When about one half of an aqueous solution has evaporated, it is often advantageous to repass it through a flannel or horsehair strainer, to remove the flocculi that generally form by the action of the heat and air. This is especially necessary with vegetable solutions prepared without boiling, and should be adopted whenever a smooth and slightly extract is desired.
II. Reduction of Solutions:—The reduction of the solution to the proper consistence is effected by evaporation. The mode in which this is performed varies for different extracts. The London College directs that, “unless otherwise ordered, the evaporation should be conducted as quickly as possible, in a broad, shallow pan, placed in a water bath, until a proper consistence is acquired for forming pills; stirring assiduously with a spatula towards the end of the operation.” The Dublin College orders that “all simple (aqueous) extracts (EXTRACTA SIMPLICIORA), unless otherwise ordered, are to be prepared by boiling the vegetable matter in 8 times its weight of water, till the liquid is reduced to one half; the liquor is then to be expressed, and, after a short time allowed for defecation, to be decanted, filtered, and evaporated in a steam or water bath, until it begins to thicken, and then finally inspissated by a reduced heat, with continual stirring, until a consistence for forming pills be attained.” The instructions of the Edinburgh College are similar, with the one important exception, however, of ordering the evaporation to be conducted in a water bath saturated with chloride of sodium.
Though the water bath has the sanction of the London College, it is ill adapted for the purpose to which it is here ordered to be applied, as from its low evaporative power the advantages which are derived from its equable temperature are vastly overbalanced by the lengthened exposure of the solution in a heated state to the action of the atmosphere. It has been shown that a vegetable extract so prepared is inferior in quality to a similar one formed by rapid evaporation in a shallow pan over a naked fire, or placed in a sand bath, provided proper care is taken, and assiduous stirring is adopted during the whole time of the exposure to heat. In practice, however, the use of a naked fire is perfectly inadmissible, as the least neglect on the part of the operator would probably lead to the incineration of the whole. These objections are obviated by the addition of the 1⁄5th part of salt to the water of the bath, which raises its boiling-point to 2183⁄4° Fahr., when the temperature of the contained extract is fully 212°; the remaining 6° being lost by the interposition of the substance of the evaporating vessel.
On the large scale, the evaporation of infusions or decoctions for extracts is usually conducted in very wide, shallow, copper or tinned-copper pans, having steam-tight jackets of cast iron, and heated by steam ‘playing’ between the two.
The rapid deterioration which vegetable juices and solutions undergo by exposure to the air, especially at high temperatures, has led to the introduction of apparatus, by which they may be concentrated without contact with the atmosphere, and at a less degree of heat than is required for that purpose in open vessels. Such is the method, commonly called ‘Barry’s process,’ in which the air is removed from certain
air-tight refrigerators by the introduction of steam, which is then condensed by the application of cold, by which means a partial vacuum is obtained. Another process for attenuating the atmosphere over the surface of fluids during evaporation is by the action of an air-pump. This plan was introduced by Howard, and is commonly applied to the concentration of syrups in the sugar refineries. Extracts obtained by either of these methods are said to be prepared ‘in vacuo,’ and are found in practice to be immensely superior to the common extracts of the shops, and consequently require to be exhibited in proportionably small doses.
‘The American Journal of Pharmacy’ for September, 1877, contains a new process for the preparation of extracts without heat, by Professor Herrara. We extract the following from the Professor’s paper:—
“The results of my observations have satisfied me that, when the water partially congeals, the dissolved principles remain in solution in the mother liquors, and that two or three congelations are generally sufficient for obtaining the solutions concentrated enough to finish the extract by exposure upon plates to the heat of the sun, or of a drying closet, heated to about 30° cent. (86° Fahr.). The extracts prepared by this method accurately represent the properties of the plants, and the principles which are changed by the influence of heat remain unaltered; even the volatile constituents are not dissipated, though most of the water be removed by freezing. Owing to the small cost of the necessary apparatus, it appears to me that my process for preparing extracts should be preferable even in those countries where ice is less readily obtainable than combustibles.
“Extract of conium prepared with unpurified juice by the process mentioned, has preserved the characteristic odour of conia, and by dissolving it in water. I have obtained a solution exactly representing the juice of the plant in appearance and properties, and giving when heated an abundant coagulation, proving that even albumen had remained unaltered. 1750 grams of cow’s milk at 9° R., left, after three congelations, 750 grams of a liquid having a density of 148, and by evaporation in the sun this left a dry extract of milk, which again formed that liquid on being dissolved in water. A number of other liquids similarly treated, gave corresponding results, and it seems to me, therefore, that medicinal extracts are best prepared by congelation. It may be objected that the medical juices should be previously purified, but it should be remembered that coagulated albumen always encloses a considerable portion of the active principles, and that the heat necessary to effect the coagulation and the evaporation by means of a water-bath is sufficient to change many principles; also that the extracts thus prepared are sometimes inert or less active. The careful experiments made by Orfila and the clinical experience of others demonstrate that extracts prepared with unpurified juice are stronger.
“The apparatus employed by me is the so-called sorbetière;[294] for larger quantities the apparatus of Gougaud is preferable. The frigoric mixture is composed of ice and sodium chloride, or preferably of crystallised calcium chloride. After a large portion of the solution has congealed, the mass is enclosed in a cloth and subjected to pressure, the press-cake of ice is broken and again pressed, to separate the mother liquor as completely as possible, and the congelation is repeated two or three times, with the precaution that it is not carried far enough to cause the precipitation of the sparingly soluble principles. The mother liquor is then put into shallow dishes and exposed to the heat of the sun or of a drying room, the temperature of which does not exceed 30° C. (86° F.) until the extract has attained the desired consistence.”