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A highly interesting process for the conversion of acetic acid and acetates to citric acid by yeast was patented by Boehringer (10). This process is based on the original discovery of Wieland and Sonderhoff (53) who found that aerated yeast cultures produced both succinic and citric acids from acetates. A dilute solution (0.2 N) of sodium or calcium acetate is fermented with yeast under highly aerobic conditions maintained by stirring the solution vigorously in the presence of air or oxygen. Eight hundred and Bty grams of citric acid and 230 grams of succinic acid were obtained from 6000 grams of acetic acid during a fermentation period of 12 hours. The chief importance of this discovery lies in the fact that a submerged yeast growth is employed for the process, making possible the use of tanks or vats as culture vessels. The reported yield of citric acid is low, but further research might reveal conditions that would lead to im- provements in this respect. LABORATORY METHODS OF CULTURING SURFACE MOLDS IN TUBES AND FLASKS Zender (56) recently patented a process for the production of citric acid by mold fermentation in which the acid may be crystallized directly from the culture solution, thus eliminat- ing the usual step of first separating the acid as the calcium salt. The mold-fermented culture solution is fermented with yeast in the presence of a soluble calcium compound to remove the residual carbohydrate, the calcium is removed with oxalic acid, and the citric acid is then recovered by crystallization. Properties and Uses of Citric Acid Citric acid was first isolated in the solid state by Scheele in 1784 from lemon juice. It crystallizes in large rhombic prisms containing one molecule of water of crystallization and has the formula CsHg07,H20. The anhydrous form of the acid, which is now commercially available, crystallizes in monoclinic prisms. It is readily soluble in water, moderate:y in alcohol, but only sparingly in ether. The water of crystal- lization of the hydrated form is entirely lost at 130"; it melts at 153" and, on further heating above 175", decomposes into aconitic and itaconic acids, citraconic anhydride, carbon dioxide, and acetone, Chemically, citric acid is a tribasic acid forming three series of salts which are well defined. The alkali salts are soluble in water, the others are mostly in- soluble. Citric acid is used in the manufacture of citrates, flavoring extracts, confectionery, soft drinks, effervescent salts, as a silvering agent and as an ink ingredient in engraving, in dyeing and calico printing, and in medicine. According to the United States Department of Commerce, about 65 per cent of the domestic consumption was recently reported to be used for medicinal purposes, including the manufacture of citrates, 15 per cent in foods, 9 per cent in candy, and smaller amounts in silvering, engraving, as an ink ingredient, and in dyeing and calico printing. Recent research on synthetic resins of the alkyd type revealed the importance of citraconic and itaconic acids and their anhydrides as raw materials. Substantially better yields of these products from citric acid were obtained recently; an enormous potential use of this acid (9) is thus indicated. The use of citric acid in the manufacture of edible synthetic ester resins which may be employed as substitutes for chicle was suggested by Ellis (21). The use of citric acid in Italy in the manufacture of 1-phenyl-3-methylpyrazolone which is used as an interme- diate in the production of antipyrine and certain azo colors was reported by Corbellini (17). Natural Citric Acid Industry Citric acid occurs in many plant tissues, especially those of the citrus variety. In citrus fruits and in sloes, cran- berries, etc., it occurs free and with very little malic acid; in fruits such as the cherry, strawberry, and raspberry it occurs with an approximately equal proportion of malic acid. The chief commercial sources for the natural variety of acid are lemons, limes, and pineapples. Another potential source of large quantities of citric acid is tobacco waste but, so far as is known, no commercial development of a process making use of this material has yet occurred, although several patents have been granted to Shmuk (62). Italy still produces the largest quantity of natural citric acid; the only other important sources are California, the West Indies, and Hawaii. The Sicilian citrus by-products industry was described in detail by Cruess (I@, Molinari (Sti), and Galeano (b7), so that only the more important features of this industry will be mentioned here. The larger part of the citrus fruit grown in Sicily is pri- marily for export in the fresh state. The normal annual production of lemons in Sicily is 300,000 tons, but, unless the fresh fruit market is unfavorable, little normal fruit goes into the by-product industry. However, the proportion of in- ferior fruit grown in Italy is very high (30 to 50 per cent) so that large quantities are available for the production of lemon oil and calcium citrate. Most of the Italian calcium citrate was formerly exported, but since 1921 large quantities of this raw material have been consumed for the manufacture of citric acid in the domestic citric acid industry.