This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
2. They give blackish-blue or blackish-green colors with ferric salts, a fact made use of in the manufacture of ink.
3. In alkaline solution the tannins and many of their derivatives readily absorb oxygen becoming dark in color.
4. They precipitate gelatin from solution and form insoluble compounds with gelatin-yielding tissues, a property which enables them to convert hide into leather.
5. They are precipitated from solution by many metallic salts such as copper or lead acetates, or stannous chloride.
Astringent qualities due to tannins. The astringent properties give a slightly bitter taste to some foods, the degree depending on the amount of tannins present. Thatcher states that "Tannins are of frequent occurrence in green fruits imparting to them their characteristic taste. They nearly always disappear as the fruit ripens." Two explanations are suggested for the disappearance of the tannins in ripening fruit. The anthocyan pigments may be derived from the tannins, so that as the fruit colors the tannins disappear; or the tannins may be changed into an insoluble form and therefore are not so apparent to the taste.
Persimmons contain a very large amount of tannin. Green persimmons have a bitter, astringent, puckery taste, that is not easily forgotten, once they have been tasted. Gore's work has shown that most of the tannin of persimmons is enclosed in cells, which he terms giant tannin cells. By artificial processing in carbon dioxide, or by ripening, the membrane containing the tannins becomes hard and insoluble so that the astringent taste is not evident.
Blue or purple discoloration in English walnuts. English walnuts are more astringent than other nuts and often develop color changes that are unattractive when combined with other foods. When they are combined with apples in a salad they often develop a blackish blue or purple color. This may come from having prepared the apples with an iron knife, the acid and enzyme of the fruit acting on the iron to produce ferric salts. When this small amount of ferric salt comes in contact with the tannin of the nut and particularly with the skin the purplish color develops. In nut bread, English walnuts often produce a dark color.
Tannins may produce a gray shade in sugar. Zerban has found that the color of dark greenish cane juice, which produces a sugar with a gray shade, may be due to tannins, oxidizing enzymes, and iron. When the cane is crushed the acid of the juice forms some ferrous salts with the iron of the roller. Oxidizing enzymes of the juice oxidize the ferrous salts to ferric ones and they combine with the tannins of the juice. A similar change occurs when fruits are pared with iron knives and especially when the juice is left to stand on the knife and it darkens.
Tannins and discolorations in canned goods. Kohman has reported that the discoloration, a darkening in color, often found in spots in canned sweet potatoes is due to tannins. The tannin combines with the iron of the can to produce the dark color. Oxygen is necessary to oxidize the ferrous iron to ferric, so that, unless oxygen is found in the can, either through not exhausting all of the air in processing or to a leak in the can, the discoloration does not occur.
Salsify probably contains large amounts of tannic substances. It turns dark when peeled even if put under water unless a little vinegar is added to the water to acidify it. Salsify or oyster plant and sometimes other vegetables such as carrots stain the fingers when they are pared. This may be due in part to tannins. Lemon juice is effective in removing or lightening such stains because of its acidity.
Other discolorations with tannins. Brownish or black discolorations of vegetables in vinegar have been caused by the tannin of the vinegar combining with the iron of the food. The use of a tannin-free vinegar does away with this difficulty. Greenish black spots in chocolate ice cream were traced to the tannin supplied by the chocolate or cocoa and the iron from rusty spots in the can. In green-colored beer the iron came from exposed pipes, the tannin from the hops.
According to Atwater, cherry juice, now commercially marketed, when mixed with gelatin for a molded dessert, sometimes gave a gummy, purplish precipitate. The precipitate was caused from combination of the tannin of the juice with the protein, gelatin; the purplish color came from combination with metals.
Discoloration of pared fruit, considered at greater length in the next section, is caused by tannin compounds in combination with enzyme action.
Tea. Black tea is darker than green tea because of oxidation of tannins in the leaf during drying and fermentation. It is also less astringent than green tea because the tannins are in a less soluble form.
Tannins also give precipitates of calcium, magnesium, and iron tannates, when these minerals are present in the water. The film on coffee or tea is from these tannates. When lemon or orange is added to tea, the film usually disappears, for in acid the tannins often lose the darker color acquired in an alkaline medium.
Tannins lower the surface tension of water, and the water in which vegetables containing tannins are cooked is apt to froth or foam.