EXPERIMENT STATION BULLETINS. . 555 



without question, a protection to bacteria within the pellicle. In cream 

 the fat itself may have protective properties or the protection may be in- 

 direct and due to a large amount of dry matter with a comparatively 

 low percentage of water. Kaw milk may offer greater or less protection 

 than milk sterilized by heat. iWhether the protective property of casein 

 is due to the formation around the bacterial cell of an acid or rennet 

 curd, brought about during the exposure through the agency of the 

 enzymes liberated by the cell, is a question for further study. The pos- 

 sibility that the reducing action of lactose or the action of reductase 

 may have an influence can not be ignored. The presence of products 

 and of bj^-products of bacterial groAvth undoubtedly has something to 

 do with a bacterium's maximum endurance of heat. It must be admitted 

 that the death of bacteria, when subjected to heat, may be postponed or 

 hastened by the presence of enzymes (reducing, oxidizing and hydrolytic) 

 of animal and of bacterial origin. It is not improbable that the age of an 

 individual cell i. e. the two cells the moment after division and the cells 

 hardened with age; that the physical condition of the cell i. e. a cell in 

 the act of division, a cell rapidly synthesizing vital protoplasm, a cell 

 resting but maintaining life, an involuted form or a stage in the life 

 cycle; and that the formation within the cell or the exudation from 

 the cell wall — permitted or even favored by the nature of the medium 

 upon which the cell is nourished — of protective substance i. e. the stor- 

 ing of fats, carbohydrates, sulfur, etc., within the cell and the excretion 

 of waste products, gums, muculent substances, etc., are determining 

 factors in raising or lowering the resistance of cells to the devitalizing 

 action of high temperatures. In the pasteurization of dairy products 

 and in the control of infectious diseases the resistance of microorganisms 

 to heat is a question of great magnitude and a problem for research. 



SUMMARY. 



1. The temperature of cream during pasteurization by the holding 

 process in a "Perfection" pasteurizer (with coil rotating) was found to 

 be almost constant throughout the whole mass : The temperature in all 

 parts of the cream at a stated time showed a variation of less than one 

 degree. 



2. The thermal death point, determined in bouillon, of some of the 

 non-spore bearing bacteria isolated from pasteurized milk and cream is 

 higher than the pasteurization temperature (62.5° C. or 145° F. for 20 

 min.), while many have a thermal death point below the temperature of 

 pasteurization. 



3. The casein and fat in milk ofiCer some protection to bacteria that 

 are subjected to high temperature during the death point determina- 

 tion. 



4. No pellicle was observed to form in any of the tubes of milk dur- 

 ing the exposure. The plain cotton plugged tubes, the tubes with paraf- 

 fin oil on the surface of the medium and the Sternberg bulbs gave con- 

 cordant results in the thermal death point determinations. 



5. The addition of small amounts of lactic acid to a milk suspension 

 of the bacteria did not change their thermal death point. 



