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Оглавление| Table I—The Solubility of Caffein | ||||
| Solvent | Sp. Gr. of Solvent | Temperature of Solution | Solubility: Grm. Caffein per 100 Grm. of Saturated Solution | Sp. Gr. of Saturated Solution |
| Water | 0.997 | 25 | 2.14 | —— |
| Ether | 0.716 | 25 | 0.27 | —— |
| Chloroform | 1.476 | 25 | 11.0 | —— |
| Acetone | 0.809 | 30–1 | 2.18 | 0.832 |
| Benzene | 0.872 | 30–1 | 1.22 | 0.875 |
| Benzaldehyde | 1.055 | 30–1 | 11.62 | 1.087 |
| Amylacetate | 0.860 | 30–1 | 0.72 | 0.862 |
| Aniline | 1.02 | 30–1 | 22.89 | 1.080 |
| Amyl alcohol | 0.814 | 25 | 0.49 | 0.810 |
| Acetic acid | 1.055 | 21.5 | 2.44 | —— |
| Xylene | 0.847 | 32.5 | 1.11 | 0.847 |
| Toluene | 0.862 | 25 | 0.57 | 0.861 |
The similarity between caffein and theobromin (the chief alkaloid of cocoa), xanthin (one of the constituents of meat), and uric acid, is shown by the accompanying structural formulæ.
These formulæ show merely the relative position occupied by caffein in the purin group, and do not in any wise indicate, because of its similarity of structure to the other compounds, that it has the same physiological action. The presence and position of the methyl groups (CH3) in caffein is probably the controlling factor which makes its action differ from the behavior of other members of the series. The structure of these compounds was established, and their syntheses accomplished, in the course of various classic researches by Emil Fischer.[137]
Formula for Caffein, Showing Its Relation to the Purin Group
Gorter states that caffein exists in coffee in combination with chlorogenic acid as a potassium chlorogenate, C32H36O19, K2(C8H10O2N4)2·2H2O, which he isolated in colorless prisms. This compound is water-soluble, but caffein can not be extracted from the crystals with anhydrous solvents. To this behavior can probably be attributed the difficulty experienced in extracting caffein from coffee with dry organic solvents. However, the fact that a small percentage can be extracted from the green bean in this manner indicates that some of the caffein content exists therein in a free state. This acid compound of caffein will be largely decomposed during the process of torrefaction, so that in roasted coffee a larger percentage will be present in the free state. Microscopical examination of the roasted bean lends verisimilitude to this contention.
Planter's Bungalow with Coffee Trees in Flower, Mysore
Coolies Bagging Coffee on the Drying Grounds COFFEE SCENES IN BRITISH INDIA
| Table II—Coffee Analyses | ||||||||
| Santos Green | Santos Roasted | Padang Green | Padang Roasted | Guatemala Green | Guatemala Roasted | Mocha Green | Mocha Roasted | |
| Moisture April 20th | 8.75 | 3.75 | 8.78 | 2.72 | 9.59 | 3.40 | 9.06 | 3.36 |
| Moisture September 20th | 8.12 | 6.45 | 8.05 | 6.03 | 8.68 | 6.92 | 8.15 | 7.10 |
| Ash | 4.41 | 4.49 | 4.23 | 4.70 | 3.93 | 4.48 | 4.20 | 4.43 |
| Oil | 12.96 | 13.76 | 12.28 | 13.33 | 12.42 | 13.07 | 14.04 | 14.18 |
| Caffein | 1.87 | 1.81 | 1.56 | 1.47 | 1.26 | 1.22 | 1.31 | 1.28 |
| Caffein, dry basis | 2.03 | —— | 1.69 | —— | 1.39 | —— | 1.44 | —— |
| Crude fiber | 20.70 | 14.75 | 21.92 | 14.95 | 22.23 | 15.23 | 22.46 | 15.41 |
| Protein | 9.50 | 12.93 | 12.62 | 14.75 | 10.43 | 11.69 | 8.56 | 9.57 |
| Protein, dry basis | 10.41 | —— | 13.68 | —— | 11.53 | —— | 9.41 | —— |
| Water extract | 31.11 | 30.30 | 30.83 | 30.21 | 31.04 | 30.47 | 31.27 | 30.44 |
| Specific gravity, 10 percent extract | 1.0109 | 1.0101 | 1.0107 | 1.0104 | 1.0105 | 1.0404 | 1.0108 | 1.0108 |
| Bushelweight | 47.0 | 28.2 | 45.2 | 27.8 | 52.2 | 27.2 | 48.8 | 30.2 |
| 1,000 kernel weight | 103.60 | 120.20 | 167.30 | 151.35 | 189.20 | 165.80 | 119.52 | 100.00 |
| 1,000 kernel weight, dry basis | 119.1 | 115.7 | 154.1 | 147.2 | 171.0 | 160.1 | 108.6 | 96.6 |
| Dextrose | —— | 0.72 | —— | 0.81 | —— | 0.54 | —— | 0.46 |
| Caffetannic acid | .58 | 17.44 | 15.37 | 16.93 | 16.27 | 17.13 | 15.61 | 16.89 |
| Acidity by titration apparent | 1.50 | 2.08 | 1.47 | 2.00 | 1.39 | 2.13 | 1.11 | 1.87 |
As may be seen in Table II,[138] the caffein content of coffee varies with the different kinds, a fair average of the caffein content being about 1.5 percent for C. arabica, to which class most of our coffees belong. However, aside from these may be mentioned C. canephora, which yields 1.97 percent caffein; C. mauritiana, which contains 0.07 percent of the alkaloid (less than the average "caffein-free coffee"); and C. humboltiana, which contains no caffein, but a bitter principle, cafemarin. Neither do the berries of C. Gallienii, C. Bonnieri, or C. Mogeneti contain any caffein; and there has also been reported[139] a "Congo coffee" which contained no crystallizable alkaloid whatever.
Apparently the variation in caffein content is largely due to the genus of the tree from which the berry comes, but it is also quite probable that the nature of the soil and climatic conditions play an important part. In the light of what has been accomplished in the field of agricultural research, it does not seem improbable that a man of Burbank's ability and foresight could successfully develop a series of coffees possessed of all the cup qualities inherent in those now used, but totally devoid of caffein. Whether this is desirable or not is a question to be considered in an entirely different light from the possibility of its accomplishment.
| Table III—Caffein in Different Roasts | |||
| Rio | Santos | Guatemala | |
| Green | 1.68% | 1.85% | 1.82% |
| Cinnamon | 1.70 | 1.72 | 1.80 |
| Medium | 1.66 | 1.66 | 1.56 |
| City | 1.36 | 1.66 | 1.46 |
The variation in the caffein content of coffee at different intensities of roasting, as shown in Table III[140] is, of course, primarily dependent upon the original content of the green. A considerable portion of the caffein is sublimed off during roasting, thus decreasing the amount in the bean. The higher the roast is carried, the greater the shrinkage; but, as the analyses in the above table show, the loss of caffein proceeds out of proportion to the shrinkage, for the percentage of caffein constantly decreases with the increase in color. If the roast be carried almost to the point of carbonization, as in the case of the "Italian roast," the caffein content will be almost nil. This is not a suitable coffee for one desiring an almost caffein-free drink, for the empyreumatic products produced by this excessive roasting will be more toxic by far than the caffein itself would have been.
Caffein-free Coffee
The demand for a caffein-free coffee may be attributed to two causes, namely: the objectionable effect which caffein has upon neurasthenics; and the questionable advertising of the "coffee-substitute" dealers, who have by this means persuaded many normal persons into believing that they are decidedly sub-normal. As a result of this demand, a variety of decaffeinated coffees have been placed on the market. Just why the coffee men have not taken advantage of naturally caffein-free coffees, or of the possibility of obtaining coffees low in caffein content by chemical selection from the lines now used, is a difficult question to answer.
In the endeavor to develop a commercial decaffeinated coffee the first method of procedure was to extract the caffein from roasted coffee. This method had its advantages and its disadvantages, of which the latter predominated. The caffein in the roasted coffee is not as tightly bound chemically as in the green coffee, and is, therefore, more easily extracted. Also, the structure of the roasted bean renders it more readily penetrable by solvents than does that of the green bean. However, the great objection to this method arises from the fact that at the same time as the caffein is extracted, the volatile aromatic and flavoring constituents of the coffee are removed also. These substances, which are essential for the maintenance of quality by the coffee, though readily separated from the caffein, can not be returned to the roasted bean with any degree of certainty. This virtually insurmountable obstacle forced the abandonment of this mode of attack.
In order to avoid this action, the attention of investigators was directed to extraction of the alkaloid in question from the green bean. Because of the difficulty of causing the solvent to penetrate the bean, recourse to grinding resulted. This greatly facilitated the desired extraction, but a difficulty was encountered when the subsequent roasting was attempted. The irregular and broken character of the ground green beans resisted all attempts to produce practically a uniformly roasted, highly aromatic product from the ground material.
Avoidance of this lack of uniformity in the product, and the great desirability to duplicate the normal bean as far as possible, necessitated the development of a method of extraction of the caffein from the whole raw bean without a permanent alteration of the shape thereof. The close structure of the green bean, and its consequent resistance to penetration by solvents, and the existence of the caffein in the bean as an acid salt, which is not easily soluble, offered resistance to successful extraction.
As a means of overcoming the difficulty of structure, the beans were allowed to stand in water in order to swell, or the cells were expanded by treatment with steam, or the beans were subjected to the action of some "cellulose-softening acids," such as acetic acid or sulphur dioxid. As a method of facilitating the mechanical side of extraction without deleterious effects, the treatment of the coffee with steam under pressure, as utilized in the patented process of Myer, Roselius, and Wimmer,[141] is probably the safest.
Many ingenious methods have been devised for the ready removal of the caffein from this point on. Several processes employ an alkali, such as ammonium hydroxid, to free the caffein from the acid; or an acid, such as acetic, hydrochloric, or sulphurous, is used to form a more soluble salt of caffein. Other procedures effect the dissociation of the caffein-acid salt by dampening or immersion in a liquid and subjecting the mass to the action of an electric current.
The caffein is usually extracted from the beans by benzol or chloroform, but a variety of solvents may be employed, such as petrolic ether, water, alcohol, carbon tetrachloride, ethylene chloride, acetone, ethyl ether, or mixtures or emulsions of these. After extraction, the beans may be steam distilled to remove and to recover any residual traces of solvent, and then dried and roasted. It is said[142] that by heating the beans before bringing them into contact with steam, not only is an economy of steam effected, but the quality of the resultant product is improved.
One clever but expensive method[143] of preparing caffein-free coffee consists in heating the beans under pressure, with some substance, such as sodium salicylate, with the resultant formation of a more soluble and more easily steam-distillable compound of caffein. The beans are then steam distilled to remove the caffein, dried, and roasted.
Another process of peculiar interest is that of Hubner,[144] in which the coffee beans are well washed and then spread in layers and kept covered with water at 15° C. until limited germination has taken place, whereupon the beans are removed and the caffein extracted with water at 50° C. It is claimed by the inventor that sprouting serves to remove some of the caffein, but it is quite probable that the process does nothing more than accomplish simple aqueous extraction.
In the majority of these processes the flavor of the resultant product should be very similar to natural roasted coffee. However, in the cases where aqueous extraction is employed, other substances besides caffein are removed that are replaced in the bean only with difficulty. The resultant product accordingly is very likely to have a flavor not entirely natural. On the other hand, beans from which the caffein is extracted with volatile solvents, if the operation be conducted carefully, should give a natural-tasting roast. Any residual traces of the solvent left in the bean are volatilized upon roasting.
Some of the caffein-free coffees on the market show upon analysis almost as much caffein as the natural bean. Those manufactured by reliable concerns, however, are virtually caffein-free, their content of the alkaloid varying from 0.3 to 0.07 percent as opposed to 1.5 percent in the untreated coffee. Thus, although actually only caffein-poor, in order to get the reaction of one cup of ordinary coffee one would have to drink an unusual amount of the brew made from these coffees.
The Aromatic Principles of Coffee
To ascertain just what substance or substances give the pleasing and characteristic aroma to coffee has long been the great desire of both practical and scientific men interested in the coffee business. This elusive material has been variously called caffeol, caffeone, "the essential oil of coffee," etc., the terms having acquired an ambiguous and incorrect significance. It is now generally agreed that the aromatic constituent of coffee is not an essential oil, but a complex of compounds which usage has caused to be collectively called "caffeol."
These substances are not present in the green bean, but are produced during the process of roasting. Attempts at identification and location of origin have been numerous; and although not conclusive, still have not proven entirely futile. One of the first observations along this line was that of Benjamin Thompson in 1812. "This fragrance of coffee is certainly owing to the escape of a volatile aromatic substance which did not originally exist as such in the grain, but which is formed in the process of roasting it." Later, Graham, Stenhouse, and Campbell started on the way to the identification of this aroma by noting that "in common with all the valuable constituents of coffee, caffeone is found to come from the soluble portion of the roasted seed."[145]
Comparison of the aroma given off by coffee during the roasting process with that of fresh-ground roasted coffee shows that the two aromas, although somewhat different, may be attributed to the same substances present in different proportions in the two cases. Recovery and identification of the aromatic principles escaping from the roaster would go far toward answering the question regarding the nature of the aroma. Bernheimer[146] reported water, caffein, caffeol, acetic acid, quinol, methylamin, acetone, fatty acids and pyrrol in the distillate coming from roasting coffee. The caffeol obtained by Bernheimer in this work was believed by him to be a methyl derivative of saligenin. Jaeckle[147] examined a similar product and found considerable quantities of caffein, furfurol, and acetic acid, together with small amounts of acetone, ammonia, trimethylamin, and formic acid. The caffeol of Bernheimer could not be detected. Another substance was separated also, but in too small a quantity to permit complete identification. This substance consisted of colorless crystals, which readily sublimed, melted at 115° to 117° C., and contained sulphur. The crystals were insoluble in water, almost insoluble in alcohol, but readily soluble in ether.
By distilling roasted coffee with superheated steam, Erdmann[148] obtained an oil consisting of an indifferent portion of 58 percent and an acid portion of 42 percent, consisting mainly of a valeric acid, probably alphamethylbutyric acid. The indifferent portion was found to contain about 50 percent furfuryl alcohol, together with a number of phenols. The fraction containing the characteristic odorous constituent of coffee boiled at 93° C. under 13 mm. pressure. The yield of this latter principle was extremely small, only about 0.89 gram being procured from 65 kilos of coffee.
Pyridin was also shown to be present in coffee by Betrand and Weisweiller[149] and by Sayre.[150] As high as 200 to 500 milligrams of this toxic compound have been obtained from 1 kilogram of freshly roasted coffee.
As stated above, the empyreumatic volatile aromatic constituents of the coffee are without question formed during and by the roasting process. According to Thorpe,[151] the most favorable temperature for development of coffee odor and flavor is about 200° C. Erdmann claimed to have produced caffeol by gently heating together caffetannic acid, caffein, and cane sugar. Other investigators have been unable to duplicate this work. Another authority,[152] giving it the empirical formula C8H10O2, states that it is produced during roasting, probably at the expense of a portion of the caffein. These conceptions are in the main incomplete and inaccurate.
By means of careful work, Grafe[153] came closer to ascertaining the origin of the fugacious aromatic materials. His work with normal, caffein-free coffee and with Thum's purified coffee led him to state that a part of these substances was derived from the crude fiber, probably from the hemi-cellulose of the thick endosperm cells. Sayre[154] makes the most plausible proposal regarding the origin of caffeol. He considers the roasting of coffee as a destructive distillation process, summarizing the results, briefly, as the production of furfuraldehyde from the carbohydrates, acrolein from the fats, catechol and pyrogallol from the tannins, and ammonia, amins, and pyrrols from the proteins. The products of roasting inter-react to produce many compounds of varying degrees of complexity and toxicity.
The great difficulty which arises in the attempt to identify the aromatic constituents of coffee is that the caffeols of no two coffees may be said to be the same. The reason for this is apparent; for the green coffees themselves vary in composition, and those of the same constitution are not roasted under identical conditions. Therefore, it is not to be expected that the decomposition products formed by the action of the different greens would be the same. Also, these volatile products occur in the roasted coffee in such a small amount that the ascertaining of their percentage relationship and the recognition of all that are present are not possible with the methods of analysis at present at our disposal. Until better analytical procedures have been developed we can not hope to establish a chemical basis for the grading of coffees from this standpoint.
Coffee Oil and Fat
It is well to distinguish between the "coffee oils," as they are termed by the trade, and true coffee oil. In speaking of the qualities of coffee, connoisseurs frequently use erroneous terms, particularly when they designate certain of the flavoring and aromatic constituents of coffee as "oils" or "essential oils." Coffee does not contain any essential oils, the aromatic constituent corresponding to essential oil in coffee being caffeol, a complex which is water-soluble, a property not possessed by any true oil. True, the oil when isolated from roasted coffee does possess, before purification, considerable of the aromatic and flavoring constituents of coffee. They are, however, no part of the coffee fat, but are held in it no doubt by an enfleurage action in much the same way that perfumes of roses, etc., are absorbed and retained by fats and oils in the commercial preparation of pomades and perfumes. This affinity of the coffee oil for caffeol assists in the retention of aromatic substances by the whole roasted bean. However, upon extraction of ground roasted coffee with water, the caffeol shows a preferential solubility in water, and is dissolved out from the oil, going into the brew.
The true oil of coffee has been investigated to a fair degree and has been found to be inodorous when purified. Analysis of green and roasted coffees shows them to possess between 12 percent and 20 percent fat. Warnier[155] extracted ground unroasted coffee with petroleum ether, washed the extract with water, and distilled off the solvent, obtaining a yellow-brownish oil possessing a sharp taste. From his examination of this oil he reported these constants: d24–5, 0.942; refraction at 25°, 81.5; solidifying point, 6° to 5°; melting point, 8° to 9°; saponification number, 177.5; esterification number, 166.7; acid number, 6.2; acetyl number, 0; iodin number, 84.5 to 86.3. Meyer and Eckert[156] carefully purified coffee oil and saponified it with Li2O in alcohol. In the saponifiable portion, glycerol was the only alcohol present, the acids being carnaubic, 10 percent; daturinic acid, 1 to 1.5 percent; palmitic acid, 25 to 28 percent; capric acid, 0.5 percent; oleic acid, 2 percent, and linoleic acid, 50 percent. The unsaponifiable wax amounted to 21.2 percent, was nitrogen-free, gave a phytostearin reaction, and saponification and oxidation indicated that it was probably a tannol carnaubate. Von-Bitto[157] examined the fat extracted from the inner husk of the coffee berry and found it to be faint yellow in color, and to solidify only gradually after melting. Upon analysis, it showed: saponification value, 141.2; palmitic acid, 37.84 percent, and glycerids as tripalmitin, 28.03 percent.
Carbohydrates of the Coffee Berry
There has been considerable diversity of opinion regarding the sugar of coffee. Bell believed the sugar to be of a peculiar species allied to melezitose, but Ewell,[158] G.L. Spencer, and others definitely proved the presence of sucrose in coffee. In fat-free coffee 6 percent of sucrose was found extractable by 70 percent alcohol. Baker[159] claimed that manno-arabinose, or manno-xylose, formed one of the most important constituents of the coffee-berry substance and yielded mannose on hydrolysis. Schultze and Maxwell state that raw coffee contains galactan, mannan, and pentosans, the latter present to the extent of 5 percent in raw and 3 percent in roasted coffee. By distilling coffee with hydrochloric acid Ewell obtained furfurol equivalent to 9 percent pentose. He also obtained a gummy substance which, on hydrolysis, gave rise to a reducing sugar; and as it gave mucic acid and furfurol on oxidation, he concluded that it was a compound of pentose and galactose. In undressed Mysore coffee Commaille[160] found 2.6 percent of glucose and no dextrin. This claim of the presence of glucose in coffee was substantiated by the work of Hlasiwetz,[161] who resolved a caffetannic acid, which he had isolated, into glucose and a peculiar crystallizable acid, C8H8O4, which he named caffeic acid.
The starch content of coffee is very low. Cereals may readily be detected and identified in coffee mixtures by the presence and characteristics of their starch, in view of the fact that coffee (chicory, too) is practically free from starch. On this score it is inadvisable for diabetics to use any of the many cereal substitutes for coffee. It is pertinent to note in this connection that persons suffering from diabetes may sweeten their coffee with saccharin (1⁄2 to 1 grain per cup) or glycerol, thus obtaining perfect satisfaction without endangering their health.
The cellulose in coffee is of a very hard and horny character in the green bean, but it is made softer and more brittle during the process of roasting. It is rather difficult to define under the microscope, particularly after roasting, even though the chief characteristics of the cellular tissue are more or less retained. Coffee cellulose gives a blue color with sulphuric acid and iodin, and is dissolved by an ammoniacal solution of copper oxid. Even after roasting, remnants of the silver skin are always present, the structure of which, a thin membrane with adherent, thick-walled, spindle-shaped, hollow cells, is peculiar to coffee.
The Chemistry of Roasting
The effect of the heat in the roasting of coffee is largely evidenced as a destructive distillation and also as a partial dehydration. At the same time, oxidizing and reducing reactions probably occur within the bean, as well as some polymerization and inter-reactions.
A loss of water is to be expected as the natural outcome of the application of heat; and analyses show that the moisture content of raw coffee varies from 8 to 14 percent, while after roasting it rarely exceeds 3 percent, and frequently falls as low as 0.5 percent. The loss of the original water content of the green bean is not the only moisture loss; for many of the constituents of coffee, notably the carbohydrates, are decomposed upon heating to give off water, so that analysis before and after roasting is no direct indication of the exact amount of water driven off in the process. If it be desired to ascertain this quantity accurately, catching of the products which are driven off and determination of their water content becomes necessary.
The carbohydrates both dehydrate and decompose. The result of the dehydration is the formation of caramel and related products, which comprise the principal coloring matters in coffee infusion. That portion of the carbohydrates known as pentosans gives rise to furfuraldehyde, one of the important components of caffeol.
The effect of roasting upon the fat content of the beans is to reduce its actual weight, but not to change appreciably the percentage present, since the decrease in quantity keeps pace fairly well with the shrinkage. Some of the more volatile fatty acids are driven off, and the fats break down to give a larger percentage of free fatty acids, some light esters, acrolein, and formic acid. If the roast be a very heavy one, or is brought up too rapidly, the fat will come to the surface, through breaking of the fat cells, with a decided alteration in the chemical nature of the fat and with pronounced expansion and cracking.
Decomposition of the caffein acid-salt and considerable sublimation of the caffein also occur. The majority of the caffein undergoes this volatilization unchanged, but a portion of it is probably oxidized with the formation of ammonia, methylamin, di-methylparabanic acid, and carbon dioxid. This reaction partly explains why the amount of caffein recovered from the roaster flues is not commensurate with the amount lost from the roasting coffee; although incomplete condensation is also an important factor. Microscopic examination of the roasted beans will show occasional small crystals of caffein in the indentations on the surface, where they have been deposited during the cooling process.
The compound, or compounds, known as "caffetannic acid" are probably the source of catechol, as the proteins are of ammonia, amins, and pyrrols. The crude fiber and other unnamed constituents of the raw beans react analogously to similar compounds in the destructive distillation of wood, giving rise to acetone, various fatty acids, carbon dioxid and other uncondensable gases, and many compounds of unknown identity.
During the course of roasting and subsequent cooling these decomposition products probably interact and polymerize to form aromatic tar-like materials and other complexes which play an important rôle among the delicate flavors of coffee. In fact, it is not unlikely that these reactions continue throughout the storage time after roasting, and that upon them the deterioration of roasted coffee is largely dependent. Speculation upon what complex compounds are thus formed offers much attraction. A notable one by Sayre[162] postulates the reaction between acrolein and ammonia to give methyl pyridin, which in turn with furfurol forms furfurol vinyl pyridin. This upon reduction would produce the alkaloid, conin, traces of which have been found in coffee.
Although furfuraldehyde is the natural decomposition product of pentosans, furfuryl alcohol is the main furane body of coffee aroma. This would indicate that active reducing conditions prevail within the bean during roasting; and the further fact that carbon monoxid is given off during roasting makes this seem quite probable. If one admits that caffetannic acid exists in the green bean; that upon oxidation it gives viridic acid; and that it is concentrated in the outer layers of the bean, as certain investigators have claimed, then there is chemical proof of the existence of oxidizing conditions about the exterior of the bean. In any event, however, the fact that oxidizing conditions predominate on the external portion of the bean is obvious. Accordingly, our meager knowledge of the chemistry of roasting indicates that while the external layers of the roasting beans are subjected to oxidizing conditions, reducing ones exist in the interior. Future experimentation will, no doubt, prove this to be the case.
Attempts have been made to retain in the beans the volatile products, which normally escape, both by coating previous to roasting[163] and by conducting the process under pressure.[164] However, the results so obtained were not practical, since the cup values were decreased in the majority of cases, and the physiological effects produced were undesirable. In cases where the quality was improved, the gain was not sufficient to recompense the roaster for the additional expense and difficulty of operation.
Various persons have essayed to control the roasting process automatically; but the extreme variance in composition of different coffees, the effect of changing atmospheric conditions, and the lack of constancy in the calorific power of fuels have conspired to defeat the automatic roasting machine.[165] It is even doubtful whether De Mattia's[166] process for roasting until the vapors evolved produce a violet color when passed into a solution of fuchsin decolorized with sulphur dioxid is commercially reliable.
Many patents have been granted for the treatment of coffees immediately prior to or during roasting with the object of thus improving the product. The majority of these depend upon adding solutions of sugar,[167] calcium saccharate,[168] or other carbohydrates,[169] and in the case of Eckhardt,[170] of small percentages of tannic acid and fat. In direct opposition to this latter practise, Jurgens and Westphal[171] apply alkali, ostensibly to lessen the "tannic acid" content. Brougier[172] sprays a solution containing caffein upon the roasting berries; and Potter[173] roasts the coffee together with chicory, effecting a separation at the end.
Ground Coffee Under the Microscope
The exact effect which roasting with sugars has upon the flavor is not well understood; but it is known that it causes the beans to absorb more moisture, due to the hygroscopicity of the caramel formed. For instance, berries roasted with the addition of glucose syrup hold an additional 7 percent of water and give a darker infusion than normally roasted coffee. When the green coffee is glazed with cane sugar prior to roasting, the losses during the process are much higher than ordinarily, on account of the higher temperature required to attain the desired results. Losses for ordinary coffee taken to a 16-percent roast are 9.7 percent of the original fat and 21.1 percent of the original caffein; while for "sugar glazed" coffee the losses were 18.3 percent of the original fat and 44.3 percent of the original caffein, using 8 to 9 percent sugar with Java coffee.
Grinding and Packaging
It is a curious fact that green coffee improves upon aging, whereas after roasting it deteriorates with time. Even when packed in the best containers, age shows to a disadvantage on the roasted bean. This is due to a number of causes, among which are oxidation, volatilization of the aroma, absorption of moisture and consequent hydrolysis, and alteration in the character of the aromatic principles. Doolittle and Wright[174] in the course of some extensive experiments found that roasted coffee showed a continual gain in weight throughout 60 weeks, this gain being mostly due to moisture absorption. An investigation by Gould[175] also demonstrated that roasted coffee gives off carbon dioxid and carbon monoxid upon standing. The latter, apparently produced during roasting and retained by the cellular structure of the bean, diffuses therefrom; whereas the former comes from an ante-roasting decomposition of unstable compounds present.[176]
The surface of the whole bean forms a natural protection against atmospheric influences, and as soon as this is broken, deterioration sets in. On this account, coffee should be ground immediately before extraction if maximum efficiency is to be obtained. The cells of the beans tend to retain the fugacious aromatic principles to a certain extent; so that the more of these which are broken in grinding, the greater will be the initial loss and the more rapid the vitiation of the coffee. It might, therefore, seem desirable to grind coarsely in order to avoid this as much as possible. However, the coarser the grind, the slower and more incomplete will be the extraction. A patent[177] has been granted for a grind which contains about 90 percent fine coffee and 10 percent coarse, the patentee's claim being that in his "irregular grind" the coarse coffee retains enough of the volatile constituents to flavor the beverage, while the fine coffee gives a very high extraction, thus giving an efficient brew without sacrificing individuality.
In packaging roasted coffee the whole bean is naturally the best form to employ, but if the coffee is ground first, King[178] found that deterioration is most rapid with the coarse ground coffee, the speed decreasing with the size of the ground particles. He explains this on the ground of "ventilation"—the finer the grind, the closer the particles pack together, the less the circulation of air through the mass, and the smaller the amount of aroma which is carried away. He also found that glass makes the best container for coffee, with the tin can, and the foil-lined bag with an inner lining of glassine, not greatly inferior.
Considerable publicity has been given recently to the method of packing coffee in a sealed tin under reduced pressure. While thus packing in a partial vacuum undoubtedly retards oxidation and precludes escape of aroma from the original package, it would seem likely to hasten the initial volatilizing of the aroma. Also, it would appear from Gould's[179] work that roasted coffee evolves carbon dioxid until a certain positive pressure is attained, regardless of the initial pressure in the container. Accordingly, vacuum-packing apparently enhances decomposition of certain constituents of coffee. Whether this result is beneficial or otherwise is not quite clear.
Brewing
The old-time boiling method of making coffee has gone out of style, because the average consumer is becoming aware of the fact that it does not give a drink of maximum efficiency. Boiling the ground coffee with water results in a large loss of aromatic principles by steam distillation, a partial hydrolysis of insoluble portions of the grounds, and a subsequent extraction of the products thus formed, which give a bitter flavor to the beverage. Also, the maintenance of a high temperature by the direct application of heat has a deleterious effect upon the substances in solution. This is also true in the case of the pumping percolator, and any other device wherein the solution is caused to pass directly into steam at the point where heat is applied. Warm and cold water extract about the same amount of material from coffee; but with different rates of speed, an increase in temperature decreasing the time necessary to effect the desired result.
It is a well known fact that re-warming a coffee brew has an undesirable effect upon it. This is very probably due to the precipitation of some of the water-soluble proteins when the solution cools, and their subsequent decomposition when heat is applied directly to them in reheating the solution. The absorption of air by the solution upon cooling, with attendant oxidation, which is accentuated by the application of heat in re-warming, must also be considered. It is likewise probable that when an extract of coffee cools upon standing, some of the aromatic principles separate out and are lost by volatilization.
The method of extracting coffee which gives the most satisfaction is practised by using a grind just coarse enough to retain the individualistic flavoring components, retaining the ground coffee in a fine cloth bag, as in the urn system, or on a filter paper, as in the Tricolator, and pouring water at boiling temperature over the coffee. During the extraction, a top should be kept on the device to minimize volatilization, and the temperature of the extract should be maintained constant at about 200° F. after being made. Whether a repouring is necessary or not is dependent upon the speed with which the water passes through the coffee, which in turn is controlled by the fineness of the grind and of the filtering medium.
The Water Extract
Although many analyses of the whole coffee bean are available, but little work has been reported upon the aqueous extracts. The total water extract of roasted coffee varies from 20 to 31 percent in different kinds of coffee. The following analysis of the extract from a Santos coffee may be taken as a fair average example of the water-soluble material.[180]
