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Porges ($9) found that sodium nitrate was superior to either ammonium nitrate or ammonium sulfate as a nibrogen source in a concentration of 0.4 Der cent. and acid was roughly proportional to this ratio. Doelger and Prescott (20) found a similar relation in studies on the pro- duction of citric acid. Large-Scale Operation In present industrial practice shallow aluminum pans are used for the prodnetion of citric acid from sugars by mold fermentation. The pans must be constructed with high- purity aluminum t.o prevent corrosion and to avoid the harm- ful effect of other metals such as copper,iron, etc. Theproc- ess consists essentially in tile inoculation with spores of a suitable organism of the sterile sugar solution contained in the aluminum culture vessels. A continuous felt of mycelium forms over the entire surface of the solution within 2 days, and formation of citric acid occurs rapidly aft.er the fourth day. The fermentation is usually complete in 7 to 10 days after inoculation. The solution is then drained off, the myeelium is pressed to remove any acid present in the tissues, and the acid is either crystallized directly after a yeast fermentation of the residual carbohydrate or first separated from the solution as the calcium salt from which the acid may be recovered by crystallization after treatment with sulfuric acid. The yield of citric acid obtained is approxi- mately 60 per cent by weight of the sugar taken, Little is known regarding the actual details of industrial operation of the fermentation, but the difficulties involved rnnst be enormous. May et al. (S4), in semiplant-scale st.ndies on the production of gluconic acid, used aluminum pans 43 X 43 X 2 inches (109 X 109 X 5 em.) which had an optimum charge capacity of 48 liters of culture solution. Based on a 7-day process and a 50 per cent weight yield of acid from 20 per cent sucrose solutions, it is calculated that from 12,000 to 16,000 pans of this capacity would be re- quired to manufacture the estimated 7,000,000 pounds of fermentation citric acid produced annually in the United States. It is not surprising that the details of such a process are kept secret. MIICHAXISM STUDIES. Most of the recent literature dealing with citric acid has been concerned with efforts to explain the mechanism of the reactions involved in the con- version of glucose and other sugars to citric acid. Clial- lrnger (14) and May and Herrick (3s) summarized the various theories which had been proposed up to about 1930. The more recent theories have been concerned with mecha- nisms involving acetic acid as one of the important inter- mediate substances. The acceptance of this acid as an intermediate in the process naturally led to the assumption that its formation was preceded by a breakdown of glucose similar to that encountered in the alcoholic fermentation of yeast. Chrzaszcz and Tiukow (16) proposed that citric acid is formed from acetic acid through the following steps: acetic acid -+ succinic acid 3 fumaric acid -+ malic acid. The latter by condensation with another molecule of acetic acid then forms citric acid, according to their view. Bernhauer (6) proposed a similar scheme in which succinic acid reacts with acetic acid to form tricarballylic acid which, on oxida- tion, yields aconitic and finally citric acid. However, in spite of all the favorable evidence to support these views, it has been shown that mechanisms involving acetic acid as an intermediate in the citric acid process, in which it is assumed that the initial reactions leading to its formation follow the general chemical equation which repre- sents the alcoholic fermentation, can be ruled out on purely quantitative evidence. Butkevich and Galvskaya (la), Chrzaszcz and Peyros (15),