This garlic clove turned blue-green when pickled with onion (and green
cherry tomatoes.) Though poisonous-looking, it is safe to eat. The hue
comes from chlorophyll-like chemicals in the onion and garlic.
Last summer, when I pickled a few pints of green cherry tomatoes, the cloves of garlic tucked in the jars turned a lurid blue-green—the sort of weirdly beautiful shade that copper turns when exposed to weather. I was so alarmed that I emailed Rick Field of Rick’s Picks, who had given me the recipe. ‘Is it poisonous?” I asked. “Will I die if I eat it?” “Don’t worry,” said Rick “That happens all the time. It’s perfectly safe to eat.”
In today’s New York Times (“The Curious Cook: When Science Sniffs Around the Kitchen,” Wednesday, December 6, 2006, pp. D1 and D11), Harold McGee writes about his own experience with another bizarre color change. “…I was really rattled the first time I pureed raw garlic, onion and ginger together in the blender to make chicken in yoghurt from Madhur Jaffrey’s “Invitation to Indian Cooking.’ When I fried the puree, the entire mass turned turquoise blue.”
McGee notes that in northern China, aged fresh garlic is left in vinegar for a week to make an “intentionally intensely green” Laba garlic pickle traditionally served with New Year’s dumplings. According to chemists at the China Agricultural University in Beijing, aged garlic is saturated with a chemical that turns garlic green when released by the acetic acid in the vinegar. “The pigment itself turns out to be a close chemical relative of chlorophyll, which gives all green leaves their color,” he writes.
The color change in garlic and garlic-onion blends is created, McGee says, by the “same handful of sulfur compounds and enzymes that give the allium family its unique pungent flavors. Under the right conditions, these chemicals react with each other and with common amino acids to make pyrroles, clusters of carbon-nitrogen rings.” Essentially, these rings absorb different wavelengths of light and may appear green or blue, depending on their structure. To eliminate the blue hue of an onion-garlic blend, simply raise the heat in the pan—it will turn, according to Ms. Jaffrey, “a more acceptable pinkish-brown...”
Harold McGee is the author of On Food and Cooking: The Science and Lore of the Kitchen, a superb reference book which includes a excellent analysis of the flavor components in spices and herbs. His website is www.curiouscook.com. Today’s article is the first in a series of columns on the science of cooking, also called “The Curious Cook,” which McGee will write for The Times.