Why are Fruits and Vegetables brightly colored and What are the Chemical Names of Plant Pigments?

The kaleidoscope of brightly colored fruits and vegetables, the red tomatoes, watermelons, strawberries, and beets; the orange carrots, sweet potatoes, pumpkins, apricots, and mangoes; the yellow lemons and squashes; the bluish-purple grapes, plums, and cabbages; and all the green beans and leafy vegetables, are due to a variety of phytochemicals that can be classified into three main groups, chlorophylls, carotenoids, and flavonoids, the last of which includes anthocyanins and anthoxanthins.

A phytochemical, from the Greekphyton, meaning “plant,” is any chemical compound produced by plants. But lately the term has been appropriated by health foodists to mean any plant chemical, beyond the nutritious proteins, carbohydrates, fats, minerals, and vitamins, that may be deemed “good for you.” That includes many of the red, orange, yellow, green, and blue pigments in fruits and vegetables, which do indeed have known health benefits. But nicotine and cocaine are also phytochemicals, are they not?

Chlorophyll needs no introduction. It is a green compound, each of whose molecules contains a magnesium atom. But chlorophyll has taught us that it’s not easy staying green. When chlorophyll molecules are reconfigured (denatured) by heat, their magnesium atoms are released, converting the chlorophyll into pheophytin and pyropheophytin, the dishearteningly olive-drab colors of overcooked green vegetables.

Carotenoids range in color from yellow to orange and red. Orange beta carotene is converted into vitamin A in the body. Carotenoids beautify everything from carrots to corn, peaches, citrus fruits, squash, paprika, saffron, tomatoes, watermelons, and pink grapefruit. The last three, especially tomatoes, contain the fat-soluble carotenoid lycopene, an antioxidant that has been touted as a possible preventive of prostate cancer.

Anthocyanins are water-soluble pigments found in grapes, berries, plums, eggplant, cabbage, cherries, and autumn leaves. Purple or blue in alkaline environments, they turn red in acidic media.

With not too many exceptions, you can think of the carotenoids as the yellow, orange, and bright red colors in vegetables and the anthocyanins as the blue, purple, and dark red colors.

Among the less common food colors are betalains, the intensely red, water-soluble pigments in beets. When beets are sliced before cooking, much of their color dissolves out into the water. But when unpeeled and cooked whole, they remain as defiantly red as Fidel Castro.

And what is Nature’s purpose in painting all these vegetables such pretty colors? It’s not just for still-life painters. The bright colors attract animals, who eat the plants, to their mutual benefit. The animals benefit from the healthful antioxidant properties that Nature has built into many of the colored chemical compounds, while the plants benefit by the animals’ pollinating their flowers and spreading their seeds.

The characreristic red color of tomatoes, often attributed to lycopene, actually results from a combination of carotenoid pigments, of which lycopene is only the most abundant. The color of the fruit doesn’t always correlate with the amount of carotenoids, much less with the amount of lycopene. So one can’t draw valid conclusions about lycopene content from the redness of a tomato. Nevertheless, all tomatoes are pretty good sources of lycopene.

To justify my heading, I am obliged to explain that the blue color of violets and the red color of roses are both due to an anthocyanin that is an acid-base indicator. It is red in the slightly acidic rose petals and blue in the slightly alkaline violet petals.