Why Do Walnuts Turn Bread Purple?
Walnuts contain a chemical compound called juglone, which reacts with the bread dough’s acidity or presence of yeast to cause a color change from yellow to blue or purple. The juglone compound is also responsible for the bitter taste of the green outer layer that surrounds the nut meat. The amount of juglone varies in walnuts according to the type of tree and has varying effects on different foods.