You will remember in my earlier post about purple sweet potatoes that it’s difficult to bake with them, because the anthocyanin that provides the color reacts with baking soda (or baking powder, or anything alkaline) and turns the resulting mixture blue (or green, or blue-green). It will also react with acids and turn red. This is a pretty common science experiment; you can find one version of it here, using red cabbage. Try it at home!
Well, I did, and not with cabbage juice, either. I decided to make my favorite sweet potato dessert recipe, Sweet Potato-Chocolate-Nut Cake (original recipe from The Victory Garden Cookbook), using purple sweet potatoes. Which, by the way, are all harvested as of several weeks ago, mostly cured (a few are sprouting, but I’ll just use those up quickly), and even more delicious. I baked a bunch of them to produce the two cups of puree needed for this recipe (it says mashed in the recipe, and normally I would, but even after baking the insides of these guys are pretty firm, and there were a lot of them, so out came the food processor). Here’s what the baked and peeled potatoes looked like:
Still very, very purple. But, lo and behold, when I mixed the batter up fully, it began to acquire that suspicious bluish tint (not the third of the batter with chocolate mixed in, of course). And when the cake was baked, sliced and served, here’s what resulted:
I mean… magic, right? No, chemistry! And actually, I don’t think it looks too bad. I’ve made some baked goods before – the smearier sort of blueberry muffins come to mind – where the chemical reaction produced a blue-green reminiscent of mold, which is not pleasant at all. This cake is a nice forest green, with brown chocolate bits for an even more foresty feel. It’s just rather odd to tell people it was made from purple sweet potatoes.
Anyway, I’d still like to achieve a bread or muffin or something that’s really purple, so after eating some cake I started searching for answers. I’m going to give you several interesting links below, but first: the solution may be to balance the alkalinity of the baking soda/powder with some acid, adding (as the red velvet cake creator below did) ingredients such as cream of tartar, yogurt, buttermilk, vinegar or lemon juice, depending on the recipe and its desired taste profile. I will report back on the results.
Blue Food Roundup! This is a nice explanation of how various anthocyanins produce blue/purple pigments and how cooking affects this.
Natural Red Velvet Cake. Recipe using beet puree (and no food coloring) and also a lot of nice stuff about chemistry and ingredient choices.
Chemistry in Cake. Another post about the whys of red velvet cake.
Kitchen Chemistry – Anthocyanins and Blue Food. More interesting kitchen trials involving blue and purple and pink.