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:—
“Without delay Sowelylim begins his preparations for coffee. These open by about five minutes' blowing with the bellows, and arranging the charcoal till a sufficient heat has been produced. Next he places the largest of the coffee-pots, a huge machine, and about two-thirds full of clear water, close by the edge of the glowing coal-pit, that its contents may become gradually warm while other operations are in progress. He then takes a dirty knotted rag out of a niche in the wall close by, and having untied it, empties out of it three or four handfuls of unroasted coffee, the which he places on a little trencher of platted grass, and picks carefully out any blackened grains, or other non-homologous substances commonly to be found intermixed with the berries when purchased in gross; then, after much cleansing and shaking, he pours the grains so cleansed into a large open iron ladle, and places it over the mouth of the funnel, at the same time blowing the bellows and stirring the grains gently round and round till they crackle, redden, and smoke a little, but carefully withdrawing them from the heat long before they turn black or charred, after the erroneous fashion of Turkey and Europe; after which he puts them a moment to cool on the grass platter. He then sets the warm water in the large coffee-pot over the fire aperture, that it may be ready boiling at the right moment, and draws in close between his own trouserless legs a large stone mortar with a narrow pit in the middle, just enough to admit the black stone pestle of a foot long and an inch and a half thick, which he now takes in hand. Next pouring the half-roasted berries into the mortar he proceeds to pound them, striking right into the narrow hollow with wonderful dexterity, not ever missing his blow till the beans are smashed, but not reduced into powder. He then scoops them out, now reduced to a sort of coarse reddish grit, very unlike the fine charcoal powder which passes in some countries for coffee, and out of which every particle of real aroma has long since been burned or ground. After all these operations . . . . . he takes a smaller coffee-pot in hand, fills it more than half with hot water from the larger vessel, and then, shaking the pounded coffee into it, sets it on the fire to boil, occasionally stirring it with a small stick as the water rises, to check the ebullition and prevent overflowing. Nor is the boiling stage to be long or vehement; on the contrary, it is and should be as slight as possible. In the interim he takes out of another rag-knot a few aromatic seeds called heyl, an Indian product, but of whose scientific name I regret to be wholly ignorant, or a little saffron, and after slightly pounding these ingredients, throws them into the simmering coffee to improve its flavour, for such an additional spicing is held indispensable in Arabia, though often omitted elsewhere in the East. Sugar, I may say, would be a totally unheard-of profanntion. Last of all, he strains off the liquor through some fibres of the inner palm-bark, placed for that purpose in the jug-spout, and gets ready the tray of delicate party-coloured grass and the small coffee-cups ready for pouring out.”
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Fig. 3.—Napier's Coffee Apparatus.
