292 BACTERIOLOGICAL CHEMISTRY 



Oxalic Acid. — Oxalic acid was recognised quite early 

 in the form of crystals of calcium oxalate in many moulds. 

 Wehmer in 1891 showed that it was a fermentation 

 product and could be produced by Aspergillus niger 

 from a variety of substrates, including glucose. He 

 demonstrated that large yields of calcium oxalate can 

 be obtained by maintaining the medium more or less 

 neutral by the addition of calcium carbonate, which 

 precipitates the oxalic acid as fast as it is formed. Other 

 species of Aspergillus and some species of Penicillium also 

 give oxalic acid in good yield. 



Citric Acid. — Citric acid was first described as a mould 

 product by Wehmer in 1893, who claimed that it was pro- 

 duced only by two species of Penicillium-like organisms 

 for which he created the genus Citromyces. However, he 

 himself showed later that citric acid formation is not 

 so restricted, but is, in fact, a common characteristic of 

 many species of Penicillium and Aspergillus, particularly 

 of the black species of the latter. By suitable adjustment 

 of the conditions of fermentation, namely, by growth in 

 a medium at pH 1 to 2 instead of the more usual ^H 6 to 7, 

 it is possible to suppress almost entirely the production 

 of oxalic acid by A. niger and to obtain citric acid as 

 almost the sole product. The strong acidity inhibits the 

 growth of bacteria, yeasts and most other fungi, which 

 makes the process very useful and easy to control 

 industrially. It is used in America for the large-scale 

 production of citric acid. 



Gluconic Acid. — Gluconic acid seems to be an almost 

 constant product of fermentation by A. niger. It was 

 first reported as such by Molliard in 1922. He showed in 

 subsequent work that it was possible to produce oxalic, 

 citric or gluconic acids at will as the main product by 

 varying the proportion of the mineral constituents of 

 the medium. Reduction of the amount of phosphate 

 and nitrogen to a minimum gave greatly increased yields 

 of gluconic acid, as much as 80 to 90 per cent, being 



