All the anthocyanins so far isolated have fallen into one of three groups. They are illustrated as the chlorides. The number of hydroxy groups attached to the side benzene ring is made the basis of the classification. The glucosides of these anthocyanidins are called pelargonins, cyanins, and delphinins.

One of the reactions that the anthocyanins have in common is their color changes. Onslow states that the pigment should be pure to test the color reactions, for in the plant the pigment is found with other substances that may modify the reaction. In an acid solution, pure anthocyans are usually red; in an alkaline one they are violet or blue, but if flavones or flavonols are present, a green color is obtained through the mixture of the blue and yellow. Yet the solutions of many fruit juices show typical color changes. In ordinary solutions of plant pigments they become green, then yellow, and sometimes brown upon the addition of alkali. If the alkali is very weak, or with salts with a weak alkaline reaction, a blue color may form and the green may never develop, or the blue may be intermediate between the red and the green.

Color changes of fruit juices. Pratt and Swartout state that the solutions of many fruit pigments act as indicators; that the solutions are easily prepared and stable; that the liquid indicators can be used in titrating acids, but not bases, for in a solution no more than moderately alkaline they soon decompose, all of them producing a brown color which does not change when acid is added. They found that apricots, peaches, pears, persimmons, and tomatoes failed to yield pigments that could be used as indicators. It is interesting that the pigment of cactus holds its red color even in a distinctly alkaline medium. They also state that "the pigment of red beets remained red through the acid range and into the alkaline range at least as far as pH 13.0." In their conclusion they recommend that the greatest usefulness of the indicators is in test papers.

pelargonidin chloride

Pelargonidin chloride

cyaninidin chloride

Cyaninidin chloride

delphinidin chloride

Delphinidin chloride

Fruit colors and punch. Combinations of fruit juices for punch can yield beautiful, clear colors, or ugly, muddy ones. If a red color is desired, use red- or blue-colored juices and keep the reaction acid by the addition of lemon juice. For a purple shade, choose fruit juices nearly neutral in reaction and do not add lemon juice. A blue color can usually be intensified by the addition of canned pineapple juice. (See the following paragraph.) Alkaline water may or may not give a bluish tinge to red fruit juices, depending on the alkalinity of the water and the acidity of the juice. Orange juice should be added to red or blue fruit juices only when a brownish or magenta shade is desired, for often this combination is not attractive. From the colors produced, or unless very small proportions of one color are used, the red and yellow, blue and yellow, or green and yellow combinations should be avoided.

Table 16 The pH Range with Color Changes of Fruit Juices (Pratt and Swartout)

Fruit source

Color change

pH range

Apples

Red to yellowish-green

6.2- 7.2

Blackberries

Red to dark grayish-blue

6.0- 7.4

Blueberries

Reddish-purple to greenish-purple

6.2- 7.2

Cactus

Red to faint purple

9.0-12.0

Cactus

Faint purple to reddish-brown

12.0-13.0

Cherries

Red to bluish-purple

6.0- 7.2

Grapes

Red to purple

5.0- 6.6

Grapes

Purple to gree0n

6.6- 7.6

Plums

Red to yellowish-green

6.2- 7.2

Pomegranates

Red to purple

6.0- 6.8

Pomegranates

Purple to green

6.8- 7.6

Strawberries

Red to yellowish-green

6.2- 7.2

The Color of Fruit Juices in Neutral, Acid, and Alkaline Mediums

(Pratt and Swartout)

Fruit source

Neutral tint

Acid tint

Alkaline tint

Apples

Grayish-purple

Red

Green

Blackberries

Purple

Red

Bluish-green

Blueberries

Purple

Red

Blue

Cherries

Reddish-purple

Red

Bluish-green

Cranberries

Faint purple

Red

Light green

Grapes

Purple

Red

Bluish-green

Plums

Faint purple

Red

Light green

Pomegranates

Purple

Red

Bluish-green

Strawberries

Reddish-purple

Red

Light green

The color change obtained by the addition of canned pineapple juice to grape, wild grape, blackberry, raspberry, or loganberry juices cannot be explained on the basis of acidity alone. Even if these juices have had a large quantity of lemon juice added they usually turn blue or the original blue shade is intensified and particularly after the juices have been mixed and left standing a short time. Tin salts from the canned pineapple may be one cause for the color change. In addition, proteins, tannins, and ferric salts may play a role in causing color changes. Also many of the salts of the anthocyanins have characteristic colors which are independent of mild changes in acidity. Many organic substances also have characteristic color changes.

Violet colorations in canned fruits. Culpepper and Caldwell have reported the cause of violet coloration of some fruits canned in tin containers. The red anthocyan pigments have the property of combining with tin, forming salts that are violet colored. The salts are formed when the material containing the pigment is heated with tin. They find that "the amount of the violet compound formed is determined by the amount of pigment present, and by the degree of acidity of the medium, low acidity favoring its formation, high acidity depressing or suppressing it." The addition of an alkali intensifies the violet color; the addition of acid restores the original red color. The violet color is deepened by standing in the air after opening the can.

Color changes of red vegetables. Clark gives in his list of indicators that red cabbage extract is red at pH 2.4 and green at pH 4.5. However, the results in this laboratory have not agreed with those of Clark. In general there is lack of agreement in reports of pH at which the pigment of red cabbage turns blue. There are probably two reasons for this. The antho-cyanins combine with metals to form salts, the particular metal influencing the color reactions. Also the color developed at a given pH may depend upon the time of exposure at that pH. The cabbage itself, when cooked in water or juice pressed from the cabbage, shows changes at varying reactions, which have varied slightly at different times. This is probably due to other constituents and their concentration in addition to the red cabbage coloring. Temperature and time of standing before the determinations were made also affect the pH.

Usually at a pH of 2.4 to about 4.0 the color is red, showing gradual changes through blue-red, purple or violet, red-blue, and finally blue. These changes occur over a rather wide range of pH, the blue developing at pH 6 or above. The green color develops with greater alkalinity at about pH 7 to 9. With still higher concentrations of alkali a yellow or brown color develops.