198 Effect of Heat. 



60 C. for five minutes. All are killed at 60 C. for ten minutes. 

 However, the great majority of these micro-organisms are killed 

 by the time the milk reaches 60 C. 



So far as can be judged from the meager evidence at hand, 60 

 C. for twenty minutes is more than sufficient to destroy the in- 

 fective principle of Malta fever in milk. The Micrococcus meliten- 

 sis is not destroyed at 55 C. for a short time ; the great majority 

 of these organisms die at 58, and at 60 all are killed. 



Milk heated at 60 C. and maintained at that temperature for 

 twenty minutes may therefore be considered safe so far as con- 

 veying infection with the micro-organisms tested is concerned. 



Ayers states that the best method of pasteurization at the 

 present time, and the one which should be used, is the holder proc- 

 ess, in which the milk is held for 30 minutes. For this process a 

 temperature of 63 C. (145 F.) is to be advised, since that temper- 

 ature gives a margin beyond that sufficient to destroy pathogenic 

 organisms, while at the same time it leaves in the milk the maxi- 

 mum number of lactic-acid-producing organisms which cause the 

 souring of the milk. When using the flash process, the milk should 

 be heated to at least 160 F. Since there is almost always a 

 fluctuation in the temperature during pasteurization, care should 

 be taken to see that the temperature never drops below 71 C. 

 (160 F.) in the flash process. Trans.] 



Exposure at 50 degrees of temperature for 15 minutes or at 

 70 C. for 10 minutes is sufficient to destroy the virus of foot-and- 

 mouth disease. The virus is destroyed instantaneously at 85 

 deg. C. 



All of these advantages may also be obtained from subjecting 

 the milk in the household to heating for a short time without 

 'boiling, when through occasional stirring the formation of the 

 scum upon the surface is prevented. Therefore the purchase of 

 raw milk, whose fresh condition can be readily controlled, should 

 be generally recommended, and the destruction of bacteria should 

 then be carried out by simple heating. 



The observation recorded by Schut appears to be worthy of consideration, namely 

 that relatively low temperatures rapidly destroy bacteria, when applied simultaneously 

 with a lowering of the pressure. In heating the milk at 70 deg. C. the disturbing scum 

 formation was omitted. As accepted by Sehut, in this process the steam penetrates 

 into the body of bacteria, which explains the more efficient action of this method. 



Experiments which aim to improve the keeping qualities of 

 milk by the addition of chemical substances are very numerous. 

 In addition to improving its keeping qualities the retention of the 

 raw condition of the milk was attempted. This does not refer to 

 the adulterations which are undertaken by dealers for fraudulent 

 purposes, or which are carried out in the household, and it should 

 be considered that all additions to milk without subsequent decla- 

 ration are equal to an adulteration of food, changing it to a spoiled 

 product, possibly even converting it into material injurious to 

 health. 



