Boiled coffee is usually cloudy, due to the suspension of fine particles resulting from the disintegration of the grounds by the violence of boiling. The usual procedure in clarifying the decoction is to add the white of an egg or some egg-shells, the albumen of which is coagulated upon the fine particles by the heat of the solution, and the particles thus weighted sink to the bottom. Even this procedure, requiring much attention, does not give as clear a solution as some of the other extraction procedures employed. The conditions to which coffee is subjected during boiling are the worst possible, as both grounds and solution undergo hydrolysis, oxidation, and local-overheating, while the caffeol is steam-distilled from the brew. Many persons, who have long been accustomed to drinking the relatively bitter beverage thus produced, are not satisfied by coffee made in any other way; but this is purely a perversion of taste, for none of the properties are present which make coffee so prized by the epicure.
| Cross-section of Roasted Coffee Bean Magnified 600 Times | Coarse Grind Under the Microscope |
Steeping, in which cold water is added to the coffee, and the mixture brought up to a boil, does not subject the coffee to so strenuous conditions. Local overheating and hydrolysis occur, but not to so great an extent as in boiling; and most of the effects of oxidation and volatization of caffeol are absent. However, extraction is rather incomplete, due to lack of thorough admixture of the water and coffee.
When coffee is to be made under the best conditions, the temperature of the water used and of the extract after it is made should not fluctuate. In the pumping percolator, as in the steeping method, the temperature varies greatly from the time the extraction is started to the completion of the operation. This is deleterious. Also, local overheating of the infusion occurs at the point of application of the heat; and because of the manner in which the water is brought into contact with the coffee, the degree of extraction shows inefficiency. Spraying of the water over the coffee never permits the grounds to be completely covered with water at any one time, and the opportunity offered for channeling is excessive. The principle of thorough extraction demands that, as the substance being extracted becomes progressively more exhausted, fresh solvent should be brought into contact with it. In the pumping percolator the solution pumped over the grounds becomes more concentrated as the grounds become exhausted; so that the time taken to reach the degree of extraction desired is longer, and an appreciable amount of relatively concentrated liquor is retained by the grounds.
Medium Grind Under the Microscope
The simplest procedure to follow is that in which boiling water is poured over ground coffee suspended on a filtering medium in such a manner that the extracting water will slowly pass through the coffee and be received in a containing vessel, which obviates further contact of the beverage with the grounds. The water as it comes into contact with the ground coffee extracts the soluble material, and the solution is removed by gravity. Fresh water takes its place; so that, if the filter medium be of the proper fineness, the water flows through at the correct rate of speed, and complete extraction is effected with the production of a clear solution. Thus a maximum extraction of desirable materials is obtained in a short time with a minimum of hydrolysis, oxidation, and loss of caffeol; and if the infusion be consumed at once, or kept warm in a contrivance embodying the double-boiler principle, the effects of local overheating are avoided. Also, with the use of an appropriate filter, a finer grind of coffee can be used than in the other devices, without obtaining a turbid brew. All this works toward the production of a desirable drink.
There are several devices on the market, some using paper, and some cloth, as a filter, which operate on this principle and give very good coffee. The use of paper presents the advantage of using a new and clean filter for each brew, whereas the cloth must be carefully kept immersed in water between brews to prevent its fouling.
Contrivances operating on the filtration principle have been designed for use on a large scale in conjunction with coffee urns, and have proven quite successful in causing all of the water to go slowly through the coffee without channeling, thus accomplishing practically complete extraction. The majority of urns are still operated with bags, of which the ones with sides of heavier material than the bottom obtain the most satisfactory results, as the majority of the water must pass through the coffee instead of out through the sides of the bag. Greatest efficiency, when bags are used, is obtained by repouring until all of the liquid has passed twice through the coffee; further repouring extracts too much of the astringent hydrolysis products. The bags, when not in use, should not be allowed to dry but should be kept in a jar of cold water. The urns provided with water jackets keep the brew at almost a constant temperature and avoid the deterioration incident to temperature fluctuation.