CHAPTER XXXVni 



BACTERIOLOGY OF MILK 



The following table summarized from Van Slyke's anal- 

 yses will demonstrate that milk is an exceedingly complex 

 mixture of substances : 



rWater 87.1 rAlbumen . . 0.7 



Milk ...i f Fat 4.0 J 



I Solids ... 12.9 -^ Solids not f Proteins 3.3-) 



fat 8.9^ Milk-sugar. . 4.9 1 Casein 2.6 



I Ash (salts) . . 0.7 ^ 



100. 12.9 8.9 33 



Some of the constituents probably are in true solution. 

 Such, for example, are some of the salts and the milk sugar. 

 Others, such as the proteins, are in colloidal soliition, and 

 the fat in the form of microscopic globules forms an emul- 

 sion. It will be noted that about 87 per cent of the milk is 

 made up of water and IS per cent solids, of which about 

 4 per cent should be fat and the remainder milk sugar, 

 protein and ash. 



Fermentative and Other Changes in Milk. — Common 

 observation of the changes which occur in milk when it is 

 allowed to stand after milking, show that there are several 

 which follow each other in sequence. Ordinarily there are 

 differentiated the stages of slight souring, curdling and 

 eventual digestion of the curd. Careful laboratory studies 

 show that the number of stages is really greater than those 

 usually enumerated. They may be listed as follows : 



1. Bactericidal stage. 



2. Development of lactic acid. 



3. Neutralization and fermentation of lactic acid. 



4. Decomposition of the milk proteins. 



In addition to the stages listed, milk frequently undergoes 



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