CHAPTER VII

TANNINS

Using the term in its general application to a group of substances having similar chemical and physical properties, rather than in its limited application to a single definite chemical compound known commercially as "tannin," the _tannins_ are a special group of plant substances, mostly glucosides, which have the following characteristic properties. First, they are non-crystalline[4] substances, which form colloidal solutions with water, which have an acid reaction and a sharp astringent taste. Second, they form insoluble compounds with gelatine-containing tissues, as shown by the conversion of hide into leather. Third, they form soluble, dark-blue or greenish-black compounds with ferric salts, the common inks. Fourth, they are precipitated from their solutions by many metallic salts, such as lead acetate, stannous chloride, potassium bichromate, etc. Fifth, they precipitate out of solution albumins, alkaloids, and basic organic coloring matters. Finally, most tannins, in alkaline solutions, absorb oxygen from the air and become dark brown or black in color.

FOOTNOTES:

[4] The needle-like forms, in which commercial "tannin" comes on the market, are not true crystals, but are broken fragments of the threads into which the colloidal tannin is "spun-out" from the syrupy extracts of nutgalls, etc.

OCCURRENCE

Tannins occur widely distributed in plants. Practically every group of plants, from the fungi up to the flowering plants, contains many species of plants which show tannin in some of their tissues. Among the higher plants, tannins occur in a great variety of organs. Thus, they are found in the roots of several species of tropical plants; in the sterns, both bark and wood, of oaks, pines, hemlock, etc.; in the leaves of sumac, rhododendron, etc.; in many fruits, especially in the green, or immature, stages; and in the seeds of several species, either before or after germination. Tannins are also found in certain special structures, such as gland cells, cells of the pulvini, laticiferous tissues, etc. Further, they are especially abundant in the pathological growths known as galls, which often contain from 40 to 75 per cent of tannin and constitute the most important commercial source for these materials.

The principal commercial sources of tannin, which is used in the manufacture of inks, in the tanning of leather, in certain dyeing operations, etc., are oak-galls, the bark and wood of oak, hemlock, acacia, and eucalyptus, the bark of the mangrove, the roots of canaigre, and the leaves of several species of sumac.

CHEMICAL CONSTITUTION

Tannins are either free phenol-acids or, more commonly, glucosides of these acids. Common "tannin," when hydrolyzed, yields from 7 to 8 per cent of glucose, which indicates that it is a penta-acid ester of glucose, i.e., each glucose molecule has five acid groups attached to it. The formula for such a tannin is, therefore, as follows,

------O------- | | CH_{2}OR·CHOR·CH·CHOR·CHOR·CHOR

in which the R represents a complex phenol-acid like tannic acid, or digallic acid. These acids are derivatives of the common phenols, whose constitution will be brought to mind by the following series of formulas:

Ordinary phenol Pyrocatechol Resorcinol Hydroquinone /\ /\ /\ /\ / \ / \ / \ / \ | |OH | |OH | |OH | |OH | | | |OH | | HO| | \ / \ / \ / \ / \/ \/ \/ \/ OH

Pyrogallol Oxyhydroquinone Phloroglucinol OH OH OH /\ /\ /\ / \ / \ / \ | |OH | |OH | | | |OH | | HO| |OH \ / \ / \ / \/ \/ \/ OH

These phenols themselves do not occur as constituents of tannins, although they are often found in other glucosides, gums, etc. The following mono-carboxyl acid derivatives of these phenols are, however, found both free and in glucoside formation as constituents of many of the common tannins.

_Pyrocatechuic acid_, derived from pyrocatechol, represented by the formula,

OH /\ / \ | |OH | | \ / \/ COOH

_Gallic acid_, derived from pyrogallol, and represented by the formula,

OH /\ / \ HO| | HO| |COOH \ / \/ COOH

In most of the common tannins, however, the characteristic acids are oxy-derivatives of the so-called "tannon" group, represented by the formula, C_{6}H_{5}·CO·O·C_{6}H_{5}. For example, _digallic acid_, which is a constituent of many common tannins, is a tetra-oxy, mono-carboxyl derivative of this group, having the structural formula,

/\ /\ / \ / \ HO| |--CO·O--| |COOH HO| | HO| | \ / \ / \/ \/ OH OH

_Ellagic acid_, which is an hydrolysis product of many of the pyrogallol tannins (see below) and which produces the characteristic "bloom" on leather tanned by this type of tannins, has the following formula,

-----CO·O----- /\ /\ / \__________/ \ HO| | | |OH HO| |___CO·O___| |OH \ / \ / \/ \/

CLASSES OF TANNINS

The tannins are divided into two general classes, known respectively as the _pyrogallol tannins_ and the _catechol tannins_. These differ in their characteristic reactions as follows:

Pyrogallol variety Catechol variety Ferric salts Dark blue Greenish black Bromine water No precipitate Yellow or brown precipitate Leather Produce a "bloom" No "bloom" Conc. sulfuric acid Yellow or brown Red or pink Lime water Gray or blue ppte. Pink to brown ppte.

Pyrogallol tannins contain approximately 52 per cent of carbon; while the catechol tannins usually contain 59 per cent to 60 per cent, the difference being due to the absence of glucose from the molecule in the latter types.

The two types are distributed in plants as follows: pyrogallol tannins in oak-galls, oak wood, sumac, chestnut, divi-divi, and algaro billa; catechol tannins in the barks of pines, hemlocks, oaks, acacias, mimosas, cassia, and mangrove, in quebracho wood, canaigre roots, cutch and gambier. The so-called "pseudo-tannins" (i.e., compounds which do not tan leather but possess other properties like tannins) are found in hops, tea, wine, fruits, etc.

SOME COMMON TANNINS

Ordinary commercial "_tannin_," or "_tannic acid_," is a compound of one molecule of glucose with five of digallic acid. It is found in many plants, and is prepared commercially from the Turkish oak-galls and the Chinese sumac-galls. It exhibits all the characteristic properties which have been listed above for tannins in general and responds to all the characteristic reactions of a pyrogallol tannin. It is extensively used for the manufacture of blue-black ink, and in many technical processes.

=Catechu tannin= and =catechin= are compounds of the catechol tannin type. The latter is obtained from acacia wood, mahogany wood, mimosa wood, etc. It is not a true tannin, since it does not convert hide into leather; but when heated to 120° or above, it is easily dehydrated, forming catechu tannin which is identical with that which is obtained directly from gambier and Bombay cutch (products made by evaporating water extracts from the bark of various tropical trees). This latter is a true tannin, which is much used in dyeing and other technical processes.

"=Quercitannic acid=," obtained from oak bark, etc., is likewise a catechol tannin. It yields no glucose on hydrolysis.

A great many other tannins are known, and their possibilities for technical use in tanning, dyeing, etc., have generally been investigated; but so little has been learned about their composition and relation to the plant's own needs, that it seems unnecessary to discuss them in detail here.

PHYSIOLOGICAL USES OF TANNINS

Tannins are probably not direct products of photosynthesis. They are, however, elaborated in the green leaves of plants and translocated from there to the stems, roots, etc. Their close association with the photosynthetic carbohydrates has led many investigators to seek to establish for them some significant function as food materials, or as plastic substances in cell metabolism. Many conflicting views have been advanced, but a careful review of these leads inevitably to the conclusion that tannins probably do not serve in any significant way as food material. The glucose which is generally present in the tannin molecule may, of course, serve as reserve food material, but it seems probable that it functions as a constituent of the tannins only to assist in making them more soluble and hence more easily translocated through the plant tissues.

Some fungi, and perhaps other plants as well, can actually utilize tannins as food material under suitable conditions and in the absence of a proper supply of carbohydrates. But this does not prove that tannins can normally replace carbohydrates as food material for these species of plants.

There seems to be ample evidence that tannins are elaborated where intense metabolism is in progress, such as occurs in green leaves during the early growing season; in the rapid tissue formation which takes place after the stings of certain insects, producing galls, etc.; during germination, and as a result of any other unusual stimulation of metabolism. It may be, therefore, that tannins serve as safety accumulations of excessive condensations of formaldehyde, or other photosynthetic products, under such conditions. It seems certain that in all such cases tannins are the result of, and not (as some investigators have supposed) the causative agents for, the abnormally rapid metabolism.

It seems to be fairly well demonstrated that tannins are intermediate products for the formation of cork tissue. This may account for their common occurrence in the wood and bark of trees. Indeed, it has been shown that gallic and tannic acids are present in considerable proportions in those parts of the plant where cork is being formed. Further, that they bear direct relation to cork-formation has been demonstrated in two different ways. First, cork-like substances have been artificially produced by passing a stream of carbon dioxide through mixtures of formaldehyde with various tannic acids. Second, by various treatments of cork, decomposition compounds showing tannin-like properties may be obtained.

Some investigators have held that not only cork tissue but also other lignose, or cell-wall material, may be developed from tannins. Certain observations with _Spirogyra_ seem to indicate that tannin may play an important part in the formation of new cell walls during conjugation, as cells which are ready to conjugate are rich in tannin, which gradually diminishes in quantity until it is practically absent at the time of spore-formation. There seems to be no evidence that tannins perform any such function as this in higher plants, however.

Again, tannins may play a very important part in pigment-formation. They are very similar in structure to the anthocyanin pigments, both being made up of practically identical decomposition units, the phenolic bodies. The disappearance of tannins during the process of ripening of fruits may be connected, in part at least, with the development of the brilliant red, blue, and yellow pigments which give such rich colors to the thoroughly ripe fruits.

Finally, certain of the tannins undoubtedly serve as protective agents to prevent the growth of parasitic fungi in fruits, etc. Recent investigations show that at least some of the varieties of fruits which are resistant to the attacks of certain parasitic diseases utilize tannins for this purpose. This protective effect may be accomplished in two different ways. Either the tannin actually serves as an antiseptic to prevent the growth of the parasitic fungus within the tissues of the host plant, or it assists in the development of a corky layer which "walls-off" the infected area and so prevents further spread of the disease through the tissue. Examples of both types of protective action have recently been reported.

It is obvious that the different forms of tannins may play different rôles in plant life, and the same tannin substance may possibly serve different purposes under different conditions.

BIOLOGICAL SIGNIFICANCE OF TANNINS IN FRUITS

The presence of tannins in fruits and the changes which they undergo during the ripening process cannot fail to attract attention to their biological significance in serving to protect the fruit from premature consumption as food by animals.

Tannins are of frequent occurrence in green fruits, imparting to them their characteristic astringent taste. They nearly always disappear as the fruit ripens. The fact that during the ripening process both sugars and fruit esters, as well as attractive surface pigments, are developed has led certain investigators to the conclusion that tannins serve as mother-substances for these materials in the green fruits and are converted into these attractive agencies during ripening. There is nothing in the chemical composition of tannins which indicates, however, that they are precursors of sugars or fruit esters, although (as has been pointed out) they may give rise to anthocyan pigments.

Further, recent researches concerning the tannin of persimmons (the best-known and most striking example of the phenomena under discussion) clearly show that the tannin is not actually used up during the ripening process; that instead it remains in the ripe fruit in practically undiminished quantity; but that when the fruit is ripe, the tannin is enclosed in certain special large cells or sacs, which are surrounded by an insoluble membrane, so that when the fruit is eaten by animals the astringent tannin, enveloped in these insoluble sacs, passes by the organs of taste of the animal without causing any disagreeable effects. This walling-off of the astringent tannins can be stimulated in partially ripe fruits by treating them with several different chemical agents, the simplest method being that of placing the unripe fruit in an atmosphere of carbon dioxide gas for a short period. The artificial "processing" of persimmons to render them edible for a longer period before they become naturally fully ripe and subject to decay is now a commercial enterprise. This process is of interest because of its possible connection with the conversion of tannins into cork, under the influence of carbon dioxide gas, as mentioned in a preceding paragraph.

From these facts, it is apparent that in persimmons, and probably in other tannin-containing fruits, the process of natural selection has developed a mechanism for the secretion of tannin in green fruits, followed by a process for walling it off in harmless condition when the fruit is ripe, which serves most admirably to protect the fruit from consumption by animals before the enclosed seeds have fully developed their reproductive powers.

REFERENCES.

ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 7, Gerbstoffe, Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze, Kautschuk," 822 pages, Berlin, 1912.

ALLEN'S Commercial Organic Analysis, Vol. 5, "Tannins, Dyes and Coloring Matters, Inks," 704 pages, 6 figs., Philadelphia, 1911 (4th ed.).

COOK, M. T. and TAUBENHAUS, J. J.--"The Toxicity of Tannin," Delaware College Agricultural Experiment Station _Bulletin_ No. 91, 77 pages, 43 figs., Newark, Del., 1911.

DEKKER, J.--"Die Gerbstoffe," 636 pages, 3 figs., Berlin, 1913.

GORE, H. C.--"Experiments on the Processing of Persimmons to Render them Nonastringent," U. S. Department of Agriculture, Bureau of Chemistry _Bulletin_ No. 141, 31 pages, 3 plates, 1911; and No. 155, 20 pages, 1912.

LLOYD, F. E.--"The Tannin-Colloid Complexes in the Fruit of the Persimmon, _Diospyros_," in _Biochemical Bulletin_, Vol. 1, No. 1, pages 7 to 41, 34 figs., New York, 1911.