Chapter 30
_The Coffee-Leaf Disease_
The coffee tree has many pests and diseases; but the disease most feared by planters is that generally referred to as the coffee-leaf disease, and by this is meant the fungoid _Hemileia vastatrix_, which as told in chapter XV, destroyed Ceylon's once prosperous coffee industry. As it has since been found in nearly all coffee-producing countries, it has become a nightmare in the dreams of all coffee planters. The microscope shows how the spores of this dreaded fungus, carried by the winds upon a leaf of the coffee tree, proceed to germinate at the expense of the leaf; robbing it of its nourishment, and causing it to droop and to die.
A mixture of powdered lime and sulphur has been found to be an effective germicide, if used in time and diligently applied.
[Ill.u.s.tration: Fig. 337. Coffee. Spermoderm in surface view. _st._ sclerenchyma; _p_, compressed parenchyma. x160. (Moeller)]
[Ill.u.s.tration: Fig. 338. Coffee. Cross-section of outer layers of endosperm, showing knotty thickenings of cell walls. x160. (Moeller)]
[Ill.u.s.tration: Fig. 339. Coffee. Tissues of embryo in section. x160.
(Moeller)]
_Value of Microscopic a.n.a.lysis_
The value of the microscopic a.n.a.lysis of coffee may not be apparent at first sight; but when one realizes that in many cases the microscopic examination is the only way to detect adulteration in coffee, its importance at once becomes apparent. In many instances the chemical a.n.a.lysis fails to get at the root of the trouble, and then the only method to which the tester has recourse is the examination of the suspected material under the scope. The mixing of chicory with coffee has in the past been one of the commonest forms of adulteration. The microscopic examination in this connection is the most reliable. The coffee grain will have the appearance already described.
Microscopically, chicory shows numerous thin-walled parenchymatous cells, lactiferous vessels, and sieve tubes with transverse plates.
There are also present large vessels with huge, well-defined pits.
[Ill.u.s.tration: COFFEE LEAF DISEASE (HEMILEIA VASTATRIX)
1. under surface of affected leaf, x 1/2; 2, section through same showing mycelium, haustoria, and a spore-cl.u.s.ter; 3, a spore-cl.u.s.ter seen from below; 4, a uredospore; 5, germinating uredospore; 6, appressorial swellings at tips of germ-tubes; 7, infection through stoma of leaf; 8, teleutospores; 9, teleutospore germinating with promycelium and sporidia; 10, sporidia and their germination (2 after Zimmermann, 3 after Delacroix, 4-10 after Ward)]
Roasted date stones have been used as adulterants, and these can be detected quite readily with the aid of the microscope, as they have a very characteristic microscopic appearance. The epidermal cells are almost oblong, while the parenchymatous cells are large, irregular and contain large quant.i.ties of tannin.
Adulteration and adulterants are considered more fully in chapter XVII.
[Ill.u.s.tration: GREEN AND ROASTED COFFEE UNDER THE MICROSCOPE
Green bean, showing the size and form of the cells as well as the drops of oil contained within their cavities. Drawn with the camera lucida, and magnified 140 diameters.
A fragment of roasted coffee under the microscope. Drawn with the camera lucida, and magnified 140 diameters.]
[Ill.u.s.tration: BOGOTA, GREEN
Longitudinal--Magnified 200 diameters]
[Ill.u.s.tration: BOGOTA, GREEN
Cross Section--Magnified 200 diameters]
[Ill.u.s.tration: BOGOTA, GREEN
Tangential--Magnified 200 diameters]
[Ill.u.s.tration: BOGOTA, ROASTED
Tangential--Magnified 200 diameters]
[Ill.u.s.tration: GREEN AND ROASTED BOGOTA COFFEE UNDER THE MICROSCOPE
These pictures serve to demonstrate that the coffee bean is made up of minute cells that are not broken down to any extent by the roasting process. Note that the oil globules are more prominent in the green than in the roasted product]
CHAPTER XVII
THE CHEMISTRY OF THE COFFEE BEAN
_Chemistry of the preparation and treatment of the green bean--Artificial aging--Renovating damaged coffees--Extracts--"Caffetannic acid"--Caffein, caffein-free coffee--Caffeol--Fats and oils--Carbohydrates--Roasting--Scientific aspects of grinding and packaging--The coffee brew--Soluble coffee--Adulterants and subst.i.tutes--Official methods of a.n.a.lysis_
By Charles W. Trigg
Industrial Fellow of the Mellon Inst.i.tute of Industrial Research, Pittsburgh, 1916-1920
When the vast extent of the coffee business is considered, together with the intimate connection which coffee has with the daily life of the average human, the relatively small
True, a painstaking compilation of all the scientific and semi-scientific work done upon coffee furnishes quite a compendium of data, the value of which is not commensurate with its quant.i.ty, because of the spasmodic nature of the investigations and the non-conclusive character of the results so far obtained. The following general survey of the field argues in favor of the promulgation of well-ordered and systematic research, of the type now in progress at several places in the United States, into the chemical behavior of coffee throughout the various processes to which it is subjected in the course of its preparation for human consumption.
_Green Coffee_
One of the few chemical investigations of the growing tree is the examination by Graf of flowers from 20-year-old coffee trees, in which he found 0.9 percent caffein, a reducing sugar, caffetannic acid, and phytosterol. Power and Chestnut[102] found 0.82 percent caffein in air-dried coffee leaves, but only 0.087 percent of the alkaloid in the stems of the plant separated from the leaves. In the course of a study[103] inst.i.tuted for the purpose of determining the best fertilizers for coffee trees, it developed that the cherries in different stages of growth show a preponderance of potash throughout, while the proportion of P_2_O_5 attains a maximum in the fourth month and then steadily declines.
Experiments are still in progress to ascertain the precise mineral requirements of the crop as well as the most suitable stage at which to apply them. During the first five months the moisture content undergoes a steady decrease, from 87.13 percent to 65.77 percent, but during the final ripening stage in the last month there is a rise of nearly 1 percent. This may explain the premature falling and failure to ripen of the crop on certain soils, especially in years of low rainfall.
Malnutrition of the trees may result also in the production of oily beans.[104]
The coffee berry comprises about 68 percent pulp, 6 percent parchment, and 26 percent clean coffee beans. The pulp is easily removed by mechanical means; but in order to separate the soft, glutinous, saccharine parchment, it is necessary to resort to fermentation, which loosens the skin so that it may be removed easily, after which the coffee is properly dried and aged. There is first a yeast fermentation producing alcohol; and then a bacterial action giving mainly inactive lactic acid, which is the main factor in loosening the parchment. For the production of the best coffee, acetic acid fermentation (which changes the color of the bean) and temperature above 60 should be avoided, as these inhibit subsequent enzymatic action.[105]
Various schemes have been proposed for utilizing the large amount of pulp so obtained in preparing coffee for market. Most of these depend upon using the pulp as fertilizer, since fresh pulp contains 2.61 percent nitrogen, 0.81 percent P_2_O_5, 2.38 percent pota.s.sium, and 0.57 percent calcium. One procedure[106] in particular is to mix pulp with sawdust, urine, and a little lime, and then to leave this mixture covered in a pit for a year before using. In addition to these mineral matters, the pulp also contains about 0.88 percent of caffein and 18 to 37 percent sugars. Accordingly, it has been proposed[107] to extract the caffein with chloroform, and the sugars with acidulated water. The aqueous solution so obtained is then fermented to alcohol. The insoluble portion left after extraction can be used as fuel, and the resulting ash as fertilizer.
The pulp has been dried and roasted for use in place of the berry, and has been imported to England for this purpose. It is stated that the Arabs in the vicinity of Jiddah discard the kernel of the coffee berries and make an infusion of the husk.[108]
Quality of green coffee is largely dependent upon the methods used and the care taken in curing it, and upon the conditions obtaining in s.h.i.+pment and storage. True, the soil and climatic conditions play a determinative role in the creation of the characteristics of coffee, but these do not offer any greater opportunity for constructive research and remunerative improvement than does the development of methods and control in the processes employed in the preparation of green coffee for the market.
[Ill.u.s.tration: CROSS-SECTION OF THE ENDOSPERM OR HARD STRUCTURE OF THE GREEN BEAN]
Storage prior and subsequent to s.h.i.+pment, and circ.u.mstances existing during transportation, are not to be disregarded as factors contributory to the final quality of the coffee. The sweating of mules carrying bags of poorly packed coffee, and the absorption of strong foreign aromas and flavors from odoriferous substances stored in too close proximity to the coffee beans, are cla.s.sic examples of damage that bear iterative mention. Damage by sea water, due more to the excessive moisture than to the salt, is not so common an occurrence now as heretofore. However, a cheap and thoroughly effective means of ethically renovating coffee which has been damaged in this manner would not go begging for commercial application.
That green coffee improves with age, is a tenet generally accepted by the trade. s.h.i.+pments long in transit, subjected to the effects of tropical heat under closely battened hatches in poorly ventilated holds, have developed into much-prized yellow matured coffee. Were it not for the large capital required and the attendant prohibitive carrying charges, many roasters would permit their coffees to age more thoroughly before roasting. In fact, some roasters do indulge this desire in regard to a portion of their stock. But were it feasible to treat and hold coffees long enough to develop their attributes to a maximum, still the exact conditions which would favor such development are not definitely known. What are the optimum temperature and the correct humidity to maintain, and should the green coffee be well ventilated or not while in storage? How long should coffee be stored under the most favorable conditions best to develop it? Aging for too long a period will develop flavor at the expense of body; and the general cup efficiency of some coffees will suffer if they be kept too long.
[Ill.u.s.tration: PORTION OF THE INVESTING MEMBRANE, SHOWING ITS STRUCTURE
Drawn with the camera lucida, and magnified 140 diameters]
The exact reason for improvement upon aging is in no wise certain, but it is highly probable that the changes ensuing are somewhat a.n.a.logous to those occurring in the aging of grain. Primarily an undefined enzymatic and mold action most likely occurs, the nature of the enzymes and molds being largely dependent upon the previous treatment of the coffee. Along with this are a loss of moisture and an oxidation, all three actions having more evident effects with the pa.s.sage of time.