322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 5 



spores of Aspergillus niger, and as these germinate the surface becomes 

 covered with a frail pellicle which rapidly develops and, by the end 

 of the fermentation (8 to 12 days), becomes a fairly thick and deeply 

 intricately folded mat but still quite white. Most of the sugar has by 

 then disappeared from the solution, its place being taken by citric 

 acid practically unaccompanied by other organic acids: the solution 

 is much more acid than good lemon juice. Some of the sugar, how- 

 ever, is converted into fungus starch and some into dextrin. The 

 standard way to recover the citric acid from the fermented liquor is 

 to add milk of lime and heat nearly to boiling point, then filter off the 

 difficultly soluble calcium citrate and wash with hot water. 



Another process in which Aspergillus niger is used is the produc- 

 tion of gluconic acid, the calcium salt of which is of importance in 

 pharmacy. Here the best results are obtained from well-aerated sub- 

 merged growth, which is most economically accomplished by using 

 rotatory drums. 



Molds are able to build up their normal cell constituents from an 

 amazingly large and varied series of carbon compounds. Moreover, 

 as seen with Aspergillus niger, the same species of mold, when growing 

 in slightly different conditions, can produce different substances. 

 Citric acid, oxalic acid, gluconic acid, ethyl alcohol, and mannitol, 

 which may be regarded as the break-down products of the original 

 sugars, are formed by many species. 



There are, however, many substances which are built up by mold 

 growth, and for the most part these are highly specific products of 

 a single species, or of a few related species. The chief worker in this 

 field has been Raistrick, who, following on his work on glycerol pro- 

 duction, has been engaged with numerous collaborators in investigat- 

 ing the general biochemistry of molds. The scheme followed has been 

 to use glucose as the sole source of carbon in a synthetic culture me- 

 dium — usually Czapek-Dox medium. In this long-continued and pro- 

 ductive research, about a hundred substances previously unknown to 

 science have been prepared and many of them synthesized. It may 

 well be that some of these will be found useful in some way or other, 

 but their present interest is chiefly in giving us a picture of what hap- 

 pens inside the cell. Mold pigments, simple quinones, polyhydroxy- 

 anthraquinones and hydroxyxanthones, simple benzene compounds, 

 chlorine-containing metabolic products, derivatives of tetronic acid 

 (stimulants of bacterial growth), antibacterial and antifungal sub- 

 stances are included in the products. 4 It is a matter of phylogenetic 

 interest that the lichen acid physcion (parietin) is formed by 16 species 

 or strains in the Aspergillus glaucus series. 



* A wide range of these special chemical compounds was exhibited. 



