Why Are Red Onions Purple?

One of the things we love about cooking with alliums — the botanical family that includes garlic, shallots, leeks, and onions (via MasterClass) — is their sheer variety. From scallions to chives to Vidalia onions, these flavor-packed bulbs bring indispensable taste to soups, stocks, and all manner of dishes. If you're an avid cook, you probably know down pat which allium to use for which recipe, such as choosing green onions for scallion pancakes, leeks for the chilled potato soup known as vichyssoise, and yellow onions for French onion soup.

And if you've ever used red onions, which take well to pickling and bring a sharp heat to burgers and sandwiches, you've probably appreciated their jewel-toned skin and flesh. These beauties, in spite of the name "red" onions, have an intensely purple hue. Wondering what's responsible for the color of red onions? Read on to find out.

Red onions contain pigments called anthocyanins

According to Potandon Produce, the purplish hue found in the skin and flesh of red onions is owed to the presence of anthocyanins and flavonoids, compounds that can boost human health by acting as antioxidants that can help to neutralize free radicals in the body (via Medical News Today). Purple foods, such as purple grapes, blackcurrants, and eggplant, are often noted for being rich in these antioxidants. During the life cycle of a plant, depending on its needs, the anthocyanins present in the plant's leaves, petals, and fruits will produce colors of red, purple, and blue (via Acta Physiologiae Plantarum 35).

Anthocyanins are responsible for many different pigments, including those that make "blue" foods, such as blueberries, look blue (or, more accurately, purple) (via Kitchn). And just as so-called "blue" foods are actually more purple in color, red onions are called red even though they, too, are purple. According to Allrecipes, red onions might have earned their name because throughout history, their skins have been used to produce a natural red dye. Allrecipes also notes that red onions vary widely in their color, depending on their exact strain and growing conditions, with some onions coming out more red and others looking more purple.