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Bernhauer and Iglauer (Y), and Wells, Moyer, and May (52) reported results in which the citric acid yields obtained from glucose and sucrose greatly exceeded the highest yields obtainable from these sugars if the initial reactions followed those of the usual alcoholic fermentation of yeast. The mechanism recently proposed by Emde (ag), in which quinic acid was suggested as an intermediate, can likewise be ruled out since both the yield of citric acid and carbon dioxide production required by that theory do not agree with experimental data. Any satisfactory theory for the mechanisms of citric acid formation by fungi must take into account the quantitative data which have been accumulated, and the fact that citric acid formation by fungi from 2-, 3-, 4-, 5-, 6-, 7-, and 12- carbon compounds has been well established. In spite of all the work done on this problem, the mechanism of the mycological production of citric acid is still obscure, and no theory so far proposed accounts for all of the observed facts. The mycological production of citric acid has been widely patented both in the United States and in Europe. However, the one successful commercial process in this country was developed only after years of intensive biochemical and engineering research; consequently the actual details of operation have been closely guarded as trade secrets. The process undoubtedly consists of a shallow-pan fermentation of pure sucrose or of blackstrap molasses by a strain of A. niger; the reaction is probably completed in less than 9 days. The patent literature dates back to 1893 when Wehmer (51) secured broad patents ewering the process which he had discovered. The industrial application of Wehmer’s patent was attempted in Germany (28) about forty years ago, but, because of the numerous difficulties encountered, it was soon abandoned. In 1913 Zahorski (56) was granted a patent for the produc- tion of citric acid from sugars by the fungus Xterigmatocystis nigra. A preliminary acclimatization of the organism to high concentrations of citric acid was claimed to result in higher yields of acid than had previously been obtained. Falck (23) patented a process for the production of acids based on the use of solid starchy substrate and organisms of the Aspergillus, Penicillium, or Citromyces groups. The pro- duction of citric, malic, tartaric, and succinic acid is claimed, PATENTS. but no details as to methods of recovery or inpidual yields of the acids are given. The patent of Bleyer (8) specifies a preliminary acidification to pH 3.3 and the use of an abun- dant supply of air to maintain temperature control and to remove carbon dioxide. The patent granted to Szucs (49) in 1928 claimed the pro- duction of citric acid from molasses by fungi of the Citro- myces, Mucor, Aspergillus, or Penicillium groups. With a strain of Aspergillus it was claimed that a temperature around 20 O C. suppressed oxalic acid formation, inhibited infections, and gave high yields of citric acid. Fernbach and Yuill (24) patented a process in which the novel feature lies chiefly in the elimination of sterilization by heat through the addition of sufficient hydrochloric or sulfuric acid to the sugar solu- tions to give a pH of 1.2 to 2.5. Kanhauser (30) was granted a patent for the production of citric acid in which it is claimed that improvements in the process are effected by cultivating the organisms in alternating stages on solid agar media, on media corresponding to those used industrially, and on vegetable media containing organic acids and vitamins. It is further claimed that the presence of substances which form complexes of high molecular weight with citric acid is beneficial. According to patent specifications, Cahn (13) succeeded in producing citric acid in good yields in a short fermentation period by mold fermentation of sections of sugar-containing plants, such as sugar beets and artichokes; by fermentation of plant residues, such as sugar cane bagasse and spent sugar beet cossettes, impregnated with sugar solutions; and from sections of starch-containing material such as potato dices. It was further claimed that the fermentation may take place in closed tanks or vats. This feature would be a dis- tinct advantage over the shallow-pan process in reducing the cost and amount of equipment required and in simpli- fying manipulation procedures. Lilly (32) was granted a patent on an apparatus and method for the production of citric acid. The apparatus consists of a number of long, porous, flat, hollow tubes suspended within a closed outer casing. The outer surface of the porous tube is inoculated with a suitable organism, and a regulated stream of nutrient sugar solution is allowed to flow by gravity through the tubes. Oxygen is supplied to the organism by passing sterile air through the outer casing. Since no yield figures are given, it is not possible to determine how effectively this method operates. The patent of Frey (26) describes an apparatus for citric acid production in which a number of superimposed shallow pans are so arranged that the nutrient solution may be drawn off to a large vessel, the citric acid removed by treating with calcium carbonate, and the solution recirculated through the pans. A yield of 40 per cent citric acid in a period of 15 days is reported. In 1936 Nussbaum (37) obtained a patent for producing citric acid in which mixed cultures, especially A. niger and Mucor piriformic, are employed. The addition of mineral salts of uranium, manganese, and zinc in the proportion of 0.001 to 0.005 per cent is claimed to stimulate acid produc- tion. It is further claimed that addition of 1 to 15 per cent of citric acid favors the rapid development of the citric acid enzymes.