Penicillin was the first important commercial product produced by an aerobic, submerged fermentation. Acetone-butanol, which was a major product produced during and after World War 1, was produced in an anaerobic system and did not pose the oxygen transfer and heat transfer problems of the penicillin fermentation. In the 1930s, citric acid was produced by aerobic fermentation, but by surface fermentation in shallow trays and not by submerged fermentation in agitated deep tanks.