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.