Sometimes garlic turns blue when I cook it. Why? Is it safe to eat?
When you first asked me this, I thought it was an easy question. So easy, in fact, that I planned on conducting a little kitchen experiment to document both the reaction and its counter-reaction. But things didn’t work out that way, as I’ll explain below. But first, let’s answer your question. The simplest answer is that garlic contains a set of pigments known as anthocyanins. Under certain conditions, the anthocyanins appear blue-green. They are harmless to eat, and may even go away depending on what you do to the dish next. Harold McGee explained his take on this in his inaugural “Curious Cook” column in the New York Times:
I hear every year from cooks who have been alarmed at seeing normally pale garlic turn bright green and even blue, sometimes when the cloves are pickled whole, sometimes when they’re chopped and cooked with other ingredients. I’d often been puzzled by little blue-green specks when I made garlic bread with loaves of sourdough, but I was really rattled the first time I puréed raw garlic, onion and ginger together in a blender to make chicken in yogurt from Madhur Jaffrey’s “Invitation to Indian Cooking.” When I fried the purée the entire mass turned turquoise blue.
I asked a couple of Indian friends who happen to be plant biologists whether they knew what was going on. They said they had never seen the blue purée, because Indian cooks don’t grind onions and garlic together. They grind or chop them separately and usually fry the onions first. . . .
According to chemists at the China Agricultural University in Beijing, aging the garlic gives it a chance to accumulate large quantities of one of the chemicals that generate the color; fresh garlic doesn’t green much at all. And a strong green color develops in Laba garlic only with acetic acid, the main acid in vinegar (also found in sourdough), because it’s especially effective at breaching internal membranes and mixing the cell chemicals that react together to create the green pigment. The pigment itself turns out to be a close chemical relative of chlorophyll, which gives all green leaves their color.
Two recent reports from the House Foods Corporation in Japan detail exactly how the garlic and garlic-onion pigments develop. Their creators are 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. These rings can be linked together into multipyrrole molecules.
The ring structures absorb particular wavelengths of light, and thus appear colored. The two-pyrrole molecule looks red, the three-pyrrole molecule looks blue and the four-pyrrole molecule looks green, as does its cousin tetrapyrrole, the chlorophyll molecule. Like chlorophyll, all the pyrrole pigments are perfectly safe to eat.
Now, I was familiar with the explanation that says that the blue-green apparance is more likely to take place in acid environments. In my own experience I noted that garlic only tends to turn blue when cooked in lemon juice, usually in a metal pan. In fact, the anthocyanin/acid relationship is so strong that you can actually use some vegetables—the classic is red cabbage—as a home pH meter. (You can learn more about the chemistry of red cabbage color changes in this episode of Distillations, a chemistry podcast.)
It was with all of this in mind that I had what I thought was a brilliant idea. If acid turns garlic blue, shouldn’t baking soda, a base, turn it back? So for you, dear readers, I attempted to conduct a scientific experiment.
First I added lemon juice to minced garlic in a glass bowl. Nothing happened.
I transferred the contents to a metal pan. Nothing happened.
I heated it up, thinking maybe the reaction needed a jump start. Still nothing happened.
I added white vinegar, thinking maybe my lemons weren’t strong enough. Nothing happened.
We ate dinner, giving the garlic and acid time to get to know each other. Still no blue.
At this point, I did some additional research and learned that garlic actually turns blue for two reasons. Certainly the anthocyanins are the main culprit. But garlic also contains a lot of sulfate that, when combined with copper, creates copper sulfate, an insoluble, bright turquoise compound. (Again: still fine to eat.) Apparently the trace amounts of copper in water and other foods can be enough to trigger the reaction.
So, I added water. Still no blue.
More research: what food stuffs have high proportions of copper, I wondered? Leafy green vegetables, like kale, chard, and spinach, and certain nuts, like sesame seeds.
I dumped a big pile of sesame seeds into the pot and kept cooking. At this point the kitchen smelled terrible. But still no blue.
Table salt, perhaps, with its iodine content? No go.
This went on and on for about an hour before I finally gave up. So while I can certainly tell you what turns garlic blue, I cannot tell you whether you can turn it back by adding baking soda. If it happens to you, give it a try, and let me know.