All You Want to Know : Pigments and Cooking
There are several characteristics of vegetables that influence the way we cook them. One of these is colour. No matter what colour a raw vegetable is, we want to preserve as much as possible of that colour. Vegetables may be grouped by colour into four categories:
RED: beets, red cabbage, red beans
GREEN: beans, broccoli, asparagus, peas artichokes, okra, brusselsprouts, spinach, parsley.
YELLOW: carrots, yams, squashes, sweet potatoes,
WHITE: potatoes, turnips, celery, cauliflower, onions, mushrooms, cucumbers, zucchini
These colours come from the pigments in the vegetables. Certain pigments react to acids, alkalis, or heat during cooking, undergoing chemical changes that cause the vegetable to change colour. It is important to know which pigments are susceptible to colour change and how to deal with them during the process of cooking.
Red Vegetables: The pigments in red vegetables are known as anthocyanins. These pigments are red in an acid medium but will change to blue or purple in an alkaline medium. They are also water soluble and therefore can draw out the colour into the cooking medium. egs.: Beetroot- one would have to cook it with the skin on, so that the colour does not leach out into the water. Cook covered so the acids do not evaporate. Red cabbage in an alkaline medium will not only lose its redness, but will also turn purple, blue or green.
Green Vegetables; Chlorophyll’s action to acid is the opposite of the red vegetables. They keep their colour best if cooked in a slightly alkaline medium. It would seem ideal to include baking soda in the cooking medium to increase alkalinity-Not So! If only colour were the quality desired then this approach would be fine. Baking soda has other less desirable effects. It tends to destroy such nutrients as vitamins, and also makes the vegetables more mushy and sometimes gives it a bitter taste. Note- the dark green colour in canned green vegetables is because of prolonged cooking at high temperatures to prevent botulism.
Yellow Vegetables; The pigments here are known as carotene’s, and there are several types which range from the yellow in corn, to the orange in carrots, to the red in tomatoes. They do not suffer any colour loss in acidic or alkaline medium, however if the vegetable is overcooked then it can lose some amount of colour.
White Vegetables; The Flavones are the pigments in these veggies! They remain white in acid, but turn yellow in an alkaline medium, so a bit of lemon juice or a cover on the pot is in order.
NOTE; the pH value of a material indicates its acidity or alkalinity. Acids are characterized by hydrogen ions(H+), while alkalis or bases by hydroxyl ions (OH). To test the pH value of a material a scale of numerical value ranging from 1 (for very acidic) to 14 (for very alkaline) has been developed and is in common use. The whole pH scale has been so devised that each whole step represents a tenfold change in the degree of acidity. Thus a solution with a pH of 5 is ten times more acidic than a solution with a pH of 6. Similarly a solution with a pH of 4 is hundred times more acidic than a solution with a pH of 6. -salad greens go limp and discolour when salad dressings are poured over them, a long time before service. This is because the acids first of all destroy the chlorophyll. The reaction here takes a long time, because there is no heat involved in this reaction. Secondly the acids and the seasonings in the dressings draw out the moisture in the leaves- this leaves the salad all limp. -fresh milk has a pH of 6.6. As it ages the milk sugar lactose is converted to lactic acid, and at a pH of around 6.4 the change in the pH along wit the other changes can be detected in taste. We say that milk is sour at a pH of around 5 where the coagulation of protein takes place and the milk has curdled. -table wines have a pH of between 3 to 3.5 and beers a pH of about 4 to 5 . -the pH of living muscle is about neutral, but becomes acidic after death due to the formation of lactic acid from glycogen. -did you know that egg white is alkaline while egg yolk is acidic?
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