Organic acids

Small quantities of amino acids such as glutamic acid, tryptophan, and lysine are produced by microbial fermentations and used in food flavoring. However, microbes are also used to produce bulk organic acids, the most important of which are acetic and citric acids. Acetic acid is the active ingredient of vinegar, and although vinegar itself is a complex mixture of many compounds and flavors, the key reaction in its production is the microbial oxidation of alcohol to acetate. Vinegar has been produced for thousands of years, but in modern controlled processes, Gluconobacter or Acetobacter are used to carefully convert any alcohol-containing liquid (e.g.  Wine, cider, fermented malt) into acetic acid.  The fermentation occurs in vats or the Bacteria are immobilized onto wood shavings so that a simple trickle filter column is made. Alcoholic liquid is poured in at the top, trickles past the Bacteria, where the oxidation takes place, and then product can be collected at the bottom.

Citric acid is used very widely to give acidity to foods and to enhance fruit flavors and forms a major constituent of some carbonated drinks. It is produced by an aerobic fermentation of sucrose by the mold Aspergillus niger as a secondary metabolite. The organ- ism overproduces citrate during stationary phase to sequester iron, and this has been exploited and enhanced by biotechnologists.