2 GENERAL CONDITIONS AFFECTING BACTERIA IN MILK 



to the bacteriology of the dairy than further extensive additions to 

 the already formidable number of species. Hence our investiga- 

 tions have been specially devoted to such a general study rather 

 than to the isolation of supposed new organisms, or of such as do 

 not exactly answer to any of the already published descriptions of 

 bacteria. We would indeed go so far as to suggest that the more the 

 conditions of growth, culture, and environment are studied the more 

 is it likely that there will be a reduction rather than an increase 

 in the number of clearly differentiated species of milk organisms. 



However that may be, and however much or little such a result 

 will in the future be due to clearer nomenclature, we are convinced 

 that a careful study of those conditions which in one way or 

 another affect all bacteria, and especially bacteria in milk, is of the 

 first importance to the bacteriologist in a clear understanding of 

 their role. 



General Properties and Composition of Milk 



It is desirable then to state briefly the chief physical and chemical 

 properties of milk.^ Milk consists, like the blood, of two main 

 elements, a fluid or milk plasma^ and suspended therein innumerable 

 minute globules of fat. It is therefore when fresh a perfect emul- 

 sion, and its white colour is produced, as in other emulsions, by 

 reflection of light from the surface of the globules. In addition to 

 the fat globules there are also in suspension smaller particles of 

 proteid matter.^ 



The globules of fat vary in size, but probably the average 

 diameter of the globules in cow's milk is about g^Vir of ^'^ inch. 

 The size varies considerably with different breeds, and even with 

 different cows. As a rule the fat globules of Jersey and Guernsey 

 milk are large and uniform, and of the Ayrshire small and 

 variable. Lloyd has, however, pointed out that the presence of 

 exceptionally large fat globules in milk is rare, and uniformity 

 in size is the rule (varying from -oi to -0016 mm. in diameter). 

 He states that in Jersey milk the globules appear to have a narrow 

 range of variation in size whilst in Shorthorn the range is con- 

 siderable.^ The number of globules in a given volume of milk 

 varies greatly according to their size and in the percentage of fat. 



^ It is only necessary in a book of this character to refer to those points which 

 have a relationship direct or indirect to bacteria in milk. The general 

 chemistry of milk will be found recorded in Foods: Their Composition and 

 Analysis (Wynter Blyth), 1903, pp. 189-310. 



2 Text-book of Physiology, edited by E. A. Schafer, F.R.S. {W. D. Halli- 

 burton, F.R.S., p. 125.) 



'^ fpurnal of Bath and West of England Society, vol. xii. (1902), p. 128. 



