Why do walnut breads sometimes turn purple? Can anything be done to prevent this?
Theories abound on why yeast breads that contain walnuts (or pecans) turn purple. After reading a paper in the Journal of Agricultural and Food Chemistry, we decided to pursue the one that seemed most plausible: that the iron in flour is reacting with gallic acid found in walnut skins to create a color change.
To confirm this theory, we combined walnuts, water, and (to minimize variables) crushed-up iron supplements from the vitamin aisle and watched the water immediately turn a deep purple. We repeated the experiment with pecans, which were equally reactive, and almonds and hazelnuts, both of which triggered a slightly less dramatic color change.
So why don’t all baked goods containing these nuts turn purple? The answer has to do with acidity and time. The purple tint appears only in an acidic environment, and most baked goods aren’t acidic enough to cause the reaction. But thanks to the acids produced by yeast as it ferments, bread dough does have what it takes. In addition, since gallic acid leaches out slowly, rising bread dough gives time for the effect to happen.
Only tiny amounts of gallic acid are needed to bring out the purple. The tint can’t be prevented completely, but it can be minimized by blanching the walnuts in boiling water for 1 minute before using.