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.
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cytss_eca 2016年8月14日 HKT下午4时36分07秒
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.
http://www.thefreelibrary.com/Why+do+cooked+vegetables+change+color+...+...
http://www.webexhibits.org/causesofcolor/7A.html