~Biological requirements.~ _1. Enhanced keeping quality._ In commercial practice the essential biological requirement is expressed in the enhanced keeping quality of the pasteurized milk. This expresses in a practical way the reduction in germ life accomplished by the pasteurizing process. The improvement in keeping quality depends upon the temperature and time of exposure, but fully as much also on the way in which the pasteurized product is handled after heating. The lowest temperature which can be used with success to kill the active, vegetative bacteria is about 140 deg. F., at which point it requires about ten minutes exposure. If this period is curtailed the temperature must be raised accordingly. An exposure to a temperature of 175 deg. F. for a minute has approximately the same effect as the lower degree of heat for the longer time. The following bacteriological studies as to the effect which a variation in temperature exerts on bacterial life in milk are of importance as indicating the foundation for the selection of the proper limits. In the following table the exposures were made for a uniform period (20 minutes): _The bacterial content of milk heated at different temperatures._ Number of bacteria per cc. in milk. 45 deg. C. 50 deg. C. 55 deg. C. 60 deg. C. 65 deg. C. 70 deg. C. Unheated 113 deg. F. 122 deg. F. 131 deg. F. 140 deg. F. 149 deg. F. 158 deg. F. Series I. 2,895,000 ---- 1,260,000 798,000 32,000 5,770 3,900 Series II. 750,000 665,000 262,400 201,000 950 700 705 Series III. 1,350,000 1,100,000 260,000 215,000 575 610 650 Series IV. 1,750,000 ---- 87,360 ---- 4,000 3,500 3,600 It appears from these results that the most marked decrease in temperature occurs at 140 deg. F. (60 deg. C.). It should also be observed that an increase in heat above this temperature did not materially diminish the number of organisms present, indicating that those forms remaining were in a spore or resistant condition. It was noted, however, that the developing colonies grew more slowly in the plates made from the highly heated milk, showing that their vitality was injured to a greater extent even though not killed. _2. Destruction of disease bacteria._ While milk should be pasteurized so as to destroy all active, multiplying bacteria, it is particularly important to destroy any organisms of a disease nature that might find their way into the same. Fortunately most of the bacteria capable of thriving in milk before or after it is drawn from the animal are not able to form spores and hence succumb to proper pasteurization. Such is the case with the diphtheria, cholera and typhoid organisms.