THE FERMENTATION OF INDIGO 119 



Bacillus cyaneo-fuscus is very susceptible both to desiccation on which 

 account it is not found in atmospheric dust and high temperatures, as also to 

 acids. This explains why the microbe cannot be cultivated from the blue granules 

 in old ripe cheese, since it is no longer alive therein, but has succumbed under 

 the influence of the lactic acid (of which Edam cheese, for example, contains 

 between 1.3 and 1.8 per cent.) produced from lactose by the other bacteria in 

 the cheese during the ripening process. Considered from this standpoint, the 

 good result obtained by so-called ropy whey (a culture of long thread and lactic 

 acid bacteria, more particularly described in chapter xxix.) as a preventive of 

 blue grain becomes obvious. Since, as already stated, oxidising agents destroy 

 the colour, it has been proposed by De Vries to decolorise blue cheese by ex- 

 posing it for some time to the action of oxygen in a closed vessel. 



This Bacillus cyaneo-fuscus (not infrequently met with in natural waters and 

 soils) is the sole species that as yet has been positively identified as capable of 

 producing blue grain in cheese. It is uncertain whether this faculty is also 

 inherent in other organisms, though Hollman, who, in his " Handboek voor den 

 Kaasmaker " ('' Cheesemaker's Handbook "), deals thoroughly with this disease, 

 ascribes the responsibility for its appearance to Bacillus cyanogenus an assump- 

 tion which, however, according to the inoculation experiments conducted in this 

 connection by Adametz and Beyerinck, is untenable. 



For the sake of completeness it should be recorded that, according to Beyer- 

 inck's report, Bacillus cyaneo-fuscus has been found to make itself unpleasantly 

 apparent in a glue factory by giving rise to black glue, which, by reason not 

 only of its undesirable colour (due to the pigment developed by the microbe), 

 but also of its diminished setting power, was thus seriously depreciated in value 

 and merchantable quality. The source of infection was discovered in a dirty 

 pipe previously used for the conveyance of ditch water, and afterwards for 

 delivering the finished glue into the setting pans. When this pipe was cleansed 

 the evil disappeared entirely. 



96. The Fermentation of Indigo. 



As is well known, the indigo so highly prized for dyeing, and constituting such 

 an important article of the world's commerce, is obtained from certain species of 

 the Leguminous genus Indigofera. The province of Bengal alone produces from 

 Indigojera tinctoria over twelve millions of pounds of indigo per annum, the 

 value being about ^2, 000,000 sterling. Some five hundred thousand workers 

 make a livelihood in that province by the cultivation and treatment of this plant. 

 The colouring matter does not exist ready formed in the plant, but is developed 

 therefrom by the fermentation of a glucoside constituent known as indican. The 

 plants are cut down shortly before flowering-time, and are left immersed in a 

 five to eight-fold quantity of water, to undergo a continuous fermentation for 

 eight to fifteen hours at an atmospheric temperature of 25-35 0., the liquid 

 gradually becoming yellow in colour, with an alkaline reaction, and throwing up a 

 blue-violet scum. The most important of the fractional processes constituting this 

 fermentation is the splitting up of the indican into sugar (indiglucin) and indigo 

 white, which remains in solution in the alkaline liquor. The liquor is then drawn off 

 into another vessel and beaten with rods, whereby numerous and fresh points of 

 attack are presented to the air and the indigo white (C 16 H 12 N 2 O 2 ) is oxidised by 

 atmospheric oxygen into indigo blue or indigotin (C 16 H 10 N 2 2 ), which is precipitated 

 as an insoluble body and deposited at the bottom of the vessel. The sediment is 

 then boiled in a pan, strained through cloths, dried, and brought into commerce 

 in the form of irregular fragments, cubes, balls, &c. 



Several noteworthy observations on. the organisms taking part in this 



