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How did Louis Pasteur discover molecular asymmetry?

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Painting by Albert Edelfelt showing Louis Pasteur in his laboratory, holding a glass flask and examining it intently, surrounded by scientific equipment on a workbench. — Louis Pasteur In the 19th century Louis Pasteur's work established the germ theory of disease, proving that microorganisms cause fermentation and sickness, which led to the development of pasteurization, aseptic techniques in medicine, and effective vaccines for diseases such as rabies. (Painting by Albert Edelfelt, 1885.)

Louis Pasteur discovered molecular asymmetry while investigating why tartrates and paratartrates behaved differently toward polarized light, despite having identical chemical properties. While tartrates rotated the plane of polarized light, paratartrates did not.1

Pasteur observed that tartrate crystals exhibited asymmetric forms corresponding to their optical activity. Upon examining crystalline paratartrate, he discovered it was a mixture of two types of crystals that were mirror images of one another. By manually separating these crystals, he found that each type exhibited its own right- or left-handed asymmetry. He concluded that the paratartrate was optically inactive because it contained a balanced mixture of both forms.12

This discovery revealed that molecules can exist in non-superimposable, mirror-image arrangements, a concept known as chirality. This finding established the foundation of stereochemistry and led Pasteur to believe that asymmetry is a fundamental characteristic of living matter.21