Pasteur showed that yeast was responsible for turning sugar into alcohol. In the 1850s, he investigated why beetroot-based alcohol and wine sometimes fermented properly and sometimes became sour. His experiments demonstrated that alcoholic fermentation was linked to living yeast cells, not merely to the chemical decomposition of sugar.
Yeast converts sugar into ethanol and carbon dioxide when oxygen is limited. Pasteur’s work helped establish that different microorganisms produce different chemical changes: yeast produces alcohol, while other microbes can produce lactic acid and spoil wine or beer.
A common mix-up is confusing yeast with mold. Both are fungi, but the yeast used in alcoholic fermentation is primarily Saccharomyces cerevisiae, a single-celled microorganism. Pasteur’s fermentation studies also helped lay foundations for microbiology and eventually influenced food preservation through pasteurization.