Читать книгу The Manufacture of Chocolate and other Cacao Preparations - Paul Zipperer - Страница 9

The varieties of the cacao tree which yield the beans at the present time occurring in commerce are.

Theobroma cacao, Linné the true cacao, spread over the widest area, and almost exclusively cultivated on plantations, with many varieties (Crillo, Forastero etc.) and Theobroma bicolor, a party-coloured cacao tree the seeds of which are mixed with Brazilian and Caracas beans.

Theobroma speciosum Wildenow, which yields, like Theobroma cacao, Brazilian beans (magnificent tree).

Theobroma quayanense, yielding Guiana beans.

Theobroma silvestre or forest cacao.

Theobroma subincanum, white-leaved-cacao, and Theobroma microcarpum, small-fruited cacao, are met with as admixtures in Brazilian beans.

Theobroma glaucum, grey cacao, fruits of which variety are found among Caracas beans.

Theobroma angustifolium the narrow-leaved and Theobroma ovatifolium, oval leaf, may be regarded as characteristic of Mexican cacao.

Before describing the commercial kinds of cacao, a knowledge of which is of first importance to manufacturers, it is desirable to consider the beans in regard to external form and microscopial structure, in order that the use of some indispensable scientific expressions in the subsequent description of particular commercial kinds of cacao may be intelligible.

The bean, page 3 Fig. 2 C-G, consists, according to Hanousek15, of a seed-shell, a seed-skin and the embryo or kernel with the radicle. The oval-shaped seed is generally from 16 to 28 mm. long, 10 to 15 mm. broad and from 4 to 7 mm. thick. At the lower end of the bean there is a depressed, flattened and frequently circular hilum visible, from which a moderately marked line extends up to the apex of the bean where it forms the centre of radiating longitudinal ribs—vascular bundles-extending to the middle of the bean through the outer seed-coating back to the hilum.

The outer seed shell (cf. Fig. 3) is of the thickness of paper, brittle, scaly externally and reddish brown, lined with a colourless translucent membrane peeling to the so-called silver membrane (previously but falsely known as seed envelope) and penetrating into the convolutions of the kernel in irregularly divided folds. The shells of some of the better sorts of beans, such as Caracas, are frequently covered with a firmly adherent, dense, reddish-brown powder, consisting of ferruginous loam originating from the soil on which the beans have been dried and serving as a protection against the attacks of insects. But opinions are divided as to, the utility of this process.

The fermented kernel consists of two large cotyledons occupying the whole bean; it is of fatty lustre, reddish grey or brown colour and often present a superficial violet tinge; and under gentle pressure readily breaks up into numerous angular fragments the surfaces of which are generally bordered by the silver membrane. The fragments can be easily recognised when laid in water. At the contact of the lobes there is an angular middle rib and two lateral ribs are connected with the radicle at the broader end of the bean. The ripe fresh-gathered cacao-kernel is undoubtedly white and the reddish brown or violet pigment is formed during the fermenting of the bean. But there is also a white cacao, though seldom met with. According to information furnished by Dr. C. Rimper of Ecuador, it is of rare occurrence and is not cultivated to any great extent. In Trinidad also a perfectly white seeded cacao, producing large fruit and fine kernels, was introduced from Central America by the curator of the Botanic Gardens in 1893.

The microscopic structure of the shell, Fig. III., presents no remarkable peculiarity that requires to be noticed here.

The delicate inner membrane (fig. 3) coating the cotyledons and penetrating into their folds consists of several layers. Connected with it are club-shaped glandular structures, fig. 4, consisting of several dark coloured cells that are known as the Mitscherlich particles. According to A. F. W. Schimper16 they are hairs fallen from the epidermis (fig. 4) of the cotyledon and do not originate, as was formerly supposed, in the inner silver membrane.

These structures, named after their discoverer, were formerly supposed to be algae, or cells of the embryo sac, unconnected with the tissues of the seed cells. They are, however, as true epidermoid structures, similar to the hairs of other plants.

Fig. 3. Cross Section of Shell of Cacao Bean (Tschirsch).

gfb	vascular bundles	fe	endocarp, or inner coat of fruit	st	sklerogenous, or dry cells
co	cotyledon	se	epicarp, or skin	is	silver membrane
pc	ducts	sch	mucilagenous, or slime cells	co	cotyledon
f	pulp	lp	parenchyma, or cellular tissue	gfb	vascular bundles

These Mitscherlich particles are not only characteristic of the seed membrane, but also of the entire seed as well as the preparations made from it. Wherever cacao is mixed with other materials, its presence may be ascertained by microscopical detection of these structures, which are peculiar to cacao.

In the large elongated, hexagonal cells of the seed membrane there are two other structures to be seen with the aid of high power (250 fold), one appearing as large crystalline druses, while the other consists of extremely fine needles united in bundles.

Fig. 4.

Cross section of the cotyledon, showing “Mitscherlich particles” (Moeller).

By addition of petroleum spirit the former, consisting of fat acid crystals, are dissolved, the latter, remaining unaltered, are considered by Mitscherlich to be theobromine crystals, since their crystalline form closely resembles that of theobromine. A more scientific explanation has not been forthcoming.

The cotyledons are seen under the microscope to consist of a tissue of thin walled cells, without cavities, lying close together, and here and there distributed through the tissue, cells with brownish yellow, reddish brown, or violet coloured contents. These latter are the pigment cells which contain the substance known as cacao-red and analogous to tannin; it, together with theobromine, gives rise to the delicate taste and aroma of cacao. The other cells of the tissue are filled with extremely small starch granules the size of which rarely exceeds 0.005 mm.; with them are associated fat, in the form of spear-shaped crystals, and albuminoid substances.

In order to discriminate between these substances they must be stained by various reagents. According to Molisch17, theobromine may be recognised, in sections of the seed, by adding a drop of hydrochloric acid and after some time an equal drop of auric chloride solution (3 %) After some of the liquid has evaporated, bunches of long yellow crystals of theobromine aurochloride make their appearance. On addition of osmic acid the fat is coloured greyish brown. On addition to the microscopic section a drop of iodine solution, or better iodozine chloride, the starch becomes blue, while albuminous substances are coloured yellow. Cacao starch granules are very small and cannot well be mistaken for other kinds, except the starch of some spices such as pimento or that of Guarana, prepared from the seeds of Paulinna sorbilis. According to Möller the blue iodine colouration of cacao starch takes place very slowly and it is probably retarded by the large amount of fat present; but the point has been contested by Zipperer and later investigators.

In order to make the starch granules of cacao and the cells containing cacao-red distinctly visible under the microscope, it is advisable to immerse the section in a drop of almond oil, because the addition of water renders the object indistinct in consequence of the large amount of fat present. Another excellent medium for the microscopic observation of cacao is the solution of 8 parts of chloral hydrate in 5 parts of water, as recommended by Schimper.18

By these means it may easily be seen that the pigment or cacao red in different sorts of cacao varies more or less in colour.

To complete the account of the microscopic characters of the cacao cotyledon, mention must be made of the small vascular bundles, generally spiral, that are distributed throughout the tissues of the cotyledons and are readily made visible by adding a drop of oil or a drop of chloral hydrate solution.