CupScout

_How the beans may be examined under the microscope, and what is
revealed–Structure of the berry, the green, and the roasted
bean–The coffee leaf disease under the microscope–Value of
microscopic analysis in detecting adulteration_

The microscopy of coffee is, on the whole, more important to the planter
than to the consumer and the dealer; while, on the other hand, the
microscopy is of paramount importance to the consumer and the dealer as
furnishing the best means of determining whether the product offered is
adulterated or not. Also, from this standpoint, the microscopy of the
plant is less important than that of the bean.

[Illustration: Fig. 331. Coffee (_Coffea arabica_). I--Cross-section of
berry, natural size; _Pk_, outer pericarp; _Mk_, endocarp; _Ek_,
spermoderm; _Sa_, hard endosperm; Sp, soft endosperm. II--Longitudinal
section of berry, natural size; _Dis_, bordered disk; _Se_, remains of
sepals; _Em_, embryo. III--Embryo, enlarged; _cot_, cotyledon; _rad_,
radicle. (Tschirch and Oesterle.)]

_The Fruit and the Bean_

The fruit, as stated in chapter XV, consists of two parts, each one
containing a single seed, or bean. These beans are flattened laterally,
so as to fit together, except in the following instances: in the
peaberry, where one of the ovules never develops, the single ovule,
having no pressure upon it, is spherical; in the rare instances where
three seeds are found, the grains are angular.

The coffee bean with which the consumer is familiar is only a small part
of the fruit. The fruit, which is the size of a small cherry, has, like
the cherry, an outer fleshy portion called the pericarp. Beneath this is
a part like tissue paper, spoken of technically as the parchment, but
known scientifically as the endocarp. Next in position to this, and
covering the seed, is the so-called spermoderm, which means the seed
skin, referred to in the trade as the silver skin. Small portions of
this silver skin are always to be found in the cleft of the coffee bean.

The coffee bean is the embryo and its food supply; the embryo is that
part of the seed which, when supplied with food and moisture, develops
into a new plant. The embryo of the coffee is very minute (Fig. 331,
II, _Em_)[101]; and the greater part of the seed is taken up by the food
supply, consisting of hard and soft endosperm (Fig. 331, I and II, _Sa_,
_Sp_). The minute embryo consists of two small thick leaves, the
cotyledons (Fig. 331, III, _cot_), a short stem, invisible in the
undissected embryo, and a small root, the radicle (Fig. 331, III,
_rad_).

[Illustration: Fig. 332. Coffee. Cross section of bean showing folded
endosperm with hard and soft tissues. x6. (Moeller)]

_Fruit Structure_

In order to examine the structure of these layers of the fruit under the
microscope, it is necessary to use the pericarp dry, as it is not easily
obtainable in its natural condition. If desired, an alcoholic specimen
may be used, but it has been found that the dry method gives more
satisfactory results. The dried pericarp is about 0.5 mm thick. Great
difficulty is experienced in cutting microtome sections of pericarp when
the specimen is embedded in paraffin, because the outer layers are soft
and the endocarp is hard, and the two parts of the section separate at
this point. To overcome this, the sections might also be embedded in
celloidin. When the sections are satisfactory, they may be stained with
any of the double stains ordinarily used in the study of plant
histology.

[Illustration: Fig. 333. Coffee. Cross section of hull and bean.
Pericarp consists of: 1, epicarp; 2-3, layers of mesocarp, with 4,
fibro-vascular bundle; 5, palisade layer; and 6, endocarp; _ss_,
spermoderm, consists of 8, sclerenchyma, and 9, parenchyma; _End_,
endosperm (Tschirch and Oesterle)]

A section cut crosswise through the entire fruit would present the
appearance shown in Fig. 333. The cells of the epicarp are broad and
polygonal, sometimes regularly four-sided, about 15-35 µ broad. At
intervals along the surface of the epicarp are stomata, or breathing
pores, surrounded by guard cells. The next layer of the pericarp is the
mesocarp (Figs. 333, 334, 335), the cells of which are larger and more
regular in outline than the epicarp. The cells of the mesocarp become as
large as 100 µ broad, but in the inner parts of the layer they become
very much flattened. Fibrovascular bundles are scattered through the
compressed cells of the mesocarp. The cell walls are thick; and large,
amorphous, brown masses are found within the cell; occasionally, large
crystals are found in the outer part of the layer. The fibro-vascular
bundles consist mainly of bast and wood fibers and vessels. The bast
fibers are as large as 1 mm long and 25 µ broad, with thick walls and
very small _lumina_. Spiral and pitted vessels are also present.

[Illustration: Fig. 334. Coffee. Surface view of _ep_, epicarp, and _p_,
outer parenchyma of mesocarp. x160. (Moeller)]

The layer next to this is a soft tissue, parenchyma (Fig. 333, 5; Fig.
334, _p_). The parenchyma, or palisade cells as they are called, is a
thin-walled tissue in which the cells are elongated, from which fact
they receive their name. The walls of these cells, though very thin, are
mucilaginous, and capable of taking up large amounts of water. They
stain well with the aniline stains.

The endocarp (Fig. 336) is closely connected with the palisade layer and
has thin-walled cells that closely resemble, in all respects, the
endocarp of the apple. The outer layer consists of thick-walled fibers,
which are remarkably porous (Fig. 333, 6; Fig. 336) while the fibers of
the inner layer are thin-walled and run in the transverse direction.

_The Bean Structure_

Spermoderm, or silver skin, is not difficult to secure for microscopic
analysis; because shreds of it remain in the groove of the berry, and
these shreds are ample for examination. It can readily be removed
without tearing, if soaked in water for a few hours. The spermoderm is
thin enough not to need sectioning. It consists of two
elements–sclerenchyma and parenchyma cells. (Figs. 333, 337, _st_,
_p_).

[Illustration: Fig. 335. Coffee. Elements of pericarp in surface view.
_p_, parenchyma; _bp_, parenchyma of fibro-vascular bundle; _b_, bast
fiber; _sp_, spiral vessel. x160. (Moeller)]

Sclerenchyma forms an uninterrupted covering in the early stages of the
seed; but as the seed develops, surrounding tissues grow more rapidly
than the sclerenchyma, and the cells are pushed apart and scattered. The
cells occurring in the cleft of the berry are straight, narrow, and
long, becoming as long as 1 mm, and resemble bast fibers somewhat. On
the surface of the berry, and sometimes in the cleft, there are found
smaller, thicker cells, which are irregular in outline, club-shaped and
vermiform types predominating.

Parenchyma cells form the remainder of the spermoderm; and these are
partially obliterated, so that the structure is not easily seen,
appearing almost like a solid membrane. The raphe runs through the
parenchyma found in the cleft of the berry.

The endosperm (Figs. 333; 338) consist of small cells in the outer part,
and large cells, frequently as thick as 100 µ, in the inner part. The
cell walls are thickened and knotted. Certain of the inner cells have
mucilaginous walls which when treated with water disappear, leaving only
the middle lamellae, which gives the section a peculiar appearance. The
cells contain no starch, the reserve food supply being stored cellulose,
protein, and aleurone grains. Various investigators report the presence
of sugar, tannin, iron, salts, and caffein.

The embryo (Fig. 331, III) may be obtained by soaking the bean in water
for several hours, cutting through the cleft and carefully breaking
apart the endosperm. If it is now soaked in diluted alkali, the embryo
protrudes through the lower end of the endosperm. It is then cleared in
alkali, or in chloral hydrate. The cotyledons shown have three pairs of
veins, which are slightly netted. The radicle is blunt and is about 3/4
mm in length, while the cotyledons are 1/2 mm long.

[Illustration: Fig. 336. Coffee. Sclerenchyma fibers of endocarp. x160.
(Moeller)]

_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.

[Illustration: Fig. 337. Coffee. Spermoderm in surface view. _st._
sclerenchyma; _p_, compressed parenchyma. x160. (Moeller)]

[Illustration: Fig. 338. Coffee. Cross-section of outer layers of
endosperm, showing knotty thickenings of cell walls. x160. (Moeller)]

[Illustration: Fig. 339. Coffee. Tissues of embryo in section. x160.
(Moeller)]

_Value of Microscopic Analysis_

The value of the microscopic analysis 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
analysis 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.

[Illustration: COFFEE LEAF DISEASE (HEMILEIA VASTATRIX)

1. under surface of affected leaf, x 1/2; 2, section through same
showing mycelium, haustoria, and a spore-cluster; 3, a spore-cluster
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 quantities of tannin.

Adulteration and adulterants are considered more fully in chapter XVII.

[Illustration: 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.]

[Illustration: BOGOTA, GREEN

Longitudinal--Magnified 200 diameters]

[Illustration: BOGOTA, GREEN

Cross Section--Magnified 200 diameters]

[Illustration: BOGOTA, GREEN

Tangential--Magnified 200 diameters]

[Illustration: BOGOTA, ROASTED

Tangential--Magnified 200 diameters]

[Illustration: 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]