68 THE CORNELL READING-COURSES 



now known, however, that in the process of splitting up the milk-sugar 

 other by-products are produced. In the simple lactic type of fermentation 

 these secondary products are not very important. It should be noted, 

 however, that in the souring of milk by different species of bacteria, 

 correspondingly different by-products may result. In consequence of 

 this the souring is often accompanied with by-products that are undesirable, 

 if not injurious, to the consumer. In such cases the deleterious substances 

 are often produced before the quantity of acid is sufficient to cause curd- 

 ling. In fact, the by-products themselves may become harmful while 

 the milk is still considered sweet and wholesome. The most telling truth 

 that comes to us from all inquiries on the subject is, that different bacteria 

 causing souring in milk produce very different effects on the milk itself, 

 as is shown in the rapidity of the souring and in the types of fermentation 

 accompanying it. 



Much has been written concerning disease-producing bacteria in milk. 

 They belong to two distinct classes, namely: (i) The specific bacteria 

 of certain diseases of cattle, which may, if the animal is suffering from 

 disease, gain entrance to the milk. In this class may be mentioned 

 tuberculosis, foot-and-mouth disease, and possibly anthrax. (2) The 

 bacteria of certain human diseases, such as typhoid fever and diphtheria, 

 and the virus of scarlatina and measles. A large number of epidemics 

 of these diseases has been traced to the milk supply; through it the infec- 

 tions occurred. The explanation of this is, that in cases in which the 

 diseases existed among the attendants or in their homes, sufficient care 

 was not taken in handling milk to prevent the entrance of the disease 

 germs. In the case of typhoid fever the water used in rinsing utensils 

 may be contaminated. In cases of "diphtheria it often happens that those 

 who have recently apparently recovered from the disease but still have 

 the bacilli in their throats, are engaged in milking or in otherwise handling 

 the milk, when, by sneezing or coughing, the bacilli from the throat may 

 be introduced into the milk. The sad experiences of the past are teaching 

 the importance of taking reasonable precautions against such infection. 



When digestive disorders, especially among children, follow the use 

 of milk containing many bacteria, the immediate cause is quite as likely to 

 be the acids and other by-products that have been produced in the milk 

 by various forms of bacteria, as the activities within the digestive tract 

 of any one or more species of the micro-organisms consumed. We must 

 look to the effect of bacteria on the milk itself for the cause of many, 

 but not all, of such ailments. It is to prevent those effects that pasteur- 

 ization is employed. 



Milk is sterilized or pasteurized for two purposes: to keep it sweet for 

 a longer time than would otherwise be possible, and to kill all harmful 



