CHAPTER XVI 



SULPHITATION 



The discharge of the colouring matters of cane juices by acids has been 

 akeady mentioned, and to this property is due in the main the extended 

 use of sulphurous acid in the manufacture of white and yellow consumption 

 sugars. Sulphurous acid in addition is a reducing agent, and it may have some 

 further action on the colouring matters due to this property ; granted, how- 

 ever, that such an action obtains, the results would be only temporary, the 

 colour being restored on exposure to the air follo\\'ing on oxidation. To such 

 a cycle may be ascribed the darkening which is frequently obser\''ed when 

 plantation white sugars are stored for an}^ length of time. Apart from the 

 action of sulphurous acid on the natural colouring matters, it has a specific 

 action on the ferric salts, which lind their way into the juices from the pipes 

 and containers. These ferric salts form very dark-coloured compounds 

 \vith the polyphenols expressed from the cane, and also ^^^th the lime-reducing 

 sugar decomposition products, which, according to Schneller,^ are akin in 

 structure to the polyphenols. These dark-coloured bodies are nothing else 

 than inks. The ferrous compounds to which they are reduced b}' sulphurous 

 acid are, however, colourless, and Harloff and Schmidt^ distinctly state 

 that these do not crystallize with the sugar, so that in their absence there is 

 no darkening of sugar due to tliis cause. Sulphurous acid shares with any 

 other acid the power of protecting juices from discoloration on boihng. 

 As already stated, the action of alkalies on reducing sugars results in the form- 

 ation of dark-coloured bodies, and a darkening to this cause is well known to 

 occur between the defecated juice and the syrup in the ordinan,- defecation 

 process. Such a darkening also occurs in the carbonation process and is 

 here caused by the permanent alkalinity due to the potassium carbonate.* 

 Suspended calcium carbonate due to bad filtration is also sufficient to cause 

 this change. If then the juices coming from the second carbonation, or 

 the first where only one is used, be rendered acid before evaporation they will 

 maintain their hght colour and will be especially adapted for the making 

 of white sugars. Such a scheme, using sulphurous acid as the acid, was 

 introduced into Java by Harloff, and is known as the acid thin-juice process. 

 Acid thin-juice processes without carbonation had, however, been used for 

 many years previously in Louisiana, Demerara and Mauritius, although no 

 detailed account of them seems to have been given. 



The use of sulphurous acid was first suggested by Proust^ in 1809, and its 

 application forms the subject of French patent 2543, 1829, granted to 

 Dubrunfaut. The earhest British patent is that of Stolle (7573, 1838), 

 which describes its apphcation much as it is now used. Its introduction 

 is, however, due to Melsens,* who in 1849 published a paper which had a 



* In the Battelle process, with complete elimination of the reducing sugars, this discoloration should not occur. 



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