Some microorganisms such as yeast produce energy in oxygen-free environments through anaerobic cellular respiration. Anaerobic cellular respiration is where oxygen is not used. Yeast produces energy by converting sugars into alcohol, gas or acetic acid. This process is called fermentation. Ethanol is a by-product of fermentation. Humans have used fermentation to produce food and drink since the Neolithic Age. Fermentation makes yoghurt from milk by producing lactic acid. It also produces alcoholic beverages such as wine from grapes.
