You open a jar of pickles and the garlic cloves have gone blue, or a blue-green. It looks wrong, but it is not a sign the jar has gone off. It is a colour change, and the garlic is still fine to eat.
Garlic goes into a lot of my jars, from ogórki kiszone to my fermented radishes, carrots and beetroot, so here is what is going on.
The short answer
Garlic turns blue or green when it sits in something acidic, like vinegar or the sour brine of a ferment. Compounds in the garlic react and change colour. Oregon State University Extension, in its guide to pickle problems, lists blue or purple garlic and says simply: use the pickles.
What is going on in the jar
Garlic is full of the sulphur compounds that give it its smell, and the colour change in pickled garlic involves one of them. Garlic also carries water-soluble pigments, and pigments like these change colour completely with acid. It is the same kind of thing that turns red cabbage blue. The exact reaction is not fully worked out, but acid is what sets it off.
What makes it more likely
- Young garlic. Garlic that is not fully mature is more likely to turn.
- Copper. Copper in the water, or a copper pan, can react with the sulphur compounds in garlic to make a blue compound.
- Acid. Vinegar pickles and sour ferments both give it the acid it needs.
How to stop it happening
Blanch the garlic before it goes in the jar. A quick dip in boiling water keeps the colour change to a minimum. Use a stainless steel or enamel pan rather than copper for anything that goes in the jar with it.
Garlic in oil is a different story
Colour is not the thing to worry about with garlic. Garlic kept under oil is. Oil keeps the air out, and that is exactly what the bacteria that cause botulism like. If you want to keep garlic in oil, soak it in vinegar or lemon juice for a few hours first, and keep the jar in the fridge.
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