554 



STATE BOARD OF AGRICULTURE. 



ACIDIFIED MILK. 



When the thermal death point of three strains of Bad. lactis acidi 

 was determined in sterile separated milk to which was added normal 

 lactic acid (commercial) no effect of the acid was noticed. (Table V.) 



TABLE v.— THERMAL DEATH POINT DETERMINED IN ACIDIFIED MILK. 



(Average of 20 determinations). 



*A degree (°) means per cent N/10. 



GENEKAL DISCUSSION. 



As further work on several points of this subject is well under way a 

 detailed discussion of the many factors will not be undertaken at this 

 time; yet it will not be out of place to mention some of the factors which 

 may or may not influence the resistance to heat (thermal death point) 

 of Ibacteria in milk. It is understood that the composition of milk from 

 different cows is not the same and that the milk from the same cow at 

 different milkings or even at the same milking is not uniform. If media 

 of different composition have anything to do with raising or lowering 

 the thermal death point of bacteria therein, then all factors which 

 modify milk or cause variation in its composition may influence directly 

 or indirectly, favorably or unfavorably the ability of bacteria to survive 

 high temperatures. Physical and chemical conditions of the medium in 

 which bacteria are subjected to heat are not without influence upon the 

 thermal death point. Media with low or high specific heat would sub- 

 ject the bacteria to sudden or retarded changes in temperature when 

 they are placed in the hot water bath and when they are cooled by plac- 

 ing in cold water. A sudden chilling of bacteria, as when the thin wall 

 capillary tube is plunged into cold water after the ten minutes exposure, 

 may be decidedly detrimental. The use of sealed containers as Sternberg 

 bulbs or capillary tubes might have an influence because the exposure to 

 heat during the determination would create an internal pressure which, 

 in itself, is a potent factor in hastening death. The evaporation in un- 

 sealed tubes would lower the surface temperature at the time of expos- 

 ure, thus permitting the bacteria in the surface to survive when all be- 

 low the surface have succumbed. A possible indirect effect of evapora- 

 tion may be the production of a drier heat in the surface of the medium 

 due to the concentration of its soluble and suspended matter. The form- 

 ation of a pellicle on raw milk due to the coagulation of albumen, to sur- 

 face drying and to oxidation when milk is heated in contact with air is, 



