CHAPTER XXXII 

 THE BACTERIAL AND HEALTH ASPECTS OF PASTEURIZATION^ 



MILTON J. ROSENAU 

 Harvard Medical School, Boston, Mass. 



PASTEURIZATION AND THE BACTERIAL FLORA OF MILK 



It is well known that the bacterial population of milk is both large and miscella- 

 neous. The flora often reach the magnitude of incredible numbers. From the stand- 

 point of health, however, it is the kind rather than the numbers that concern us. Milk 

 is pasteurized primarily for the purpose of making it safe for human consumption. 

 There is another benefit, however, namely, the reduction of the miscellaneous bac- 

 teria always found in milk. This reduction of the germ life in the milk probably also 

 has some sanitary value, particularly for infants in the summertime. The subject 

 is ably discussed by Prucha in an article which has just appeared.^ The most exten- 

 sive work is that of Rogers^ and of Ayers and Johnson,-) who include a large bibli- 

 ography. 



Pasteurization ordinarily reduces the bacterial count in milk 99 per cent; however, 

 under certain conditions, the milk may become heavily contaminated with bacteria 

 that are resistant to pasteurizing temperatures. Prucha points out that resistant spe- 

 cies of bacteria are likely to get into milk during a heated spell with a prolonged 

 rainy season, making the pastures muddy. In this case, in spite of attempts to keep 

 the cows clean, the milk becomes seeded with fairly large numbers of spore-produc- 

 ing soil bacteria. 



It is well known in all bacteriological laboratories that samples of pasteurized 

 milk are encountered with bacterial counts above the legal limits. How to interpret 

 such counts is often puzzling. The bacteria that survive pasteurization in large num- 

 bers have a tendency to produce very small colonies on standard agar plates. The 

 term "pin-point colonies" has been coined to describe this phenomenon. 



Prucha studied 108 different cultures and found that 45 of these were organisms 

 that usually make up the predominating bacterial flora in raw milk, and all of them 

 were killed at 145° F. for thirty minutes. Thirty-four cultures invariably present in 

 the raw milk, but usually in very small numbers, survived pasteurization, but none 

 of these would be classed as thermophiles. They probably belong for the most part 

 to the group of bacteria described by Ayers and Johnson as acid-producing, heat- 

 resisting bacteria. Most of them are cocci and produce very small pin-point colonies. 



' The author has drawn upon his chapter on pasteurization in the fifth edition of Preventive 

 Medicine and Hygiene, and acknowledgment is herewith made to D. Appleton & Co. The reader is 

 referred to that volume for a more extensive treatment. 



'Prucha, M. J.: Am. J. Pub. Health, 17, 356. 1927. 



3 Rogers, L. A.: Bur. Anim. Hiisb., U.S. Dept. Agric, Bull. 73. 1905. 



^ Ayers, S. H., and Johnson, W. T.: ibid.. Bull. 126. 1910; ibid.. Bull. 161. 1913. 



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