For the next 20 years Pasteur studied various fermentation processes & came to the conclusion that all fermentation processes are accompanied by the formation of microorganisms. He also concluded that different fermentation processes contain different microbes. Moreover these occurrences of microbes are specific for specific fermentation (as evident by specific end products like alcohol and lactic acids). Pasteur differentiated these organisms microscopically according to shape & size and their favorable environmental conditions. For example, organisms from alcohol fermentation prefer acidic condition whereas that from lactic acid fermentation grows in neutral pH. So to grow lactic acid microbes one should use some neutralizing agents to prevent the acidic effect due to the formation of lactic acid (Pasteur used calcium carbonate i.e. chalk for this purpose).