378 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



and made of such material that they can be thoroughly sterilized 

 •either with water which is actually boiling or in steam. 



The MiLKiEte. — No fdod material requires greater care and cleanli- 

 ness on the part Si those handling it than does milk. All persons having 

 to do with the handling of this delicate food product should constantly 

 keep in mind that clean hands and clothing and extreme cleanliness in 

 every operation is very necessary if milk of good quality is .to be ob- 

 tained, -j, 



Group'^'r Types of Microorganisms Found in Milk 

 AND Their Sources 



In studying the types of bacteria found in milk, it is convenient 

 to arrange them in groups based upon their action on the milk and 

 their effect upon persons consuming it. There are certaiii types of 

 organisms which »re very troublesome to the milk dealer but which are 

 not injurious to the consumer. Other species which may be of little or 

 no significance from their action on the mUk are of greatest significance 

 from the standpoint of the consumer since most of the disease organisms 

 which may be carried by milk have no appreciable action upon it. Still 

 other forms are of but little importance to either the dealer or the con- 

 sumer and others are troublesome to both. 



General Significance of Acid-fprming Bacteria. — Of all the 

 bacteria that find their way into milk, those that are able to ferment the 

 milk sugar, producing from it different kinds and amounts of acids, find 

 more favorable conditions for growth at ordinary temperatures, 15° to 

 45°, than do those belonging to other groups. Because of their greater 

 rapidity of growth and because of the inhibiting effect of their by-prod- 

 ucts upon the other groups of bacteria, the acid types tend to predom- 

 inate in milk and the specific change which they produce, the souring, 

 is of such common occ.urrence that it is often looked upon as something 

 inherent in milk. 



Groups of Acid-forming Bacteria.* — The acid-forming bacteria 

 that are constantly present in milk represent many kinds which differ in 

 morphology, in cultural characteristi9s, and in their products of fermen- 

 tation. They may be divided into four groups that vary greatly as far 

 as their importance in the handling of mUk is concerned. If milk is pro- 

 duced under clean conditions and is kept at temperatures ranging from 



* Prepared by E. G, Hastings. 



