SPECIAL INDUSTRIAL FERMENTED PRODUCTS 657 



used protected the skins against overbating and injury but at the 

 same time permitted effective removal of the lime and very satisfactory 

 bating or softening of the texture. Similarly, good results were ob- 

 tained by Noble at the University of California by using pure cultures 

 of B. subtilis in hay infusions or other cheap and suitable liquids. 

 The practice which he preferred was to heat an infusion of hay to boiling 

 and to allow this to stand until a good growth of B. subtilis had 

 developed. 



Patented mixtures of a pure culture of Bacillus erodiens or other 

 suitable organisms and a suitable nutrient medium in dry and soluble 

 form have been successfully employed to replace dung bates. A mix- 

 ture of pancreas extract and ammonium sulphate has been sold under 

 a trade name and used with fair success. It therefore appears that 

 the use of the dung infusions is not essential. 



At one time the skins from the bating liquor were transferred to 

 a fermenting infusion of bran known as the " drench" for 18 to 24 

 hours. Acid formed by B. jur juris and similar organisms removed the 

 last traces of lime. At the present time a " pickle" consisting of dilute 

 sulphuric acid and sodium chloride is commonly used to remove the 

 lime left in the skins from the bate liquor. The calcium sulphate so 

 formed crystallizes in the pickle vats as a hard incrustation. 



The skins are now ready for the tan pits. They enter the old and 

 more dilute tan liqu'ors first. In these the bacteria carried over on the 

 hides from the bate, etc., adapt themselves. Some lactic acid is formed 

 and tends to plump the skins and facilitate the penetration of the tannin. 

 Dilute tannic acid solutions must be used at first to permit deep pene- 

 tration of the tannin. The concentration of tannin is increased in 

 the succeeding pits until a saturated solution is reached. The extrac- 

 tive matter from the tan bark in the dilute liquors supports a varied 

 growth of lactic bacteria, yeasts and molds, while sufficient tannin 

 is present to check effectively the growth of putrefactive organisms. 

 Tan bark is used for ordinary leather while sumach and other light 

 colored tannin extracts are used for lighter colored leather. 



Tannin may be replaced by chrome alum or potash alum, or other 

 mineral tanning materials. Some of the toughest and most resistant 

 leathers are of this type. 



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