BACTERIA IN FOODS 



207 



filtration removes some of the essential nutritive elements 

 of milk, has caused the process to be but little adopted. 

 Dr. Seibert states that if milk be filtered through half an 

 inch of compressed absorbent cotton, seven-eighths of the 

 contained bacteria will be removed, and a second filtration 

 will further reduce the number to one-twentieth. One 

 quart of milk may thus be filtered in fifteen minutes. 



The common methods now in vogue for the protection of 

 milk are based upon gcrmicidal temperatures. Low temper- 

 atures, it is true, do not easily destroy life, but they have a 

 most beneficial effect upon the keeping quality of milk. At 

 the outset of the process of cooling, strong currents of air are 

 started in the milk-can, which act mechanically as deodor- 

 isers. But if the temperature be lowered sufficiently, the 

 contained bacteria become inactive and torpid, and event- 

 ually are unable to multiply or produce their characteristic 

 fermentations. At about 50 F. (10 C.) the activity ceases, 

 and at temperatures of 45 F. (7 C.) and 39 F. (4 C.) 

 organisms are deprived of their injurious powers. If it hap- 

 pens that the milk is to be conveyed long distances, then 

 even a lower temperature is desirable. The most important 

 point with regard to the cooling of milk is that it should 

 take place quickly. Various kinds of apparatus are effective 

 in accomplishing this. Perhaps those best known are Law- 

 rence's cooler and Pfeiffer's cooler, the advantage of the 

 latter being that during the process the milk is not exposed 

 to the air. It must not be forgotten that cooling processes 

 are not sterilising processes. They do not necessarily kill 

 bacteria; they only inhibit activity, and under favourable 

 circumstances the torpid bacteria may again acquire their 

 injurious faculties. Hence during the cooling of milk 

 greater care must be taken to prevent aerial contamination 

 than is necessary during the process of sterilising milk. No 

 cooling whatever should be attempted in the stable; but, 

 on the other hand, there should be no delay. Climate 



