Which chemical acid did Pasteur study in crystals from wine barrels?

The chemical acid Pasteur studied in crystals from wine barrels was tartaric acid.

Pasteur’s interest began with tartrate crystals deposited during winemaking. In 1848, while studying salts of tartaric acid, he noticed that apparently identical crystals could possess mirror-image forms. By painstakingly separating the two kinds by hand, he showed that their solutions rotated polarized light in opposite directions.

This work became a landmark in stereochemistry. Pasteur’s observation connected a crystal’s asymmetrical shape with the three-dimensional arrangement of its molecules, helping establish the idea of molecular chirality. It also helped distinguish naturally occurring tartaric acid from its optically inactive racemic mixture.

Wine drinkers may still encounter the same chemistry as harmless “wine diamonds”: potassium bitartrate crystals that precipitate during storage or form on a cork. They are not shards of glass, and they should not be confused with acetic acid, lactic acid, or citric acid, which have different chemical and biological roles.