It isn’t exactly an earth-shaking problem, but you would think someone would have solved it by now. Why does pickled garlic sometimes turn blue? People get unnerved when they pick up a jar in which some of the cloves have a blue tinge. Surely, it must be spoiled, they think. Well, no. We’ve known for a long time that blue garlic, while it may seem unappetizing, is completely safe to eat. The puzzle has been why cloves turn blue, and why it only happens to some cloves, in some jars.
Believe it or not, the blue garlic question has been investigated for over fifty years, but it was only recently that a paper published in the Journal of Agricultural and Food Chemistry finally got to the heart of the matter. Oh, it isn’t that explanations haven’t been offered before. They have. But they’ve been wrong! Two basic theories have been advanced in the scientific and the popular literature. One held that the discoloration was due to the buildup of copper sulfate. Not an unreasonable idea, given that copper sulfate is indeed blue, and that copper can be present in our water supply. The theory was that sulfur compounds known to be present in garlic could convert to sulfate, which then reacts with copper in water to form the colorful copper sulfate. Since copper concentrations in water vary, the problem wasn’t expected to occur all the time. Sounds reasonable, but this explanation is wrong. Experiments can readily show that doping water with extra copper before immersing garlic cloves does not necessarily produce the blue color.
The other theory advanced was that garlic contains compounds called anthocyanins, which change color depending on the acidity of the surroundings. While anthocyanins in red cabbage may be susceptible to such changes, those in garlic are not. It is simple enough to show that pickling garlic with different amounts of vinegar—that is, different amounts of acid—does not correlate with blue color formation.
So what, then, is the answer? As a team of experts from the Czech Republic and the us found, the discoloration is due to pigments that form between sulfur compounds in garlic and amino acids. Isoalliin is found in garlic, and when garlic tissue is disrupted, as happens in processing, an enzyme is liberated and reacts with it to form thiosulfinates, compounds that then react with the garlic’s naturally occurring amino acids to form blue pigments. The age of the garlic determines how much isoalliin there is in the first place, and the nature of the processing determines how much enzyme is liberated. Some of the mysteries still have to be cleared up, but you can rest assured that there is no harm in consuming your blue garlic. The pigments that form by the reaction of the thiosulfinates with amino acids are not toxic.
So that mystery is solved. But another color conundrum that I’ve been asked about over the years remains. Try this. Take a gold ring and slowly rub it across your cheek or the back of your hand. Some of you will see a black line, just as if you had used a pencil on the skin. This is known as “black dermographism,” which translates literally to “black writing on the skin.” In common language it is usually referred to as “gold smudge.”
Now, if you are looking for a little fame and fortune, just find the cause of this problem. There are all sorts of theories, but not one of them can explain all cases. This may be because not all cases have the same cause. Let’s start by listing the facts. The effect has been noted with all gold jewelry, irrespective of karat value. Women experience it more than men, and more if they are wearing makeup. Some women claim they see it more at certain times of the month, or if they are stressed, or if they have been eating acidic foods.
There is no doubt about the connection to cosmetics. Any finely powdered metal will appear to be black, and cosmetics contain abrasives that can act like fine sandpaper on gold. Many cosmetics contain zinc oxide, titanium oxide, calcium carbonate, and iron oxide as pigments. Both titanium dioxide and zinc oxide are harder than gold or silver, so as the gold is drawn across the face small amounts will abrade and discolor the skin. twenty-four-karat gold makes more intense marks because it is the softest form of gold and is the most readily abraded.
But what happens with men or women who don’t wear any makeup, and still experience gold smudge? We may be looking at a different cause here. Gold jewelry is not pure gold; the metal is alloyed, usually with copper and silver. Both of these metals can undergo a chemical reaction with sulfur compounds in the skin, stemming from the breakdown of sulfur-containing proteins. Both copper sulfide and silver sulfide can form black marks on the skin. The breakdown of the proteins in question may be hormonally controlled, which might explain why some women experience gold dermography more noticeably at certain times of the month. These reactions are more likely when the skin is more acidic, since copper and silver from the ring then become more soluble.
I have searched the scientific literature, and it seems that nobody has engaged in a systematic study of the causes of gold smudge. It is high time that science paid attention to such important issues, and I would welcome offers from prospective scientists to take up this challenge. If you have any comments, or any theories about gold smudge, please let me know. I, for one, intend to start testing everyone I come across. So if you see people with black marks on their faces, you’ll know that I’m on the job.