This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Heat decomposes chlorophyll, an olive-green color being produced. The extent of decomposition depends upon the time of heating and the temperature reached. With a very short cooking period little destruction may occur, but with a longer time all the chlorophyll may be decomposed. At lower temperatures the change is less rapid and at higher temperatures more rapid. Thus in a pressure cooker the change is rapid, since the temperature is high and acidity is not decreased because the volatile acids are retained. Increasing the alkalinity by using alkaline water or adding soda retards the color change by heat. However, the addition of soda is not advisable, because it softens the cellulose rapidly so that the vegetable with slight over-cooking becomes mushy or even slimy and the destruction of some of the vitamins is hastened.
The cooking of green vegetables that contain enough acid to taste sour, like sorrel, sour grass, and dock, always produces this olive-green color no matter how cooked or for how short a time. Their acid content is too high to be neutralized by the alkaline salts of water or the addition of baking soda in small amounts. Green fruits like gooseberries, green grapes, and green plums develop an olive-green color when cooked. If the fruits are mature they may also contain yellow pigments that modify the color to a certain extent.
If acid is to be added as a seasoning to green vegetables it is preferable to add it when served, for there will not be so long a time for the acid to react on the chlorophyll and less brown color will develop. When vegetables are cooked with added acid, the softening of the vegetable is prevented to a slight extent.
Replacing magnesium in chlorophyll products. It is very difficult to replace the magnesium of the magnesium-free chlorophyll products. It can be done chemically by using a very reactive substance, magnesium methyl iodide, but this is impracticable to use in cooking processes. However, Will-statter states that the acetate salts of some metals such as copper, iron, and zinc will combine with phaeophytin and give very bright green-colored products. This is such a good test for small quantities of these metals that it is necessary in isolating chlorophyll to be extremely careful that it does not come in contact with them. If to the green vegetable to which acid has been added and in which the olive-green color has developed, Experiment 17A, 6, copper acetate is added, a vivid green color will develop on standing.
Carotinoids. Carotinoids is a term applied to the pigments giving yellow and orange coloring to fruits and flowers, carotene, C40H56, and xanthophyll, C40H56O2. The carotinoids are not soluble in water, and the pigments are not affected, to any great extent, by the concentration of acids or alkalies used in food preparation. The darkening produced by the action of alkalies by caramelization of the sugar in vegetables, like carrots, must not be confused with a change in pigment color. Carotene is easily oxidized when exposed to air. Xanthophyll can also be oxidized in the same way. One of the characteristics of the carotinoids is the intensity of their coloring. The red pigment found in tomatoes is lycopin, an isomer of carotene.
Flavones and Flavonols. Flavones and flavonols are amphoteric pigments found in vegetables and petals of flowers. They are also found in the cell sap of the epidermis and underlying tissue of plants. Onslow gives the following formulas:
As they occur in nature they often have other or additional hydrogen atoms replaced by hydroxyl groups. The position of the hydroxyl groups markedly influences the intensity of color, the color usually being deeper if two hydroxyl groups are in the ortho position to each other. The flavone and flavonol pigments are yellow in color. In plants they often occur as glucosides, one or more of the hydroxyl groups being combined with the sugar, and then the color is less intense. They give an intense yellow color with alkalies. Sometimes the quantity of them found in white vegetables is so minute that they are not visible. Adding an alkali or holding them over ammonia vapor intensifies the color and they become visible. The color changes of the flavones cannot be observed so readily in the yellow, red, or green vegetables.
Rice cooked in alkaline water usually has a yellow tinge, and sometimes it is rather deep in hue or has a green tint. Rice from the same source as that cooked in alkaline water, but cooked in distilled water, has a snowy white appearance.
The cooking of cauliflower, white cabbage, and particularly white onions in alkaline water often causes the yellow color to develop. Some white onions cooked in the hard water at Ames have developed nearly a sulfur yellow in color, whereas portions of the same onions cooked in distilled water remained white, or retained their natural yellow tinge.
Potatoes must often contain flavones, as they sometimes develop a yellow or green color in alkaline water. Mashed potatoes that have a strong alkaline taste may be improved in flavor by adding a very small amount of cream of tartar.
Flavones and flavonols with iron salts turn green and then brown. This may explain some color changes that occur when foods are cooked in chipped enamel utensils having an iron base.
Anthocyanins. The anthocyans include a group of pigments that, like sugars, have similar composition and properties yet have individual differences. As a rule they occur as glucosides in different parts of the plant and are known as anthocyanins in this combination. When the glucoside is hydrolyzed by boiling with dilute acid the non-glucosidal pigment portion is called anthocyanidin. The anthocyanins are soluble in water. Onslow states that occasionally they crystallize from the cell sap. Most but not all of the anthocyanins are soluble in alcohol. The anthocyan pigments give the blue, purple, violet, and red shades to different parts of the plant. They are very widely distributed. Onslow states that the plant that does not produce them is the exception rather than the rule. They occur in apples, cherries, currants, grapes, blueberries, red and black raspberries, in some varieties of peaches and plums, and in red cabbage, radishes, beets, and other fruits and vegetables.