156 CONCENTRATED FOOD-STUFFS 



enzyme. When linseed cake is macerated with water and allowed 

 to stand at blood-heat, hydrocyanic acid is very frequently pro- 

 duced, sometimes in considerable quantity, owing to the hydrolysis 

 of the glucoside by the ferment. 



149. Qualitative Examination for HCN. 



A few grams of the cake are soaked with water, and 

 kept, in a corked flask, in a warm place for a few hours. 

 The prussic acid may be detected — 



1. By the smell. 



2. By suspending over the mixture a piece of filter 



paper soaked in sodium picrate solution. In 

 the presence of HCN the yellow colour of the 

 paper changes to orange red. 



3. By steam distilling (see below) and testing as 



follows : — The distillate is made alkaline with 

 caustic soda, and two drops each of ferrous 

 sulphate and ferric chloride solutions added. 

 The mixture is just warmed, well shaken, and 

 acidified with a few drops of strong HCl. If 

 hydrocyanic acid is present a blue colour of 

 Prussian blue is produced, which in time settles 

 down as a precipitate. 



Estimation of Prussic Acid in Linseed Cake. 



This method may be employed for any prussic-acid-producing 

 feeding stuff (Java beans, sorghum, cassava, etc.). It distinguishes 

 between the total prussic acid which can be formed and the 

 "free," or "available," prussic acid which is actually produced 

 when the substance is soaked in water. 



150. {a) Total Prussic Acid. 



This figure, representing the amount of cyanogenetic 

 glucoside present in the cake or other feeding stuff, is 

 determined as follows: — A known weight (about 25 grams) 

 of the ground material is re-percolated with hot alcohol in 



