Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/133

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The physiological and dietetic value of coffee depends principally upon the alkaloid caffeine which it contains, in common with tea, cocoa, maté or Paraguay tea, guarana, and the African kola nut. Its commercial value is, however, determined by the amount of the aromatic oil, caffeone, which develops in it by the process of roasting. By prolonged keeping it is found that the richness of any seeds in this peculiar oil is increased, and with increased aroma the coffee also yields a blander and more mellow beverage. Stored coffee loses weight at first with great rapidity, as much as 8 per cent. having been found to dissipate in the first year of keeping, 5 per cent. in the second, and 2 per cent. in the third; but such loss of weight is more than compensated by improvement in quality and consequent enhancement of value. In the process of roasting, coffee seeds swell up by the liberation of gases within their substance, their weight decreasing in proportion to the extent to which the operation is carried. Roasting also develops with the aromatic caffeone above alluded to a bitter soluble principle, and it liberates a portion of the caffeine from its combination with caffetannic acid. Roasting is an operation of the greatest nicety, and one, moreover, of a crucial nature, for equally by insufficient and by excessive roasting much of the aroma of the coffee is lost; and its infusion is neither agreeable to the palate nor exhilarating in its influence. The roaster must judge of the amount of heat required for the adequate roasting of different qualities, and while that is variable, the range of roasting temperature proper for individual kinds is only narrow. In Continental countries it is the practice to roast in small quantities, and thus the whole charge is well under the control of the roaster; but in Britain large roasts are the rule, in dealing with which much difficulty is experienced in producing uniform torrefaction, and in stopping the process at the proper moment. The coffee-roasting apparatus is usually a malleable iron cylinder mounted to revolve over the fire on a hollow axle which allows the escape of gases generated during torrefaction. Messrs W. and G. Law of Edinburgh have introduced a very ingenious adaptation of the cylinder whereby a compound simultaneous horizontal and vertical motion is secured, causing the seeds to be tossed about in all directions and communicating a uniform heat to every portion of the cylinder. The roasting of coffee should be done as short a time as practicable before the grinding for use, and as ground coffee especially parts rapidly with its aroma, the grinding should only be done when coffee is about to be prepared. Any ground coffee which may be kept should be rigidly excluded from the air. While Arabia produces the choicest variety of coffee, the roasting of the seeds and the prepararion of the beverage are also here conducted with unequalled skill. Mr W. G. Palgrave gives the following account of these operations in his Central and Eastern Arabia:—

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—Napier's Coffee Apparatus. There is no doubt that were proper attention bestowed upon the preparation of coffee it would become a much more popular beverage in Great Britain than it now is; but to obtain it in perfection much greater care is requisite than is necessary in the case of tea. To obtain coffee with a full aroma it must be prepared as an infusion with boiling water, or the water may simply be allowed to reach the boiling point after infusion and nothing more. Dr Parkes has, however, pointed out that by infusion alone much of the valuable soluble matter in ground coffee remains unextracted; and he recommends that the coffee which has already been used for infusion should be preserved and boiled, and that the liquor therefrom should be used for infusing a fresh supply. By this means the substance of the previously infused coffee and the aroma of the new are obtained together. Among the numerous devices which have been proposed for preparing coffee, none is more elegant and efficient than an apparatus constructed by Mr James R. Napier, F.R.S., for which a patent was obtained by Mr David Thomson of Glasgow. It consists of a glass globe a (fig. 3), an infusing jar b, of glass or porcelain and a bent tube c. of block tin or German silver fitted by a cork stopper into the neck of the globe and passing to the bottom of the jar, where it ends in a finely perforated disc. The apparatus also requires a spirit lamp d or other means of communicating a certain amount of heat to the globe. The coffee is infused with 