All green food pigments are from the chlorophyll molecule. If yellow and orange plant foods are remarkable for making vision possible, then chlorophyll is remarkable for making much of life on earth possible. Our oxygen-rich atmosphere can be traced back to photosynthesis, and photosynthesis needs the incredible chlorophyll molecule. But chlorophyll is a tricky molecule for the cook-- so many plant foods start out a gorgeous bright green, but if you make any mistakes handling them, that green dulls. There are several ways that chorophyll can be damaged and lose its pretty bright green color-- exposure to acidic (or basic!) substances will cause chlorophyll to become unstable, break down and become less green. Generally chlorophyll isn't soluble in water, but if the molecules are damaged (say, with heat) then the water soluble part can break off and leach out some of that green pigment. Heat and even light can damage chlorophyll molecules, leaving the once brightly green foods a more dull olive green.
Since chlorophyll is a vital part of plant life, there are endless plant foods with pretty green colors. Plants have five organs: roots, stems, flowers, fruit and leaves. All of these plant organs have green representatives that find their way to our plates. Roots are the exception -- most plants that we eat for their roots do not have much green, or green can signal a defect in taste or nutrition of the root. (The green that colors sprouting potatoes is not healthy to eat. Green shoots in the center of your garlic will taste bitter.) But there's a green exception to even that rule: certain types of radish are green and quite edible. Stems, leaves, fruit and flowers all have plenty of edible specimens it would be nearly impossible and not that interesting to try to catalog all types of green food, so instead I've grouped specific foods together based on how they are prepared along with tips about how to add some vibrant green into your cooking.
Raw Greens -
The easiest way to have a brilliant green is to serve green foods raw. Lettuces, tender green leaves and green fruits are almost always served raw. Most green fruits (grapes, melon, kiwis, cucumbers, peppers, tomatoes) are not very prone to browning. These fruits don't easily lose their green. Leaves, on the other hand, particularly fragile, delicate leaves, will easily become damaged easily when they are exposed to acid. (Look at my second image to see the effect of acid on a kale leaf) So if you have a beautiful spinach salad, wait until the last possible minute to toss it with a vinaigrette.
Cooking green foods and trying to keep them green is no simple task. There are lots of tricks people use to keep their cooked asparagus, peas and cabbage green. Though for each trick there seems to be a tradeoff. Some people boil vegetables with a clean penny (the copper helps stabilize the chlorophyll molecule). But ingesting copper isn't really that good for you, it works, but I wouldn't recommend it. Some people add baking soda to their water (the basic ph of the baking soda helps keep the chlorophyll from reacting with the acids in the plant-- but if you overdo it the vegetables can taste soapy and bitter (yuck). Most people's water is just slightly basic, which turns out to be perfect for cooking vegetables. If your water happens to be acidic, then it might be good to add just enough baking soda to tip the ph out of the acidic range. So no baking soda and no copper... what can you do? I've got three rules for green cooked vegetables: 1.Cook in lots of water. A large quantity of water helps the vegetables to cook evenly, and it also helps dilute the natural acids in the vegetable. 2. Don't overcook. You really have to watch your greens carefully just a minute or two can make the difference between tender bright greens and overcooked mushy ones. Don't trust your recipe, keep checking your veggies as they cook.(Check out my third image to see the effect of different cooking times on the color of radish greens) 3. Shock in cold water to stop cooking. If you're not going to serve the vegetables immediately, then have a big bowl of ice water ready to plunge them in straight out of the boiling pot. This stops cooking immediately, and right where you want it.
Artichokes & Avocados-
Artichokes and avocados are similar in that they are both highly prone to browning. After cutting an avocado, you should coat it with an acid, like citrus juice. Artichokes are even more prone to browning. Have a bowl of water with some lemon juice or vinegar in it ready when you cut an artichoke, and immediately plunge it into the water. Artichokes present the cook with something of a catch 22-- either coat in acid and dull the chlorophyll or keep the green, but let the artichoke brown instead. Perhaps this is why I've never seen a bright green cooked artichoke.
The chlorophyll in lime skin has some repercussions for the cook. Limes cannot be candied or dried in the same way as other citrus fruits (the chlorophyll turns brown). Limes are dried to produce loomi, a middle eastern seasoning. The transformation is incredible-- the bright green skin turns a matte brown and the inside fruit ferments and turns black. Use strips of fresh lime zest to add a green accent or garnish to a dish.
Because of chlorophyll's idiosyncrasies, green dyes can be a little tricky. You wouldn't want to boil green foods, for example, to concentrate a green dye. To make your own green dye cook tender, greens (such as spinach) for about a minute and shock them and squeeze out the water. Then puree these greens with just enough water to liquefy them, you will get a very concentrated green. Remember that you will also be adding the flavor of these greens to your dish, so if you want a mild flavor add a mild green (like spinach), if you want a concentrated flavor you can go with an aromatic green like parsley, mint or arugula If you wanted to add coloring to, say, a pistachio ice cream, pureeing a handful of spinach leaves with the cream would do the trick quite nicely. Alternately you can puree the leaves with an oil, which carries chlorophyll a little better. Strain the oil and use it as a pretty green garnish. I like to make colored/flavored oils from fresh herbs like basil and cilantro. Then you add the bright green color and distinctive herbal flavor at the same time. I typically find it easier and cheaper to buy bottled chlorophyll (look at health food stores where it is sold as a supplement). This chlorophyll does have a grassy green flavor, but if you use it in moderation as a colorant, I don't usually find it objectionable.
Pistachios and Pepitas-
These green nuts and seeds are both pretty and delicious on their own. Toasting nuts typically improves their flavor, but with green nuts and seeds the green color will degenerate with heat. Get the freshest pistachios and pepitas you can and serve them untoasted. Or look at some raw food preparations of nuts-- soaking the nuts and pureeing them will preserve their color, and let you experiment with different textures.
Not everyone likes green tea, but if you do like green tea, the powdered tea leaves called matcha have a brilliant green color. A little sprinkling of matcha makes a striking garnish to a plate.
Ever had a blueberry muffin or pancake turn green? The unstable anthocyanins in blueberries are sensitive to pH. In acidic or neutral conditions blueberries will be blue/purple. But add too much of a base and they'll turn green. Assuming that you don't want green muffins or pancakes, adjust the acidity of your recipe-- either cut back on some of the baking soda/baking powder or add some acid, like lemon juice or substitute buttermilk for milk.