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MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



Influence of Passing Milk through a Centrifugal Separator upon the Germ Content of the 



Milk and Cream. 



There is usually a marked increase in the number of bacteria which 

 will develop in plate cultures both from the skim milk and the cream in 

 excess of the number found in the whole milk. This does not mean that 

 there is an actual increase in individual bacteria in these samples due to 

 the action of the separator. What it does mean is that the small clusters or 

 groups of organisms, as they exist in the whole milk are thrown apart by 

 the force of the separator and therefore develop individual colonies in the 

 plate cultures. The effect of this breaking up of the clusters of bacteria 

 in the milk is also injurious to its keeping qualities. This is probably 

 due to the fact that the multiplication of the organisms is greater where 

 they exist in the milk as individuals than it would be had they remained 

 in their original clusters. So marked is this injurious effect upon the 

 keeping quality of the milk that in some cases the commercial milk plants 

 have been obliged to give up the practice of clarifying milk by means of 

 the centrifugal separator. 



TEMPERATURE. The temperature at which milk is kept is one of the 

 most important factors determining the development of its microbial 

 content. Every one at all familiar with milk knows that it spoils very 

 quickly if allowed to stand at warm temperatures. If, however, the milk 

 is held at temperatures of 10 or lower, the keeping quality of the milk is 

 greatly increased. Most of the ordinary species of organisms which gain 

 entrance to milk do not grow rapidly at temperatures of 10 or lower. 

 There are, however, certain species which will grow with considerable 

 rapidity at temperatures below 10, especially some of the spore-bearing 

 non-acid forms. If the temperature of the milk is allowed to rise above 

 10, the growth of the common species increases rapidly. The influence 

 of temperature upon the development of bacteria may be seen in the fol- 



