366 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



place as rapidly as in ordinarily pasteurized milk retaining its normal 

 moisture. 



POWDERED MILK. This product is produced by carrying the extrac- 

 tion of the water farther than in the case of the condensed milks. The 

 water is removed to a point where the milk solids can be reduced to a 

 powdered form. This product contains the original milk solids with a 

 very small percentage of moisture usually not more than 21/2 per cent. 

 There are several forms of powdered milk now on the market produced 

 by somewhat different methods. In some cases the moisture is removed 

 from the milk by its being exposed to a heated surface in a thin layer. 

 Sometimes the drying is done in vacuum. The resulting product is dry 

 and can be ground to the form of flour. 



Another process is to remove the moisture by spraying the milk by 

 means of an atomizer into the top of a hot chamber, the moisture being 

 removed while the fine particles of milk are falling to the floor. By 

 this process the product accumulates on the floor as a very dry flour and 

 does not require any grinding. In the first process the heat is sufficient to 

 pasteurize the milk while in the latter process it is pasteurized before 

 being subjected to the drying process. The powdered milks do not claim 

 to be sterile but are preserved against subsequent action of microorganisms 

 because of the very low percentage of moisture which they contain. It 

 is probable that there is no appreciable increase in the number of bacteria 

 in milk flour and the product will keep for a long time without undergoing 

 bacterial fermentations. 



CANNED BUTTER AND CHEESE. 



Some effort has been made to put up butter and cheese in hermetically 

 sealed cans, the purpose being to increase the keeping qualities of the 

 products and influence the flavor by controlling the development of the 

 aerobic bacteria. Only a limited amount of bacteriological work has been 

 done on these canned products and the biological changes which take 

 place in them are not very well known. 



SPECIAL MILK DRINKS MADE BY THE ACTION OF MICROORGANISMS. 



From time immemorial fermented or sour milk has been used as an 

 article of food. We are told that Abraham* placed " curdled milk" 



* Genesis 18:8. The Hebrew word "hemah" translated in the English authorized version 

 of the Bible "butter" means "curdled milk." Century Bible, Vol. Judges and Ruth, p. 72. 



