Fermentation has been used for thousands of years to produce foods and beverages. Louis Pasteur's work in the late 1800s greatly advanced understanding of fermentation and identified yeast as the agent that causes the process. Fermentation involves microorganisms breaking down organic compounds to produce energy without oxygen. It is used commercially to produce various products like alcohols, acids, enzymes, and other chemicals through aerobic or anaerobic microbial growth processes using batch, fed-batch, continuous, or immobilized cell cultures. Proper control of fermentation conditions inside bioreactors is important for consistent, high-yield production.