iv PRESERVED MILK 211 



quantity of the following solution (Schardinger's reagent) 

 5 c.c. of saturated alcoholic solution of methylene blue + 5 c.c. 

 of formalin + 190 c.c. of distilled water was added to ordinary 

 normal milk it was decolourised in about ten minutes. The 

 test is carried out by mixing 20 c.c. of milk with 1 c.c. of 

 Schardinger's reagent, placing the mixture in a water-bath at 

 45-50, and noting the time which elapses before the colour 

 entirely disappears. 



This test depends upon the presence of a specific enzyme 

 the aldehydereductase in the milk. The enzyme is destroyed 

 more or less quickly at temperatures above 70, so that if the 

 milk has been heated to 80 the colour is not discharged at all. 

 If, on the other hand, the colour disappears after about 

 30 minutes, the milk was either heated for a few minutes at 75 

 or pasteurised for 15 minutes at 70. Further information 

 relative to the degree of pasteurisation can be obtained if the 

 test proposed by Bonnema l is used. This is carried out by 

 placing the milk which is to be investigated in flasks provided 

 with rubber stoppers. The flasks, which hold 100 g. milk, are 

 sterilised and then filled to the stopper, after which they are 

 placed in an incubator at 37, or failing this a water-bath can 

 be used. The nature of the fermentation which goes on in the 

 flasks admits of an estimate of the degree of pasteurisation 

 being formed. It is possible to distinguish between the 

 following main types of fermentation : 



1. Lactic acid fermentation. Homogeneous and solid coagu- 

 lum, with few bubbles of gas and little serum, indicates that 

 the milk was unheated, or pasteurised at a low temperature. 



2. Butyric acid fermentation. The coagulum swims as a 

 cake, permeated with gas bubbles, on the upper part of the liquid. 

 The serum below is clear. There is a lively evolution of gas 

 and the smell of butyric acid. The stoppers often blow out. 

 These signs denote a milk pasteurised between 75 and 90. 



3. An intermediate form between 1 and 2 indicates that the 

 milk was pasteurised for 15 to 20 minutes at 70. 



4. Peptone fermentation. At first the milk is evenly coagu- 

 lated and in fine particles. Afterwards a clear zone forms below 

 the layer of cream and spreads gradually downwards. In this 

 case the milk was pasteurised for 10 minutes at 95 or above. 



1 Chtmiker-Zeituny, 1905, Vol. XXIX, p. 182. 



p 2 



