This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
If red cabbage is served raw with a salad dressing containing acid it is bright red or blue-red in color. If cooked in distilled water, it is violet or violet-blue in color, but often becomes blue after standing a few minutes. With a slightly alkaline water the color is blue, for the plant acids lower the pH of the water. If the water is distinctly alkaline the color becomes green if cooked about 15 minutes or longer. The addition of a little soda to distilled water, Experiment 17B, 5, gives a green color unless the cooking period is exceptionally short. The red vegetables tend to retain their color better when cooked in milk than when cooked in water. Red onions show the same color changes as red cabbage, but the colors are often muddy. Red cabbage shows color changes in handling it. When the cut edges of the cabbage come in contact with the hands or knife they turn blue. Hands and knives washed in hard water or with soap may have salts with an alkaline reaction on their surface, but the anthocyanins can also form compounds with metallic ions.
Beets do not develop the blue or green color, although the color often contains a considerable amount of blue or purple. Sometimes they turn from red to yellow when cooked. If the cooked beets are placed in acid, the red color is often restored after a short time. Blair states that beets contain two pigments, one being scarlet the other purple. The scarlet pigment is stable to heat, but the purple one fades. The scarlet pigment is stable even in the alkaline range as far as pH 9 or, according to Pratt and Swartout, even at pH 13.0. The color of canned or cooked beets depends upon the proportion of the two pigments present in the beets. This is probably an explanation of why cooked beets vary so much in color, even when cooked under the same conditions of acidity, temperature, and time. Beets have been produced in which the scarlet pigment has been increased and which do not lose color when canned at high temperatures.
Metals and anthocyan pigments. The tin salts of the anthocyanins have been mentioned, but the iron salts are even better known. In general, the iron salts of anthocyanins are blue. It is known that juice from blue hydrangeas contains a higher percentage of iron than that from pink hydrangeas even at the same pH. Some anthocyanins may not combine with iron or other modifications may occur which prevent or are necessary for the development of the blue color, for if some anthocyanins are treated with iron salts the color fades or remains red. Other metals which may combine with the anthocyanins are aluminum, zinc, and lead. Aluminum produces about the same colors as iron, although less intense. It seems reasonable to expect that, if certain salts are present in the vegetable in high concentration or are furnished by the cooking water or the cooking utensil, they will modify colors obtained at a given pH in cooking. Also, the blue color that develops on the cut surface of red cabbage in a short time after cutting with a metal knife may possibly be due to formation of anthocyanin salts with the metal.
Lathrop states that both iron and tin are "injurious to fruit pigments. The compounds of the pigments with the metals become quickly oxidized on exposure to air, with a marked increase in intensity of discoloration. Tin turns grapes, cherries, raspberries, and blackberries a deep purple and strawberries a pale red. Iron produces dull brownish discolorations. Copper and aluminum are far less injurious and are therefore used wherever fruit must come in contact with metal. Copper being somewhat injurious to the color of grape, aluminum is usually used with grape."
The application of this knowledge comes in cooking fruits, preserves, and jellies. Since tin cooking utensils are seldom used in household cooking, this metal would be injurious to color of fruits when used in taking seeds from berries with tin colanders, or when such fruits are canned in un-lacquered tin. Iron might affect the color of fruits, if such fruits are cooked in utensils from which the enamel has been chipped.
The term tannin is sometimes used to denote a whole group of substances having certain characteristics in common. Sometimes it is used to denote a particular substance, i.e., gallotanic or digallic acid. The former is the meaning used in the following paragraphs.
The tannins are widely distributed in the higher plants. Some plants are very rich in them; others contain very little. The amount in the plant will vary with different years and growing conditions. The tannins are found throughout the plant, but the woody part, the stems, and rootstocks are likely to contain larger amounts. They are found in many fruits, especially during the immature or green stages, and in the seeds of several plants. Lathrop states that stemming of grapes prior to heating for juice extraction is advisable to get rid of astringent tannin of the stems which would be detrimental to flavor.
To some extent the tannin content of fruit is not only dependent upon environmental influences, but upon inherited characteristics. The New York Experiment Station has reported a variety of peach called Sunbeam, which does not turn brown when pared. Kertesz reports its tannin content as very low, 0.0076 per cent.
Reactions of the tannins. Thatcher states that chemically the "Tannins are either free phenol-acids, or, more commonly, glucosides of these acids." The structure of the tannins is very similar to that of the anthocyan pigments. They are divided into two general classes, known as the pyro-gallol tannins and the catechol tannins. Thatcher has reported the following characteristic reactions.
Yellow or brown precipitate
Concentrated sulfuric acid.....
Yellow or brown
Red or Pink
Gray or blue precipitate
Pink or brown precipitate
Haas and Hill summarize the properties of the tannins under eight headings. Only five will be given here.
1. Tannins are mostly uncrystallizable colloidal substances with astringent properties.