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(5). The production of citric acid from sugars was believed by Wehmer (60) to be characteristic of a particular group of Penicillium-like fungi which he appropriately termed “Citro- myces.” He, likewise, believed that the formation of oxalic acid was characteristic of the Aspergilli. The separa- tion of the two groups of fungi based on this physiological difference was later abandoned after Zahorski (66) and Thom and Currie (47) had shown that citric acid was readily pro- duced by many different strains of Aspergillus niger. Nu- merous other fungi have since been studied which produce citric acid from sugars. The citric acid process has been studied in great detail, and in much of this work Aspergillus niger has been em- ployed. Thom and Church (46) pointed out that the term “Aspergillus niger does not designate a definite strain or species” but is used to designate ‘ia whole group of black Aspergilli with fundamental characters in common.” The great variations in acid production by different Aspergillus niger strains which have been recorded is, therefore, not at all surprising. The activity of inpidual strains frequently varies so much that the stabilization or maintenance of a high acid-producing capacity of the organism is recognized as one of the most difficult problems encountered in dealing 6t& the process. Doelger and Prescott (20) recently car- ried out studies with the object of determining methods for maintaining uniformity of conditions which would result in consistent yields of acid in successive fermentations. The influence of various factors which affect the citric acid fermentation has been studied by many workers. In 1917 Currie (19) made a thorough investigation of the production of citric acid by selected strains of Aspergillus niger and showed that, by properly controlling the pH and the con- centrations of the inorganic nutrient salts, the proportion of citric and oxalic acids could be varied almost at will. He found that oxalic acid formation was almost completely suppressed under conditions which were most favorable for the formation of citric acid. The importance of a preliminary acidification with hydrochloric acid to pH 3.5 for suppressing oxalic acid formation, preventing undesirable spore formation and minimizing the danger of infection by other organisms, was clearly demonstrated. It is known that contamination difficulties were at least partially responsible for the failure of early attempts made in Europe to establish the fermenta- tion process on a commercial scale. This was undoubtedly due in part to the practice of neutralizing the acid formed with calcium carbonate. Acid sterilization has without doubt played an important part in the successful coriunercial- scale development of the process, both in this country and in Europe. The work of Currie (19), which was carried out in the United States Department of Agriculture, later formed the basis for the timely commercial development of the process in the laboratories of the Charles Pfizer Company, of Brooklyn. The process is usuaUy carried out between 2.5" and 35" C., although temperatures as low as 20" and as high as 40' C. have been employed. The optimum temperature probably depends somewhat on the organism used. Aeration is un- doubtedly an important factor, although little information concerning it is available. We may assume that relatively small amounts of air are required to supply the necessary oxygen for the life processes of the organism, since Porges (S9) and Doelger and Prescott ($0) have observed that large amounts of air adversely affected acid yields. It is impossible to make any general conclusions regardiiig the kind and amount of inorganic nutrients that ndl in all cases yield the best results for citric acid production. From the vast amount of work reported, it appears that the nu- trient requirements depend to some extent on the inpidnal characteristics of the organism nsed. Aside from the carbon and nitrogen sources, the only essential elements are po- tassium, phosphorus, magnesium, and sulfur as shown by Currie (19) and by Doelger and Prescott (20). Nonever, many investigators claim that small amounts of zinc and iron am essential for the best growth of the organism, and in some instances it seems fairly well established that these elements exert a beneficial effect on acid prodnetion. The essential elements, aside from carbon, oxygen, and nitrogen, are con- veniently snpplied as KNJQ in amounts varying from 0.03 to 0.1 per cent and as MgS01.7Hz0 in amounts from 0.01 to 0.05 pep cent. The highest yields of acid have nsnally been obtained from sucrose and fructose as the carbon source (3, 4, SG); in some instances glucose appears to serve about as well (I#, 53). Sugar concentrations of 15 to 20 per cent are necessary for high yields of citric acid. Kitragen is supplied either as ammonium salts or as nitrates. In most cases ammonium nitrate in concentrations of 0.16 to 0.32 per cent has been reported to give the higliest yields of acid.