Production of lactic acid ( ) by the Lactobacillus facilitates coagulation of milk protein into curd. The milk proteins also get partially digested. The mould Aspergillus niger makes citric acid when molasses is used as the fermentation substrate. Acetic acid (vinegar) is made from ethyl alcohol by Acetobacter aceti. It oxidises the alcohol to acetic acid. Butyrate is produced as an end-product of a fermentation process solely generated by Clostridium butylicum and is used in the preparation of various butyrate esters.