Analysis of Sour Milk. 



local authorities in Great Britain in which they state that except 

 in a limited number of exceptional cases to be hereafter referred 

 to, there is no practical difficulty in accurately inferring the 

 composition of the milk when fresh from the analysis of the 

 milk when sour. 



The change which takes place in a sample of milk kept from 

 contact with air, as in a bottle nearly full of the sample, and 

 fitted with a good sound cork sealed with wax, is, as a rule, 

 comparatively slight. The causes and nature of this change 

 have been carefully studied by many observers, and they have 

 been found to be perfectly definite in character. Without 

 going into details concerning the fermentative changes to which 

 milk is liable, it may be stated that the changes which affect the 

 analysis, and, therefore, the inference to be drawn from the 

 results, are concerned with the non-fatty solids only, and more 

 particularly with the milk-sugar. The milk-sugar gives rise, 

 either proximately or remotely, to a variety of products, the 

 most important of which are lactic acid, ethyl alcohol, and acetic 

 acid ; but it can be shown that the only quantitative determi- 

 nations which need be made in order to determine the loss in 

 the non-fatty matter by keeping are the proportion of alcohol, 

 reckoned as proof-spirit, and the amount of free volatile acid, 

 together with the ammonia derived from the alteration of the 

 casein, or proteid substance, in the milk. The slight alterations 

 in weight consequent on the hydrolysis and conversion of 

 lactose into lactic acid, and the formation of certain so-called 

 bye-products of alcoholic fermentation, are partly positive and 

 partly negative in direction, but their joint effect is too small to 

 have any appreciable influence on the result. 



The entire correction, which, of course, is always additive, in 

 the case of a properly-preserved sample from three to six weeks 

 old, is fairly constant, and may be said to range from 0*2 to 0*3 

 per cent. In a few cases it has been found to be as low as o - i 

 per cent., and in very exceptional cases, as in badly-secured 

 samples, or in bottles only partially filled, it has risen to 07 or 

 0'8 per cent. 



If the fermentation has passed into the butyric acid stage, the 

 amount of free acid is greatly increased, and owing to the sepa- 

 rated casein it is sometimes impossible to get the sample into a 



