This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Chlorophyll. Chlorophyll, the green pigment of plants, plays an important role in their synthesis of carbohydrates. The cells of the mesophyll of the leaf contain chloroplasts or chlorophyll-corpuscles, the nucleus, other substances, and the cell liquid with its dissolved materials. The chloro-plasts contain four pigments, two green ones, chlorophyll a and chlorophyll b, and two yellow ones, carotene and xanthophyll.
Solubility of chlorophyll. Chlorophyll is not soluble in water. Very little green color is found in the water in which green vegetables have been cooked. Pure isolated chlorophyll is soluble in acetone, ether and benzene. In extracting the pigment from thoroughly dry leaves it is necessary to add about 20 per cent of water to the acetone or other solvent. One explanation for this is that chlorophyll is in the colloidal state in the leaf, and the mineral constituents of the leaf, dissolved in the water, peptize it, rendering it soluble. Onslow states that "the condition of chlorophyll is altered by plunging into boiling water. The pigment is then much more soluble, in ether, etc., even when the leaves are subsequently dried. It is supposed that the chlorophyll has diffused out from the plastids, and is in true solution in the accompanying waxy substances which have become liquid owing to change in temperature."
When green vegetables are dropped into boiling water a change takes place nearly instantly, the green color being intensified. Various explanations have been offered for the phenomenon. One is that the hot water has melted waxy constituents of the leaf so the chlorophyll escapes from the cell more readily or may become more soluble. Or the hot water may have dissolved salts or other substances in contact with the chlorophyll so that it diffuses more readily.
For peas, Kohman states that one factor in the intensification of the green color is the removal of air from the pea when it is dropped in the boiling water. The outer skin of the pea is transparent, the space beneath this being impregnated with air which is removed when the peas are blanched. That this change in color is caused by removal of the air can be shown by subjecting the peas to an adequate vacuum under cold water and releasing the vacuum while the peas are still under the water.
Composition of chlorophyll. Willstatter, whose work gave us the formula for and the chemical reactions of chlorophyll, reports that it exists in two forms, depending upon the degree of oxidation in the plant cells: form (a) and form (b). The former exists in the proportion of three to one of the latter.
Chlorophyll contains 2.7 per cent of the metal magnesium. It contains two ester groups, one of methyl alcohol (COOCH3) and one of phytol alcohol (COOC20H39).
Reactions of chlorophyll with alkalies. Chlorophyll is a neutral substance but gives characteristic reactions when treated with alkalies or acids. Will-statter designates the parent substance of chlorophyll as chlorophyllin. The reaction of chlorophyllin with methyl and phytol alcohols gives the ester chlorophyll. Chlorophyll, when treated in the cold with alkalies, gives alkaline salts of chlorophyllin. The color change is first brown, followed by a return of the green, but it is no longer fluorescent. When chlorophyll is saponified with hot alcoholic alkalies, isochlorophyllins are formed, which are fluorescent.
When the green-colored vegetables are cooked in water with an alkaline reaction, or in water to which a small amount of soda is added, they develop a bright, intense green color.
Reaction of chlorophyll with acids. Chlorophyll reacts with acids to give an olive-colored product, without fluorescence, called phaeophytin. The magnesium of the chlorophyll is replaced by hydrogen. From phaeophytin, Willstatter has obtained two decomposition groups: the first, designated as phytochlorins, are olive green and derived from chlorophyll a; the second, the phytorhodins, are red and derived from chlorophyll b.
The effect of heat upon chlorophyll. The chlorophyll is changed to the olive-green color by two means, (1) by hydrogen ions or an acid reaction and (2) by heat. As previously given, the hydroxyl ions, or an alkaline reaction, produces chlorophyll salts with bright green color. In general, the more acid the reaction, the more rapid is this change in color when the vegetable is heated; or, vice versa, the more alkaline the reaction, the more slowly the chlorophyll changes to olive-green. Thus in order that the bright green color be retained in cooking green vegetables, they should be cooked for as short a time as possible and contact with acids should be avoided as far as possible. It is also possible that other ions than the hydrogen and hydroxyl ions may affect the stability of the chlorophyll, for some vegetables with nearly the same pH, cooked in water from the same source, and with other conditions standardized are more stable to heat than others.
In cooking certain procedures may aid in decreasing the acidity of the cooking water. The vegetables contain both volatile and non-volatile acids, which in the plant are prevented from uniting with the chlorophyll but are liberated when the plant tissues are heated. If the cooking vessel is not covered, the volatile acids may escape with the steam, thus decreasing the acidity.
It has been found that the highest percentage of these volatile acids passes off during the first few minutes of cooking. Hence, if the cooking vessel needs to be covered for a part of the time, it is preferable to have the uncovered period the first few minutes.
Certain water, such as hard water, softened water, or water from many streams, is alkaline in reaction. Rain water, snow, or ice water is usually about neutral. If the cooking water contains alkaline salts, these salts may neutralize the non-volatile acids, and if there is a slight excess of alkaline salts the green color is intensified. To a certain extent the intensification depends upon the quantity of water used, for the larger quantity of water contains a greater quantity of alkaline salts. If the water is only slightly alkaline the plant acids may not all be neutralized and the olive-green color may develop. If the water is very alkaline and considerable water is used, not enough volatile and non-volatile acids will be liberated to neutralize the alkalinity of the water, the cover can be kept on during cooking, and the product will be bright green. With longer cooking the heat may have more effect upon the chlorophyll than the alkaline salts of the water. The addition of sodium bicarbonate (baking soda), Experiment 17A, 5, also intensifies the green color. Canned spinach, asparagus, peas, and string beans have a deep olive-green color due to the retention of the plant acids during processing and to the high temperature at which they are processed. Green vegetables like cabbage, Brussels sprouts, and spinach cooked in milk may remain a bright green color. Owing to the ease with which milk scorches and boils over there is usually less tendency to cook the vegetables too long when milk is used.