MILK 201 



mental phenomena associated with the keeping of milk, and every 

 milkman should therefore understand as a foundation of dairy prac- 

 tice that the keeping of milk is dependent upon, preventing or 

 checking the multiplication of bacteria, rather than upon simply 

 preventing their presence in milk. 



A second fact which is not so thoroughly appreciated, but is 

 equally true, is that the rapidity of growth of all species of bacteria 

 is dependent upon temperature. Within certain limits the rate of 

 multiplication rises with the increase and falls with the decrease 

 in temperature. At a temperature of freezing, bacteria do not grow 

 at all, and milk, therefore, if frozen, may be kept indefinitely with- 

 out any changes taking place therein. If the milk is kept at a few 

 degrees above freezing, the growth of bacteria begins, but at low 

 temperatures this growth is extremely slow. As the temperature 

 rises, the rapidity of bacterial growth increases. When the tem- 

 perature reaches 70, bacteria grow very rapidly; at a temperature 

 of 80 and 90 they grow more rapidly still; and at a temperature 

 of about 100 the growth of some species of bacteria is most rapid 

 of all. All this is generally understood, but it is not generally rec- 

 ognized that if the temperature is raised somewhat above these 

 higher limits, the bacteria do not grow so rapidly. If the tempera- 

 ture is raised to 120, most of these organisms find conditions un- 

 favorable to their life, and grow very slowly ; indeed many of them 

 cease to grow at all. At temperatures above this the ordinary milk 

 bacteria entirely fail to develop. From these facts it will be seen 

 that in general the growth of the kinds of bacteria that produce 

 trouble in milk will be found between temperatures of freezing and 

 a little above 100 F., and that the greater the temperature, within 

 these limits, the more rapid is the development of the bacteria, and 

 hence the more rapid the spoiling of the milk. 



From these facts of course it follows that the keeping of milk 

 will be very closely dependent upon temperature. At high tempera- 

 tures (90) milk will sour very rapidly. At somewhat lower tem- 

 peratures (70) the souring is not quite so rapid, but still it takes 

 place in a comparatively few hours. At lower temperatures still, 

 in the vicinity of freezing, the souring and all other changes may 

 be delayed for a long time; and if milk can be frozen, it may be 

 retained indefinitely without any appreciable change taking place 

 in it, (Conn. B. 26.) ^ 



Flies. The domestic fly is passing from a disgusting nuisance 

 and troublesome pest to a reputation of being a dangerous enemy to 

 human health. A species of mosquito has been demonstrated to be 

 the cause of the spread of malaria. Another kind of mosquito is 

 the cause of yellow fever, and now the house fly is considered an 

 agency in the distribution of typhoid fever, summer complaint, 

 cholera infantum, etc. 



The numbers of bacteria on a single flv may range all the way 

 from 550 to 6,600,000. Early in the fly season the numbers of 

 bacteria on flies are comparatively small, while later the numbers 

 are comparatively very large. The place where flies live also deter- 



