1 6 GENERAL CONDITIONS AFFECTING BACTERIA IN MILK 



the percentage of fat in milk, and the soHds not fat frequently 

 decrease. The average quality of milk is usually lower during the 

 spring and early summer, that is, especially during the months of 

 April, May, and June, than at other times of the year. Analyses 

 of the milk supplied to the Aylesbury Dairy Company show that, 

 on the average of ten years, the percentage of fat is lowest in May 

 and June, the months coming next being April and July. The 

 period of lowest quality may, however, vary slightly in particular 

 years. The relative poverty of milk in spring is mainly due to 

 two causes, viz., the number of freshly-calved cows coming in the 

 ordinary course into milk at that time, and the stimulating effect 

 of the first flush of grass exciting an increased flow and a conse- 

 quent lowering of quality. 



(5) Interval between times of milking. — Perhaps the greatest 

 variation in the quality of milk as regards fat is that which arises 

 from the difference in the interval of time which elapses between 

 one milking and another. That the milk yielded by the cow in 

 the morning is almost invariably less rich in milk fat than that 

 yielded in the evening is a well-known fact. It may be accepted 

 as a general axiom that the longer the interval the greater is the 

 quantity of milk and the lower is its quality, and vice versa. If 

 the day were divided into two equal periods, this difference 

 between the evening and the morning milk would be greatly 

 reduced. But such an ec^al division is, for various reasons, 

 impracticable.^ 



(6) Miscellaneous circumstances. — There are a number of minor 

 circumstances which are held to influence the composition of milk, 

 such as change of milkers, methods and rapidity of milking, ex- 

 posure to rain, unusual excitement, or temporary or trivial illnesses. 

 Thus individual animals of the same herd have a continually 

 varying milk capacity and quality. This point in regard to a cow's 

 individual variation in quality of milk supply is one of some 

 importance. It may be well illustrated by two features to which 

 attention has recently been called by Professor Rotch of Harvard.^ 

 In the first place, not only do the conditions we have discussed 

 affect the individual cow, but the quality of the milk varies 

 chemically as well as bacteriologically in the course of a single 

 milking. We shall have occasion to refer to the bacterial variations 



1 See also Droop Richmond's Annual Returns for the Aylesbury Dairy 

 Company — Jour, of State Medicine, vol. viii. (1900), p. 830, and four, of Brit. 

 Dairy Farmers Assoc, vol. xv. 



2 Brit. Med. Jour., 1902, vol. ii., p. 657. 



