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This week’s honor goes to Louis Pasteur, the French biologist and chemist best known for his invention of pasteurization. Pasteur is a towering figure in the history of science for his groundbreaking discoveries, and his contributions have helped save or improve countless lives beyond his generation.

Here is a list of his major scientific contributions and the back stories behind them.

1. He discovered the existence of molecular asymmetry.

For a number of years, scientists had been puzzled about organic chemicals such as tartaric acid. A large number of natural organic substances had been found to rotate the plane of polarized light to the left or right, while the apparently identical substances made in the laboratory did not. Pasteur, who had made a study of polarized light for his physics thesis, discovered that molecules could exist in mirror-image forms; and his study demonstrated molecular chirality and even provided the first explanation of optical isomerism. Pasteur thus laid the foundation for stereochemistry and paved the way for crucial breakthroughs in chemistry, biochemistry, and pharmaceuticals.