MILK 



193 



materials. The average composition of cow's and some other kinds 

 of milk used for food is given in the table which follows: 



Average composition of milk of various kinds. 



As the figures in the table make plain, milk of all sorts is a 

 dilute food, as it contains a large percentage of water, the lowest 

 proportion according to the figures cited being noted with reindeer 

 milk and the highest with mare's milk. The three groups of protein, 

 fat, and carbohydrates are represented by fair proportions, the quan- 

 tities of protein and fat being especially noteworthy, as it is these 

 constituents and the mineral matter or ash which to a large degree 

 give milk its peculiar value as a food for young mammals. (Dept. 

 Agr. F. B. 363.) 



Germ Life. Bacteria, Yeasts, Etc. Most of the changes that 

 take place in milk, after it is drawn from the cow, are due to minute 

 forms of plant life, principally bacterial ferments, which gain access 

 to it and in their growth and development bring about many and 

 varied changes in it and its products. As all air contains 'bacteria 

 to a greater or less extent no milk is free from them. A change 

 well-known to all is that of the souring of milk, which is due to lac- 

 tic acid bacteria gaining access to it and converting the sugar of 

 the milk into lactic or milk acid. 



These various organisms, when they gain access to milk, find 

 it a perfect food, and if the temperature and other conditions be 

 favorable to their growth they develop very rapidly. Although they 

 are extremely small, single-celled organisms, visible only under a 

 strong microscope, yet their rapidity of growth and reproduction ex- 

 plains the marked changes that often take place in milk within a 

 comparatively short space of time. Their usual method of reproduc- 

 tion is to lengthen somewhat and then divide into two. Under favor- 

 able conditions this will take place in from half an hour to an hour's 

 time. Assuming it to take place in an hour's time, and the process 

 to go on at this rate for twenty-four hours, the result would be about 

 17,000.000 organisms from the one parent organism. Of course as 

 these organisms grow they feed upon the milk and bring about many 

 changes fermentations in it which may seriously affect its flavor, 

 change its condition physically, impair its food value, and, if they 



