Blue is notoriously scarce in the palette of natural foods. And among foods that are called blue, few of them have a purely blue (not purplish or greenish) pigment. But with the scarcity of naturally blue foods, you have to take what you can get, so forgive me if some of the foods on my list are merely blueish. Up until this point the color study has been able to focus on relatively normal plant foods, and how to keep them colorful. But with blue, things are going to get weird. Researching and testing for this article I felt a little like a mad scientist-- (pH strips, mold, simmering pots of blue liquid). Most of the blue foods that I'm discussing get their pigments from anthocyanins. Most anthocyanins have unstable pigments that are affected by the pH they are exposed to. Red cabbage is the classic example-- it can turn bright red, purple, blue or dark blue-green depending on the acidity it is exposed to. Most color changing anthocyanins will lean toward the blue/purple range in basic conditions and lean toward the purple/red range in acidic ones. So you've found a blue food? Odds are, if you add an acid it will turn purple. And if you have a purple food and add acid? Odds are it will turn red. And there's one big catch to this whole fun pH color changing thing: nearly all foods are acidic. Yeah. So how are you supposed to cook with blue? Well, there are a few exceptions-- foods and preparation methods that introduce little enough acid to keep the hue blue (and cheating here and there with a pinch of baking soda). And there are a few exceptions to the variable anthocyanin rule that will allow you different cooking techniques but still give a little blue. I've grouped these blue foods into Anthocyanins (the color changers) and others. So put on your lab goggles, pull out your pH strips and get ready for some weird, blue food adventures.
Blueberries look blue when you pick them, but then they turn red/purple when they are crushed. The pigment in the skin is blue at a neutral pH, but turns red when exposed to the acid of the berries. With blueberries, I usually find the flavor much more important than the color, and the flavor is better when it is acidic. Blueberries will even turn green if they are exposed to too much of a base, such as in a pancake batter or muffin mix. To avoid this discoloration either decrease the baking soda/powder in the recipe or add more acid, such as lemon juice or buttermilk.
Blue varieties of corn are packed with anthocyanins. In acidic conditions blue corn will appear purple, in basic conditions it will be more blue. Try substituting blue cornmeal for yellow cornmeal in cornbread or tortillas.
Red cabbage is the most common natural blue food coloring here in the States. Cooked red cabbage leaves will eventually turn bluish purple if soaked in a slightly basic solution. To make a blue food dye, slice up red cabbage leaves and boil for 10-15 minutes. Strain out the cabbage, reduce the liquid until it is thick and syrupy (the cooking liquid from a whole cabbage will reduce to about a quarter of a cup. Now you have an intensely purple syrup. Add just the tiniest pinch of baking powder (you really have to go slowly here or you can turn the whole batch green). Keep adding baking soda in very small amounts until the color just turns blue. It is important to add only enough baking soda not only for the color, but for the flavor. Small amounts of baking soda have a negligible effect on taste, but add to much and it will taste terrible. Now you have a blue dye, As unappetizing as boiled cabbage and baking soda sounds, the flavor of the dye really is not that pronounced. Use it with a light touch to add blue to icings, cake batters and cookies. but remember that the color can still change. If you add it to an acidic food, it will go right back to purple.
Purple potatoes are a vibrant purple when they are raw, but when they are cooked, the balance turns to a brilliant blue-purple. I'm reaching the limits of my chemistry knowledge here, but this color change is of a different nature than other acid/base changes. Cooked purple potatoes are susceptible to color changes from acid, but much less so than red cabbage or blueberries. And, exposed to high concentrations of acid, purple potatoes bleach and turn a very light purple-- not anything like that intense purple in the raw potatoes. Purple potatoes are also not very susceptible to bleeding their colors out. All of this makes them a great way to add some unusual color to a plate, without having to worry too much about the pH. Packed with anthocyanins, purple potatoes have a leg up over white and yellow potatoes on the nutrient front. Try substituting them in your potato recipes.
Cornflowers, or bachelor's buttons, are typically bright blue. The blossoms are edible, and can be added fresh to salads or as a garnish for a dessert plate. Dried flowers can also be added as a garnish. The flavor is very mild and grassy (you're really using them for the color). Some loose leaf tea blends include cornflowers, which does make a striking display contrasted with the shriveled black tendrils of tea leaves. As always with flowers, make sure you know your supplier and can verify that the flowers have been grown organically or treated only with food-safe chemicals.
Borage is a blue-flowering Italian herb. Often grown nearby tomatoes and eggplant, the borage plant produces big hairy leaves and clusters of little five-pointed blue flowers. According to some folk medicine traditions, borage flowers are supposed to elevate your mood. And really, how could adding pretty little flowers to your plate not elevate your mood? Add fresh borage flowers to salads or as a garnish to desserts.
Butterfly Blue Pea-
The last and most spectacular of the blue anthocyanins is the butterfly blue pea flower. This pea vine produces beautiful, intensely blue flowers. Thai, Malaysian, Burmese and Chinese cooking traditions all make use of this stunning blossom. Pulut tai tai, a Malaysian sticky rice cake seasoned with coconut and pandan, is traditionally speckled blue from pea flowers. And the intricately shaped Thai dumpling, chor ladda, resembles a bright blue flower. The flowers are also used in Thailand to make a chilled herbal tea, which is thought to be refreshing and cooling. The flowers can be dried or used fresh, to make an incredibly vibrant blue infusion. I managed to get my hands on some dried butterfly blue pea flowers-- and they are remarkable. The flavor is very mild and herbal with a hint of cucumber. The only catch? The bright, electric blue will turn bright purple in the presence of acids.
Other Blue Pigments
Since I'm tiptoeing away from the plant kingdom with this inclusion, blue cheese does not get it's blue from anthocyanins. The blue comes from a mold culture added during the cheese's processing. The particular mold varies based on the type of blue cheese, but they are all in the Penicillium category. And, yes, it is that same penicillin.(though if you tried to eat enough blue cheese to cure your sinus infection, you'd likely die of heart disease first). The blue color is typically fairly dark, and is not susceptible to bleeding. When you're serving cold dishes with blue cheese, try using a string to cut your blue cheese into thin sheets-- the mottled surface can be quite attractive and much more interesting than crumbles.
Usually this chemical reaction comes up with a what on earth happened to my pickled garlic? When raw garlic is pickled, small amounts of sulfur can react with trace amounts of copper from your water or cooking implements. So the garlic starts out looking normal and then a few weeks later (mine took two) the garlic turns blue/green. If you don't want your garlic to turn blue, you simply ought to boil it briefly before placing it in the brine. Obviously this blue is not an anthocyanin, but trace amounts of copper sulfate. While the amount in pickled garlic is harmless, in larger quantities copper sulfate is toxic.
Bluefoot and Blewit Mushrooms-
If we were being picky about hues, these really look a little more purple than blue to me... but they are called blue, and they look rather fantastical, AND they have a remarkable, unique flavor. Bluefoot mushrooms are available at specialty stores in the US (and as a rare mushroom, they have an exclusive price tag). Blewit Mushrooms are related, but hard to find in the States. Blewit mushrooms are more uniformly blue/purple while bluefoots are, well, blue just at the base. Bluefoot mushrooms have a rich, woody meaty flavor. As with all flavorful mushrooms the flavor spreads beatifully when cooked with cream.