iodine value 62 to 63; unsaponifiable matter not over 0.5 per cent.; refractive index at 60° 1.4530 to 1.4550.
Lard consists of about 40 per cent. of stearin and palmitin mixed with about 60 per cent. of olein, but these proportions are subject to a little variation, and with them both melting-point and specific gravity.
Lard is liable to contain common salt, which is often added to preserve it for domestic use; it may be tested for chlorides by boiling with water, cooling, filtering the aqueous liquid, and adding silver nitrate and nitric acid. Starch, which might be added to give it a whiter appearance, could also be detected in the filtrate by solution of iodine. Sesame oil may be detected by the test detailed under 'Olive Oil' ( p. 510).
But the most frequent adulterant of lard is cotton-seed oil, which has been found in American lard, large quantities of which are imported. It may be detected by the tests described under 'Olive Oil' and by a rise in the iodine number which should not exceed 60. It should be noted that lard obtained from hogs fed upon cotton cake may give a positive result with Halphen's test.