Traditional canning information suggests that blue or green-tinted garlic is caused by iron, tin or aluminum in the water which reacts to the garlic pigments, or as a result of soil minerals which become accentuated during fermentation.

The latest horticultural and food science research pinpoints the source of the discolorations: Blue or purple pigments are caused by all amino acids (proteins) (except for cysteine, proline and 4-hydroxy-proline) in crushed, sliced, or fermented garlic. Some amino acids may react with sulphur, causing blue or green discoloration. Under acidic conditions, isoallin, a compound found in garlic, breaks down and reacts with amino acids to produce a blue-green color.

“Don’t worry, greenish-blue color changes aren’t harmful and your garlic is still safe to eat – unless you see other signs of spoilage.” – What’s Cooking America

Separation of Blue Pigments in Crushed Garlic Clove: The Color-Forming Potential of Individual Amino Acids

Model Studies on Precursor System Generating Blue Pigment in Onion and Garlic

“Under acidic conditions, isoallin, a compound found in garlic, breaks down and reacts with amino acids to produce a blue-green color. Visually, the difference between garlic cooked with and without acid can be dramatic, but a quick taste of the green garlic proved that the color doesn’t affect flavor.” From America’s Test Kitchen Newsletter, September 2004

Garlic Turning Blue or Green