Chapter 32
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 a.n.a.lyses 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-subst.i.tute" 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 const.i.tuents 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 ma.s.s 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 a.n.a.lysis 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
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 const.i.tuents 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 quant.i.ties 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 quant.i.ty 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 const.i.tuent 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 const.i.tuents 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 C_8_H_10_O_2, 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 const.i.tuents 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 const.i.tution 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 relations.h.i.+p and the recognition of all that are present are not possible with the methods of a.n.a.lysis at present at our disposal. Until better a.n.a.lytical 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 const.i.tuents of coffee as "oils" or "essential oils." Coffee does not contain any essential oils, the aromatic const.i.tuent 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 const.i.tuents 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 a.s.sists 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. a.n.a.lysis 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: d_24-5, 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 Li_2_O 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 a.n.a.lysis, 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 const.i.tuents 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, C_8_H_8_O_4, 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 subst.i.tutes 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 h.o.r.n.y 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 a.n.a.lyses 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 const.i.tuents of coffee, notably the carbohydrates, are decomposed upon heating to give off water, so that a.n.a.lysis 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 quant.i.ty 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 princ.i.p.al 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 quant.i.ty 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 p.r.o.nounced 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 const.i.tuents of the raw beans react a.n.a.logously 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 ident.i.ty.
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 role 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.