Several weeks ago I baked a batch of buttermilk biscuits with sunflower seeds in them, using a recipe from a cookbook I've had since college, Vegetarian Pleasures by Jeanne Lemlin. They were delicious fresh out of the oven, but when I broke one open the next day I noticed what appeared to be vivid green mold in the biscuit surrounding the sunflower seeds, which had themselves turned a dark greenish brown. I was alarmed and, well, perplexed. Was it because I had used roasted, salted sunflower seeds, having failed to find raw ones at my supermarket? Or because I had used baking powder and baking soda that had both expired?
A few days ago I decided to try the recipe again, this time with raw seeds found at the health food store, and brand-new baking powder and baking soda. Same delicious results fresh out of the oven. And same algae-green specks surrounding the seeds the next day. Clearly something strange was going on here.
I found the solution to my food mystery at The Kitchn, where food science writer Harold McGee explained that certain foods—sunflower seeds, carrots, blueberries and walnuts, for instance—are sensitive to changes in pH balance. When they come into contact with an alkaline substance, such as baking soda, they can change colors. I realized after seeing the list that I had encountered foods with this discoloration before, though never so vividly or jarringly as occurred in my biscuits. The discolored food, I was relieved to read, is perfectly safe to eat. McGee suggests decreasing the amount of baking soda in your recipe, or distributing it more evenly, to prevent the reaction.
Until this experience, I had never given much thought to what baking soda, or baking powder, actually does (or what the difference is between them). In its regular Nagging Question feature (which is often good for interesting tidbits of information), Chow explains that both of the white, powdery substances contain sodium bicarbonate, a leavening agent (it creates gas, causing baked goods to rise). Baking soda is the straight stuff, and is alkaline; it requires an acidic ingredient, such as buttermilk, lemon juice or brown sugar, to be activated.
Baking powder, in addition to the sodium bicarbonate, contains cornstarch to prevent clumping and acidic salts to activate the production of carbon dioxide. It can be used in recipes that don't contain enough acidic ingredients to activate the sodium bicarbonate on their own. Baking powder acts more slowly than baking soda; the salts in it only dissolve partially when they are mixed with the other baking ingredients, and don't fully work until they are heated in the oven. This is why baking powder is often called "double acting."
For a neat demonstration of the color changes that result from pH reactions, without having to eat baked goods look like they've been pulled from a dirty aquarium, check out these instructions for creating your own pH indicator using baking soda and red cabbage juice.