44. To Prepare Extracts

44. To Prepare Extracts. The process of obtaining an extract of a substance involves two distinct operations: First, the production of a solution of the soluble portion of the substance operated on; and next, the reduction of this solution to a proper consistence by evaporation. The substance is first, where practicable, reduced to coarse powder by bruising, or sliced with a knife, so that every portion may be fully exposed to the action of the solvent. Refractory substances are first softened by the solvent and then 6liced. Other substances whose nature does not require reducing, are used without preparation.

Different fluids are used for solvents, as best adapted to the solubility of the substance under treatment. Some bodies, such as fresh vegetables, yield their juice by expression alone. In the preparation of aqueous extracts, the ingredients are treated with rain or distilled water, until all the soluble matter that is desired to obtain from them is dissolved.

This is effected by either maceration, percolation, infusion, or decoction, as circumstances require: the solution thus obtained is poured off and the remaining soluble matter either pressed or washed out, and added to the solution; it is next allowed time to settle, then decanted, and strained or filtered; and if this fails to render the liquid clear, it is clarified by white of egg, and filtered; Canton flannel, first soaked in water, being generally employed for this purpose. When water acidulated with acetic acid is employed, vegetable substances are usually macerated in it in the cold, or the dilute acid is sprinkled over the bruised plant, if fresh, and the juice expressed by strong pressure.

When the principles to be extracted are insoluble, or only slightly soluble, in water, alcohol is employed, either in the form of rectified spirit, proof spirit, or diluted. These produce alcoholic or spirituous extracts; and are generally obtained by either maceration or digestion.

Ether is well adapted for obtaining extracts from bodies whose principles consist of volatile oils or resin, on account of its strong affinity for those substances. Such are termed, ethereal extracts. In nearly all cases, filtration is necessary to insure a pure extract. The means usually employed for evaporating an aqueous solution, are rapid boiling over a fire until the extract is thick enough to offer some risk of burning, and the evaporation finished over a water bath or in shallow vessels at a moderate heat, the further escape of vapor being promoted by continuous stirring with a wooden spoon or stick. It is not always advisable to heat a solution to the boiling point, but if boiling is resorted to, it cannot be done too rapidly, as the heat cannot rise above its boiling point, and rapid ebullition hastens evaporation. The fluid must never be stirred while ebullition is going on. Two fundamental rules are:- to conduct evaporation at as low a temperature as is consistent with other objects; and,- to exclude atmospheric air; or, at least, to expose the liquid to its action for as short a time as possible, as most solutions lose more or less of their active principles by heat and exposure. Solutions which will not bear boiling without loss of strength are evaporated in a vacuum, either in a closed still, or under the receiver of an air pump, in which a vessel is placed containing strong sulphuric acid; this has a powerful affinity for water and absorbs its vapor as quickly as it comes in contact with it. A good plan for evaporation, though slow, is to place the liquid in a broad shallow vessel, exposed in a stove or drying room to a temperature of about 100° Fahr., allowing free access for the air. The extracts thus evaporated arc said to be lighter in color and more transparent than by most other ways.

The method for evaporating an alcoholic or an ethereal solution is substantially the same as that pursued with an aqueous solution; except that, as a matter of economy, the vapor may be led off and condensed again.

A good extract should be free from grit, and wholly soluble in 20 parts of the solvent used for making the extract, forming a nearly clear solution; it should be of a proper consistence and of uniform texture and color, smooth and glossy in appearance; this latter can only be | arrived at by assiduous and laborious stirring as the extract thickens; and may be promoted by adding 3 or 4 per cent, each of olive oil and gum arabic, with 1 or 2 per cent, of spirit of wine. Extracts should be put into pots as soon as made, securely tied down with bladder, and kept in a dry place. Any tendency to become mouldy may be prevented by adding, the last thing before removing from the evaporating pan, a few drops of oil of cloves, or a still less quantity of creosote, dissolved in a little alcohol; or by moistening with oil of cloves or creosote, the inside of the bladder used for covering the pots.