Many microbes produce large quantities of a particular small molecule byproduct that can be readily extracted in a large scale using the fermentation technique. For example the yeast Saccharomyces cerevisiae produces ethanol, the fungus Aspergillus niger makes citric acid, Acetobacter aceti makes acetic acid, the bacteria Clostridium butylicum makes butyric acid, and the bacteria Lactobacillus make lactic acid.