Why does my garlic turn blue or green?

It is a harmless chemical reaction, not spoilage, and it is more common in pickled garlic and in dishes with acidic ingredients.

Garlic contains sulphur compounds and amino acids. When the cloves are crushed or cut, an enzyme is released that begins breaking those compounds down. Under acidic conditions the resulting products can form pigments — chains of sulphur-containing molecules that appear blue, green or occasionally blue-green depending on concentration and pH.

Several things make it more likely.

Acidity. Vinegar, lemon juice, wine and tomato all encourage it, which is why pickled garlic is the classic case.

Young or immature garlic, and garlic that has been stored cold, contain higher levels of the precursor compounds. Refrigerated garlic is noticeably more prone to it.

Trace copper, from water or from a copper or brass pan, can catalyse the reaction.

Time. The colour often develops over hours rather than instantly, which is why a pickle looks fine when jarred and startling the next morning.

It is entirely safe to eat. The flavour is unchanged, and in several cuisines the effect is produced deliberately — Chinese laba garlic is pickled specifically to turn a vivid green for the new year.

If you want to avoid it: use mature garlic stored at room temperature rather than in the fridge, use stainless steel or glass rather than copper, and if pickling, blanch the cloves briefly first to deactivate the enzyme.

A different discolouration is worth distinguishing. Brown or yellow soft spots, a sour smell, or visible mould mean the garlic has actually spoiled and should be discarded.