308 CITY MILK SUPPLY 



"1. When milk is exposed in thin layers to ultraviolet rays there was a marked 

 reduction in bacterial content. (2) That since the temperature of the exposed 

 milk never rose above 86F., the action of the rays was entirely independent of 

 the action of heat. (3) The most satisfactory method of exposure was over two 

 revolving drums, the tops of which were at a distance of 4 in. below the light tube 

 of the lamp. (4) The two factors of greatest importance in the successful appli- 

 cation of the rays were the thickness of the layer and the length of exposure. 

 A thin layer permits a more complete penetration of the rays and the longer the 

 exposure the more chance they have to act. (4) Ultraviolet rays exerted a 

 greater bactericidal action on vegetative cells in milk than on spores when ex- 

 posed under the same conditions. (6) No greater action of the rays on bacteria 

 was observed when the bacteria were weakened by pasteurization immediately 

 preceding exposure. (7) From the study of two samples of milk exposed to 

 ultraviolet rays it was apparent that the rays did not exert any specific bacteri- 

 cidal power on any particular group of bacteria in the milk. As stated before, 

 however, there was a difference on the action of the rays on bacteria in the vege- 

 tative and spore state. (8) Under similar conditions of exposure there seemed 

 to be somewhat less bacterial reduction in 15 per cent, cream than in milk which 

 was probably because the revolving drums picked up a thicker cream layer than 

 milk layer. (9) When milk was exposed under conditions suitable for a satis- 

 factory reduction of the bacteria by the ultraviolet rays there was also produced 

 an abnormal disagreeable flavor that would render the milk unsaleable. (10) 

 A large percentage of the bacteria in normally dirty and artificially infected milk 

 bottles were destroyed by exposure to the rays." 



The best results were obtained when the bottles were exposed directly 

 under the lamp, the top of the bottle being about 4 in. from the lamp tube. 

 When bottles were exposed on one side of the lamp and not directly under 

 it, poor results were obtained. It was not possible to completely ster- 

 ilize the bottles even after a 10-min. exposure. The authors concluded 

 that with quartz mercury vapor lamps of the present power and con- 

 struction it would not be possible to completely sterilize milk by the ul- 

 traviolet rays; that while by the use of large drums and many lamps 

 bacterial reductions as great as are now secured by pasteurization might 

 be obtained, there is no assurance that the disease germs would be killed, 

 since the rays do not exert a selective action on vegetative cells ; that on a 

 commercial scale it would be difficult to control the factors which influence 

 the bactericidal action of the rays and moreover the disagreeable flavor 

 imparted to the milk renders the process impracticable ; that it is doubtful 

 whether the lamps could be made to compete with the use of steam in 

 sterilizing the bottles. 



Houghton and Davis confirm the conclusion of Ayres and Johnson 

 that the untraviolet rays cannot be successfully used to sterilize milk 

 and state that in this respect their work agrees with that of Romer and 

 Sames and of Juyge. They did not find a perceptible change in the flavor 

 of the milk after it was illuminated. 



