460 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



flora of milk, pasteurized by the "holding" method, have shown that 

 some strains of the lactic acid bacteria can survive the relatively lower 

 temperatures used in this method, and that the later development of 

 the different groups of bacteria is similar to that in raw milk of equal 

 bacterial grade. 



Pasteurization at the temperatures used in the holding process does 

 not seem to cause any injurious chemical changes in the milk constitu- 

 ents, or affect its digestibility. 



Proper pasteurization gives a valuable means of rendering the milk 

 supply for our cities reasonably free from pathogenic microorganisms, 

 but, in order to insure this safety, the work must be carefully done, 

 and all later contamination avoided. Preferably, the work should be 

 done under expert, municipal supervision. Undoubtedly the ideal 

 method is pasteurization in the sealed bottle which is to be delivered to 

 the consumer, since this method reduces to the minimum the danger 

 of subsequent contamination. 



Pasteurization must not be regarded as a substitute for care and 

 cleanliness or a means of renovating old or dirty milk otherwise unfit 

 for use, but rather as an additional means of protecting the consumer 

 against disease-producing microorganisms in the milk supply. 



THE USE OF CHEMICALS. The addition of certain chemicals to milk 

 will retard the growth of bacteria. The chemicals most commonly used 

 for this purpose are calcium hypochlorite, borax and formalin. While 

 the keeping quality of milk may be materially increased by the use of 

 such chemicals, their use has been opposed by health authorities and is 

 contrary to the Pure Food Laws. If milk is handled with any degree 

 of care, there should be no need for the use of chemical preservatives. 

 They are simply a means of counteracting the unsanitary conditions of 

 the production and handling. The same results can be obtained by 

 cleanliness in the production of the milk and the use of low temperatures 

 for preventing the contamination and subsequent growth of the 

 bacteria in the milk. The developments in the production of clean 

 milk of the past few years have illustrated very clearly that the use of 

 chemical preservatives is not necessary. 



NORMAL DEVELOPMENT OF MICROORGANISMS IN MILK 



The flora of any particular sample of fresh milk is determined by the 

 conditions under which it is produced. In stables where extreme 

 cleanliness is practised the flora may be practically limited to those 



