364 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



kill all of the microorganisms 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 con- 

 siderable 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 the large percentage of sugar, producing sufficient amounts of gas to 

 cause the ends of the cans to bulge out. Such cans are known com- 

 mercially as "swell-heads." 



UNSWEETENED CONDENSED MILK. In this form of condensed milk 

 approximately the same amount of moisture is removed as in the sweet- 

 ened 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 quality of the product. After the milk 

 is condensed it is put into the can, hermetically sealed, and then placed 

 in steam sterilizers and subjected to temperatures somewhat above the 

 boiling-point. In this way the milk is heated a sufficient 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. The milk is first heated and then air under pressure 

 is forced through it. By this process the milk is heated to a temperature 

 of 60 (140 F.), and this temperature maintained for two hours, during 

 which time air is forced through the milk causing violent agitation and 



