Basically, it is the process of alcoholic fermentation. It just requires that yeast is present. In this process, sugars like glucose, sucrose or fructose are converted into cellular energy or ATP and the two waste products produced by the yeast are carbon dioxide gas and ethanol. This occurs anaerobically, thus it is a type of anaerobic respiration. Even if oxygen is present, yeasts found in baker's yeast will still carry out fermentation. They produce ethanol even under aerobic conditions as long as there is a source of energy--sugar. For example, in bread dough, the yeast will ferment the sugar and produce ethanol--an alcohol as well as carbon dioxide which makes the bread rise. In the oven, the ethanol evaporates while the bread bakes. However, if someone ferments grapes, they will produce wine and if someone ferments grains, they will make beer. In this case, the ethanol will ferment the sugars and it will produce an alcoholic beverage.