coloured greenish brown by alkalis, red by acids; acetates give a
grey to yellowish colour; tincture of iodine, stannous chloride and
mercurous nitrate give a rose to brown precipitate. Iron and copper
salts produce grey precipitates which gradually become brown to black.
Gelatine solution, containing alum, and albumin give copious yellow
precipitates.
Stains produced on linen by the colouring matter of cacao-red can
be removed by treatment with hot water and finally bleaching with a
solution of sulphurous acid.
4. ~Theobromine.~
All those materials which are regarded as stimulants, like coffee, tea,
cacao, tobacco etc., owe their action to peculiar nerve stimulating
bodies, which are present only in small quantity in the seeds or leaves
of the respective plants and are termed by chemists alkaloids and
diureides.
The physiologically active constituents of tea, coffee and cacao are
considered, even up to to-day, by many authors as alkaloids or organic
bases and especially ranked among the xanthine or purine bases. Recent
investigations, however, separate these substances from the alkaloids
in the strict sense and comprise them within a particular group of
urea derivatives under the designation of ureides; the ureides of tea,
coffee and cacao representing two molecules of urea, they are to be
qualified as “diureides
A bitter substance in the cacao bean had already been observed by
Schrader, but Woscressensky[53] in 1841 was the first to isolate the
diureide, theobromine.
Theobromine is found in the unfermented and fermented beans in two
forms; as free theobromine, which has been eliminated from the
glucoside by the ferment in the drying and fermenting processes, and in
combination with glucose and cacao-red as a glucoside, from which it
can only be separated by chemical means.
Theobromine stands in near relation to caffeine, the diureide of tea
and coffee, as will be seen from their chemical formulae—in which
theobromine is shown to contain one methyl group CH_{3}, less, its
place being taken by an hydrogen atom;
Caffeine Theobromine
____________/\____________ ______________/\______________
/ \ / \
C_{5}HN_{2}O_{3}(CH_{1})_{3}, C_{5}H_{2}N_{2}O_{3}(CH_{1})_{2},
so that in all, theobromine falls short of caffeine by only one
radical. Strecker[54] was the first to show the relation between
the two substances, when he succeeded in converting caffeine into
theobromine by the action of methyl oxide on silver theobromine for 24
hours at 100° C. Caffeine and silver iodide are then formed and can
be separated by treatment with alcohol, which dissolves the caffeine,
leaving the silver iodide undissolved.
E. Fischer[55] was shown the relation of theobromine and caffeine to
uric acid by artificial synthesis of both substances from derivatives
of both. Fischer, starting with monomethyl pseudo-uric acid, converted
it into 7-methyl uric acid by distilling it with hydrochloric acid, and
afterwards, by treating the lead salt of the latter with methyl iodide
and ether, produced 3-7-methyl-uric acid. That acid was converted into
dimethyldioxychlor-purine by treatment with a mixture of phosphorus
oxychloride and phosphoric penta-chloride, with subsequent reduction
into 3-7 dimethyl-6-amino-2-oxy-purine, from which, by the action of
nitrous acid with loss of the amine group, theobromine was finally
obtained. The synthesis of theobromine is a brilliant exploit of
Fischer’s, and it is quite possible that at no distant period, when a
simple and cheap method of production has been arrived at, synthetical
theobromine will appear commercially as a rival of the natural product.
At present there is no prospect of this being immediately realised,
and cacao shells from which theobromine is now prepared are as yet in
no danger of displacement by the new substitute, but still serve as a
useful by-product in the manufacture of cacao.
Theobromine and caffeine, like the alkaloids or plant bases, have a
distinct physiological and even toxic action if taken in too large
quantities.
From the experiments of Mitscherlich it appears that theobromine has a
similar action to caffeine, but is somewhat less active owing to its
being less soluble in the gastric juice. Mitscherlich’s experiments
with frogs, pigeons and rabbits show that 0·05 grammes killed a frog in
40 hours, 0·05 grammes a pigeon in 24 hours, and 1 gramme a rabbit in
less than 20 hours. Death resulted in all cases from cramping of the
spinal cord, producing either convulsions or subsequent paralysis.
The results of these experiments do not detract from the nutritive
value of cacao, since the human organism requires ten times as much
theobromine as rabbits to exhibit the slightest toxic symptom; in
cacao mass containing 1 % not mentioned in discussion; just a head’s
up to PP for S&R] theobromine, that would involve the consumption of
5 lbs. averdupois of chocolate at once, a practical impossibility.
Similar conditions prevail in connection with the use of tea, coffee,
and especially tobacco, where symptoms of poisoning have been
occasionally noticed (the nicotine peril of excessive smokers) but it
would seem that cacao and chocolate are the most favourably placed of
these stimulants as regards such toxic action. It appears from the
experiments of Albanese[56] Bondzynski, Gottlieb[57] and Rost[58] that
3 percent of the theobromine administered passed out in the urine
unaltered, whilst on the other hand 20-30 percent of that decomposed
in the organism is found again as monomethyl-xanthine.
The larger proportion of the monomethyl xanthine is heteroxanthine (=
7 Methyl-X) and the inferior 3 Methyl-X. The excretion of theobromine
appears to be closely connected with the quantity of urine voided,
which is especially increased by the administration of theobromine.
Since 1890, as a result of W. v. Schröder’s[59] observations in 1888,
that property of theobromine has had an extended application in
practical therapeutics; theobromine has been used as a diuretic in
kidney diseases, and, unlike all similar medicinal agents, it exercises
no influence on the heart, a circumstance which essentially increases
its therapeutic value. It can be employed for medicinal purposes,
either uncombined or in the form of salicylate, acetate and certain
double compounds, as sodium or lithium and theobromine salicylate or
acetate.
The double compounds known as diuretin, agurin and uropherin are freely
soluble in water and are therefore more readily absorbed into the
system than pure theobromine, which is only with difficulty soluble in
water. Through the establishment of theobromine as a medicinal agent,
for which we are indebted to Chr. Gram[60] and G. See,[61] cacao husks,
hitherto a waste product in the manufacture of cacao, have become of
value for the preparation of theobromine, in which many of the largest
German chemical factories are now engaged.
Fluctuations as regards the percentage of theobromine in the beans
are so extraordinary that they can only be ascribed to the lack of
prescribed and definite modes of procedure in fermenting, which
obviously necessitates differences in the resulting products.
Eminger found from 0·88-2·34 percent of theobromine in the examination
of a rather considerable number of commercial kinds of cacao beans and
in the husks 0·76 percent of the diureide: C. C. Keller[62] has also
found it in the leaves and in the pericarp. Cacao contains 0·05 to 0·36
percent of caffeine.
Theobromine is a permanent white powder, appears under the magnifying
glass as small, white, prismatic or granular crystals. At first it has
only a slightly bitter taste, which becomes more intense when it is
kept in the mouth for some length of time; and indeed, the bitter taste
of the cacao bean and its preparations is mostly due to theobromine. It
sublimes at 220 ° C. without melting. This phenomenon explains why the
over roasted bean, that is, the kernel of beans which by accident have
been heated to more than 130-150 ° C. is poorer in theobromine than
the husks. When heated to 310 ° C. theobromine melts to a clear liquid
which re-crystallizes on cooling.
One part of absolutely pure theobromine dissolves according to Eminger
in 736·5 parts of water at 18 ° C., in 136 parts at 100 ° C. in 5399
parts alcohol (90 %) at 18 ° C. in 440 parts at boiling (90 %) point
and in 818 parts of boiling absolute alcohol. It dissolves in 21000
parts of ether at 17 ° C. in 4856 parts of methyl alcohol at 18 ° C.
in 58·8 parts of chloroform at 18 ° C. and in 2710 parts of boiling
chloroform[63]. Theobromine is partly decomposed by strong alkalis but
by cautious addition of alkalis it forms compounds with them, which,
are readily dissolved by solutions of sodium salicylate, acetate or
benzoate. These double compounds under the name of diuretin, agurin and
uropherin have lately become of therapeutic value.[64]
Sodium silicate and more particularly trisodiumphosphate according
to Brissemoret[65] are great solvents of theobromine. One and a half
molecules of the latter salt can dissolve one molecule of theobromine
so that in this way it is possible to prepare a solution of nearly
2 percent. Phenol also dissolves a large quantity of theobromine,
according to Maupy,[66] who has utilised this property for the
determination of theobromine. The defatted cacao preparation is
moistened with water and extracted with a mixture consisting of 15
percent of phenol and 85 percent of chloroform.
Theobromine, like caffeine, gives the so called murexide reaction when
evaporated with chlorine water—forming amalic acid—and when a watch
glass previously moistened with a little fluid ammonia is held over the
last few drops at the end of the operation. The residue thus obtained
has a violet colour, which serves to distinguish theobromine readily
from other plant bases which do not belong to the xanthine group.
Although theobromine is the most valuable constituent of cacao beans,
the importance attached to a greater or lesser amount in the beans as a
commercial article was formerly much exaggerated.
The investigations of Dragendorff and others have shown that the
value of various stimulants like tobacco, coffee and tea, does not
entirely depend on the amount of alkaloid or diureide but partly also
on the joint action of all the constituents of those articles, and it
is particularly the aromatic bodies which determine their commercial
value. Various kinds of coffee, for example, of inferior commercial
value contain considerably more caffeine than the costly Mocca beans.
The highly prized Havana tobacco ranges lower than the Sumatra kinds in
nicotine content, and the same conclusion with regard to cacao would
probably be correct. In support of this view, attention may be directed
to the following analyses performed by Wolfram.[67]
Percentage of theobromine at 100° C.
================================+========================================
Description | % Theobromine %
================================+========+=========+=============+=======
Caracas | | 1·63 | | 1·11
Guayaquil (of considerably less | | | |
value than the first) | In the | 1·63 | In the | 0·97
Domingo | bean | 1·66 | shells | 0·56
Bahia | | 1·64 | | 0·71
Puerto Cabello (fine kind) | | 1·46 | | 0·81
Tabasco | | 1·34 | | 0·42
|————————+————————-+————————————-+———————
|Average | = 1·56% | | =0·76%
Excluding the theobromine in the shells which are not used in the
preparation of cacao, it will be seen from the above table that the
Caracas bean, which is the finest and dearest, has an amount of
theobromine which is only equal to, or even a little less, than that in
the inferior beans from Guayaquil and Domingo.
5. ~Albumin.~
On the presence of albuminous bodies in the cacao bean, varying between
14-15 percent, depends to a great extent its nutritive value. The
albumin in plants, unfortunately, is not to hand in a form suitable
for direct absorption and assimilation in the animal organism, in
fact, only a fraction of it is so available. Before considering the
nutritive value of the albumin of the cacao bean it will be well to
give attention to the general chemical and physical properties of
albumin so far as a knowledge of them will assist in the elucidation of
the subsequent matter.
Albuminous bodies or proteins occur either dissolved in the sap of
plants or in a solid in the protoplasm of plant cells; also in the
form of granular deposits (Aleuron granules[68]). In cacao they are
apparently present in the three different conditions.
The term vegetable albumen, in its more restricted sense, is meant
to designate a protein substance which is soluble in water and is
coagulable by heat. The greater part of the proteid which exists in the
seeds and sap of plants and is coagulable by heat, is not albumin but
globulin, that is to say, it is insoluble in water, though dissolved
by solutions of neutral salts. Whilst many protein substances in
aqueous solution require a temperature of 100 ° C. before coagulating,
or becoming insoluble under certain conditions, others coagulate at
65 ° C. Concentrated acetic acid dissolves all albuminous bodies with
the aid of heat, concentrated nitric acid gives a yellow coloration
(xantoprotein reaction). Albuminous substances are decomposed when
heated to 150 ° C. developing a dark colour, swelling up and evolving
an offensive smell, finally leaving behind a difficultly combustible
coaly residue.
Globulins combine with aqueous solutions of alkalis such as potash,
soda, ammonia etc. producing alkaline albuminates; with acids they
form acid albuminates or syntonins. Both have the property in common,
that whilst they are insoluble in pure water, they readily dissolve
in slightly acidulated or alkaline water, as well as in weak saline
solutions, and are then no longer coagulable by boiling.
Albuminous bodies are converted first into albumoses (proteoses), and
then into peptons by gastric and intestinal digestion or by hydrolytic
decomposition with acids or alkalis, also by the action of steam under
pressure of many atmospheres, as well as by putrefaction. Albumoses,
with the exception of hetero-albumose, are soluble in water. Peptons
dissolve entirely and in that condition are absorbed by the animal
organism.
Albumins are precipitated from their solutions by strong alcohol,
and in that way Zipperer succeeded in precipitating 4·25 percent of
albumin from the aqueous extract of Trinidad cacao, which corresponds
to about 25 percent of the total amount of albumen in the bean.
The results of his investigation have shown that generally more soluble
albumen is present in the unfermented than in the fermented bean.
Consequently, it would appear that in the finer kinds of cacao beans,
in which very careful fermentation has been carried out, the albumin,
owing to fermentative alteration, is rendered less soluble.
The constitution of albumin is still not sufficiently known, despite
the excellent experiments of E. Fischer on this subject; generally it
is regarded as having the formula:
C 52·31-54·33%
H 7·13- 7·73%
N 15·49-17·60%
S 0·76- 1·55%
O 20·55-22·98%
Accepting a mean formula corresponding to the above figures as
representation of the albumen (namely C_{72}H_{112}N_{18}SO_{22}),
it becomes possible to obtain a quantitative determination of this
constituent in the plants in which it is contained. There is, for
instance, 16 % of nitrogen here. Starting from such a standpoint,
and determining the percentage of Nitrogen contained in a plant, and
multiplying by 6·25 (i. e. 16 %), the amount of albumen is obtained.
For further particulars see paragraph 4. The albumen in cacao, as
previously mentioned, is in the form of globulin, that is, in a less
soluble form. In cacao preparations which are required for invalids,
especially those with affections of the stomach, it is important to
have the albumen in a more readily soluble condition. Various attempts
have been made with cacao preparations to obtain that result, and
later on, full illustrations and explanations will be given on this
subject. First of all, however, it is desirable to consider the
scientific methods employed to ascertain the relative digestibility or
indigestibility of albumen.
Professor Stutzer[69] of Bonn has been engaged in determining the
action of digestive ferments of the animal organism on alimentary
substances, and has worked out a method by which it is possible to
ascertain the proportion of albuminous substances which can be regarded
as digestible.
The method depends upon the fact that salivary, gastric and intestinal
digestion can be artificially imitated in the laboratory. But as the
salivary secretion only digests starch and is difficult to obtain, malt
diastase, which serves the same purpose, is used instead. On the other
hand albuminous material is only digested by juices of the stomach and
intestines as fresh obtained from the mucous membranes of the pig or
ox. If we suppose an average of 16 percent of total albumen in cocoa
powder, the following results would probably be given by Stutzer’s
method:
Of 16 % of total albumen there are on an average:
| corresponding to percentage
| of the total mass:
|
} 7·6% soluble in the stomach | 47·5% } 65%
} 2·8% soluble in the intestines | 17·5% }
Albumen: } 5·6% insoluble | 35·0%
} ——- | ———
} 16·0% | 100·0%
As shown by the experiments of Forster[70] however, artificial
digestion does not correctly represent the actual consumption of
nutriment in the human body. ~Forster’s~ experiments, in which
cacao powder was administered to healthy men, gave a much higher value,
in fact, 80 percent of the nitrogenous substance was digested, against
65 percent by Stutzer’s artificial method of digestion. The results
obtained by artificial digestion must therefore be increased in that
proportion.
6. ~Starch.~
Starch is one of the most important constituents of cacao, as on
the starch taken in conjunction with the fat and albumen depends
the nutritive value of the cacao bean. As previously stated, cacao
starch is one of the smallest kinds which occur in the vegetable
kingdom; consequently it can easily be distinguished from the starch
granules of other plants. Owing to their minuteness the concentric
rings showing the stratified structure of the starch granules can
only be distinguished with difficulty under the microscope. Cacao
starch consists usually of globular granules, generally separate, but
sometimes in aggregations of two or three. The appearance under the
microscope of the starch granules is clearly shown in fig 7, which
represents a section of Ariba cacao enlarged 750 times.[71]
[Illustration: Fig. 7.]
=a= on the above represents the intercellular spaces, =b= the cell
walls, =c= the starch granules, =d= the fat crystals, those being the
contents and structural elements of the cacao cell that the microscope
will at once distinguish.
Cacao starch has the usual properties of ordinary kinds of starch,
namely:
1. ~It is gelatinised by hot water~, that is to say, the water
penetrates between the layers of starch granules, separating them
and causing by its penetration a swelling up of the starch whereby
a transparent mass know as “starch paste” is produced. It has been
supposed that cacao starch is less easily gelatinised than the starch
of other plants. According to investigations of Soltsien’s[72], which
Zipperer unreservedly endorses, this is not the case, for under certain
essential conditions, cacao starch gelatinises just as readily as other
kinds of starch.
~The blue coloration of starch with iodine.~
This is said to take place more slowly with cacao than with other
starches, though we have always found that once the cacao starch
is gelatinised, a blue coloration appears immediately on adding a
sufficiently strong solution of iodine.
There are certainly other materials in the cacao bean, such as fat,
which by more or less enveloping the starch, prevent access of water
to the starch granules and thus hinder gelatinisation; or again, the
albumen and cacao-red may exert some retarding influence on the iodine
reaction, ~especially if the iodine solution used is very dilute~.
Yet it is impossible to describe the reaction as slow.
According to Soltsien, if a mixture of two parts of cacao bean with
one part of calcinated magnesia and water is heated, a clear-filtering
decoction is obtained, which immediately assumes the blue colour
on addition of iodine solution. On neutralising the filtrate with
acetic acid, and adding 3-4 parts of strong alcohol, its starch is
precipitated.
~By boiling with dilute acids as well as by the action of ferments
like the saliva, diastase~ etc., ~starch is converted into starch
sugar~ (~glucose~, ~dextrose~). The empirical formula for
starch is C_{6}H_{10}O_{5}, that for starch sugar is C_{6}H_{12}O_{6},
so that in the conversion one molecule of water is introduced,
wherefore its chemical nature is greatly changed, and especially in
its becoming freely soluble in water. That alteration allows of starch
being quantitatively determined, as the dextrose thus produced has the
property of reducing an alkaline solution of copper sulphate (known as
Fehling’s solution, after the discoverer); that is to say, the copper
sulphate is converted into insoluble red cuprous oxide. As dextrose
always precipitates a definite amount of cuprous oxide, the quantity of
starch present can in that way be determined.
The chemical determination of starch is only in a limited degree
effectual in the recognition of an admixture of foreign starch in cacao
preparations. If more than 10-15 percent of starch (calculated on the
crude bean) has been found, then it must be assumed that there has
been an admixture of foreign starch, but chemistry affords no means by
which foreign starch can be distinguished from the genuine starch of
the cacao bean. For that purpose the foreign starch must be minutely
observed under the microscope, which not only serves to detect its
presence, but gives an approximate estimation of the amount present,
and its origin. Great caution should be exercised, or the result may be
easily exaggerated.
7. ~Cellulose or crude fibre.~
We have already made the acquaintance of this material as the chief
constituent of the cell walls and vascular tissues. Recent chemical
investigations have shown that it consists of the anhydrides of hexose
and pentose (sugar compounds) incrustated with many impurities, such as
cacao-red, gum, mucilage etc. From a chemical point of view, cellulose
has the same formula as starch, viz. C_{6}H_{10}O_{5}, or one of its
multiples represented in formula. One of its chemical properties is
solubility in ammonio-cupric sulphate, and affinity for alkalis such
as potash, soda, ammonia, causes it to swell when they act on the cell
fibres.
Weender’s process[73] as worked out by Henneberg is the one usually
adopted for the determination of crude fibre in plants, although
recently H. Suringar, B. Tollens[74] and more particular König[75] have
pointed out that in Weender’s process the so-called pentosan, that is
to say, the sugar-like constituent of the composition C_{5}H_{10}O_{5},
which comprises a not inconsiderable portion of the crude fibre,
undergoes a disproportionate alteration, so that the analytical results
thus obtained can by no means give an accurate representation of the
amount of cellulose. The crude fibre must therefore be treated in such
manner as to eliminate the pentosan. For this purpose the various
methods of König, Matthes and Streitberger have been proposed, to which
we shall return in Book 4. Filsinger, the meritorious experimenter
on the subject of cacao, has by König’s method determined the amount
of crude fibre in a series of different varieties of cacao bean, and
obtained the following results as regards shelled and roasted beans.
percent
1. Puerto Cabello 5·37
2. Java 3·97
3. Ariba Guayaquil I 4·10
4. Ariba Guayaquil II 4·07
5. Machala Guayaquil I 4·43
6. Para 4·01
7. Surinam Guiana 3·01
8. Bahia 2·81
9. Grenada 3·10
10. Guatemala 3·50
11. Machala Guayaquil II 3·58
12. Caracas 3·65
13. Samana 4·58
14. St. Thomé A I 4·13
15. St. Thomé A II 2·95
16. St. Thomé B 3·15
17. Haiti 3·12[76]