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Why do cooked vegetables change color ... and can you control that change?

Cooking is an everyday application of science. When you understand why something happens, you can more readily count on a predictable outcome. You'll get better results when altering recipes or making up your own, and can also evaluate the significance of contrary directions in similar recipes.

This month we begin Why?, a feature for you to shape. Share with us questions, puzzling observations, contradictory information you've encountered about recipes and foods. We'll try to set the record straight. To do so, we'll call on the collective efforts of Sunset's food editors and the expertise of food scientists, primarily Dr. George K. York, extension food technologist in UC Davis's Department of Food Science and Technology.

If you have questions you'd like us to answer, write to Why?, Sunset Magazine, 80 Willow Rd., Menlo Park, Calif. 94025.

So--why do green vegetables change color when cooked?

At the first blast of heat, in hot water, steam, or fat, green vegetables get brighter, but with longer cooking the green fades.

What happens? Heat forces the gases surrounding the vegetable cells to expand and escape. As a result, you can see the green pigment, chlorophyll, more clearly. It's rather like fog fading away to let light reveal the color.

Fried vegetables, as in tempura, cook so quickly they stay quite green if eaten hot.

Boiled or steamed (including microwaved) vegetables like green beans, peas, asparagus, broccoli, cabbage, and leafy greens are ready to eat (tender to tender-crisp) while still brightly colored. But if you want to serve vegetables cold or reheat them later, you must stop their cooking with a shock to arrest color change. Drain vegetables and at once immerse them in ice water until cool; this stabilizes the chlorophyll. When reheated, the vegetables don't fade as rapidly as when first cooked. (This is why frozen green vegetables keep their color longer when cooked.)

Vegetables that take more cooking, like artichokes, lose their bright color.

Why do green vegetables turn grayish yellow?

Heat is rough on chlorophyll, which is very unstable. One reason recipes direct you to boil vegetables uncovered is so that color-destructive gases surrounding the cells can dissipate rapidly. If you cook vegetables in lots of boiling water, rather than just a little, heat is distributed faster, and vegetables have better color because there is less time for chlorophyll to fall apart.

Green vegetable color fades to olive, then to grayish yellow, as heat displaces the magnesium atoms in the chlorophyll, shifting its chemical structure and the color.

Acid has the same visual effect on chlorophyll as heat does. But another factor is at work. The acid's hydrogen atom replaces the magnesium in chlorophyll, turning the color to a yellowish graygreen. This is why green vegetables turn drab if they stand in a tart dressing for more than a few minutes. To minimize the color change, dress vegetables just before serving.

What do baking soda and copper pennies do to green vegetables?

In the past, cooks often added baking soda or copper pennies to cooking water because they made green vegetables stay brightly colored.

Baking soda is alkaline (acid's opposite), and because it prevents hydrogen from replacing magnesium in chlorophyll, the pigment gets brighter. The negative: soda rapidly breaks down pectin that holds cell walls together, making vegetables mushy.

Copper coins and unlined copper pans contain free copper or zinc ions that also replace magnesium in chlorophyll, giving the green color a bluish cast. The vegetable texture is not affected, but you might be: eating too much copper sulfate can make you sick.

Why does red cabbage turn blue when cooked?

Red cabbage and other blue-red vegetables are a war zone for two color pigments, anthocyanins (red) and betacyanins (blue). The red color needs acid (lemon juice, vinegar) to anchor it; otherwise red vegetables get the blues. For a little drama, cook red cabbage in a little water. Then add acid to taste and cook further, watching the unappetizing blue-purple color shift rapidly to red.
COPYRIGHT 1992 Sunset Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Author:Anusasananan, Linda Lau
Publication:Sunset
Date:May 1, 1992
Words:663
Previous Article:Quick ways with fresh fish.
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