Experimental cookery

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 ANTHOCYANINS 119 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 re- ported the cause of violet coloration of some fruits canned in tin con- tainers. 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 ad- dition 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 /)H 2.4 and green at />H 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 />H 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 />H may depend upon the time of exposure at that p\i. The cabbage itself, when cooked in water or juice pressed from the cabbage, shows changes at varying reac- tions, 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 /)H. Usually at a />H 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 />H, the blue developing at /)H 6 or above. The green color develops with greater alkalinity at about /)H 7 to 9. With still higher concentrations of alkali a yellow or brown color develops.