144 BACTERIOLOGICAL METHODS 



sediment must not be omitted as this will convey information re- 

 garding the sanitary conditions of the factory as well as of the 

 dairying estabhshments which supplied the milk to the factory. 



Dried or powdered milk is prepared by spraying milk (usually 

 skimmed) into a partial vacuum or by spraying it on a revolving 

 drum or on a moving belt in a partial vacuum. The dried material 

 is then placed in suitable containers. The dried milk contains 

 all of the ingredients of the milk excepting the water, the lysins 

 and certain enzymes. The fat globules are altered physically 

 but not chemically. Mixing dried milk with the required amounts 

 of water makes a liquid resembling ordinary milk. The micro- 

 scopical and bacteriological examination of dried milk is as for 

 condensed milk. Like the condensed milk it is quite free from 

 disease germs of all kinds but bacterial invasion is not excluded 

 from material which has been carelessly prepared or canned. 

 Toxins and ptomaines should be absent. The absence of moisture 

 in powdered milk prevents the ready growth of micro-organisms 

 and it may be kept in good condition for a period of 5 or 6 months 

 and even longer, in dry sterile containers stored in a cool dry 

 place. 



Attempts have been made to commercialize frozen milk but 

 so far without success. It is rather difficult to handle frozen 

 milk and the article furthermore loses the milk flavor on thawing. 



16. The Bacteriological Examination of Shellfish 



The term shellfish includes oj-sters, mussels and clams. Only 

 those species and varieties which serve as food for man are of 

 interest to the food bacteriologist. Since it has been conclusively 

 proven that shellfish, oysters in particular, have been responsible 

 for typhoid epidemics, much attention has been given to the 

 bacteriology of this class of food. In tracing such epidemics it was 

 discovered that the causative oysters had been floated or grown in 

 heavily polluted waters. In several instances contamination of 



