RELATION or MICROORGANISMS TO SPECIAL DAIRY PRODUCTS 439 



It is not intended to be sterile. The degree of heat to which it 

 is subjected is not sufficient to kill all of the micro5rganisms 

 present and it is also subject to infection after the condensing 

 is completed. Cane sugar is added to the milk, making the final 

 product contain about 25 per cent of water, 35 per cent milk solids 

 and 40 per cent cane sugar. The low percentage of moisture together 

 with the added sugar tends to preserve this product against the action 

 of microorganisms. There may be some bacterial growth, the 

 rapidity depending upon the temperature at which the product is 

 kept, but it is usually slow and milk prepared in this way will keep 

 for a considerable, time without undergoing marked bacterial changes. 

 Some gas producing bacteria exist in the milk and if cans containing 

 the organisms are allowed to remain at warm temperatures, they will 

 develop in spite of tl^e large percentage of sugar, producing suffi- 

 cient amounts of gas to cause the ends of the cans to bulge out. Such 

 cans are known commercially as "swell-heads." 



Unsweetened Condensed or Evaporated Milk. — In this form of 

 condensed milk approximately the same amount of moisture is removed 

 as in the sweetened product but no sugar is added. The decreased 

 amount of moisture tends to prevent the rapid growth of bacteria, but 

 this is not enough to guarantee the keeping quaUty of the product. 

 After the milk is condensed it is put into the can, hermetically sealed, 

 and then placed in steam steriUzers and subjected to temperatures some- 

 what above the boiling-point. In this way the milk is heated a suffi- 

 cient length of time to insure perfect sterilization of the contents of the 

 cans. If this process is properly done, the finished product contains no 

 living microorganisms and from the bacteriological standpoint the milk 

 should keep indefinitely. 



Sometimes the unsweetened product is sold in bulk in cans. In 

 this case it is subject to more or less contamination after heating and is 

 not sterile, but because of the small amount of moisture and the concen- 

 tration of the milk solids, the bacteria do not develop rapidly and if 

 kept at a cool temperature, the milk will keep several days without 

 undergoing appreciable biological fermentations. 



Concentrated Milk. — There is now on the market a form of con- 

 densed milk prepared by a different process, which is commonly known 

 as concentrated milk. By this method the water in the milk is removed 

 by means of dry air instead of by vacuum as is the case of condensed 



