CHAPTER XVIII. 



FILTRATION; COOLING; PASTEURIZATION AND 

 STERILIZATION OF MILK; PRESERVATIVES. 



THE suggestions made in Chapter XVI., 3, have 

 reference chiefly to the production of a clean milk 

 that is, they aim at protecting it from becoming con- 

 taminated with bacteria. It is, however, important to 

 deal with another side of the question, namely, the 

 methods to be adopted in order to remove or destroy 

 those organisms which unavoidably gain an entrance 

 into it, or to check their development. 



Three methods may be specially mentioned : 

 (i) The filtration of the milk ; 

 (ii) the application of low or high temperatures , 

 (iii) the introduction of " preservatives " or soluble 

 chemical substances which inhibit the mul- 

 tiplication of bacteria. 



(i) Filtration. Strainers consisting of various com- 

 binations of cotton or linen fabrics placed over fine wire 

 gauze should always be used at the cowshed or dairy, 

 to remove the particles of dung, hair, hay, and other 

 refuse which fall into the milk. Milk may be filtered 

 through layers of gravel, sand, and cloth, which remove a 

 considerable proportion of the bacteria present, although 

 they do not take out all of them. The difficulties con- 

 nected with the cleaning and renewal of such filters, as 

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