Yeast is a living organism used in baking bread and brewing beer. It was an ancient ingredient used in Egyptian cuisine. Some people are sensitive to the lactose and bacteria naturally found in milk. Louis Pasteur discovered that bacteria causes food spoilage when food is heated in an uncontrolled environment. Drinking contaminated water can spread illnesses, and chlorine can help remove bacteria. While most industrial waste is non-toxic, industries like mills and factories produce waste. Sewage bacteria can help clean water but also pollute it, and treating sewage is a long mechanical process.