Number of bacteria per cc. in milk.
| 45° C. | 50° C. | 55° C. | 60° C. | 65° C. | 70° C. | ||
| Unheated | 113° F. | 122° F. | 131° F. | 140° F. | 149° F. | 158° 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° F. (60° 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.
The organism that is invested with most interest in this connection is the tubercle bacillus. On account of its more or less frequent occurrence in milk and its reputed high powers of resistance, it may well be taken as a standard in pasteurizing.
Thermal death limits of tubercle bacillus. Concerning the exact temperature at which this germ is destroyed there is considerable difference of opinion. Part of this arises from the inherent difficulty in determining exactly when the organism is killed (due to its failure to grow readily on artificial media), and part from the lack of uniform conditions of exposure. The standards that previously have been most generally accepted are those of De Man,[137] who found that thirty minutes exposure at 149° F., fifteen minutes at 155° F., or ten minutes at 167° F., sufficed to destroy this germ.
More recently it has been demonstrated,[138] and these results confirmed,[139] that if tuberculous milk is heated in closed receptacles where the surface pellicle does not form, the vitality of this disease germ is destroyed at 140° F. in 10-15 minutes, while an exposure at 160° F. requires only about one minute.[140] If the conditions of heating are such that the surface of the milk is exposed to the air, the resistance of bacteria is greatly increased. When heated in open vessels Smith found that the tubercle organism was not killed in some cases where the exposure was made for at least an hour. Russell and Hastings[141] have shown an instance where the thermal death-point of a micrococcus isolated from pasteurized milk was increased 12.5° F., by heating it under conditions that permitted of the formation of the scalded layer. It is therefore apparent that apparatus used for pasteurization should be constructed so as to avoid this defect.
Methods of treatment. Two different systems of pasteurization have grown up in the treatment of milk. One of these has been developed from the hygienic or sanitary aspect of the problem and is used more particularly in the treatment of cream and relatively small milk supplies. The other system has been developed primarily from the commercial point of view where a large amount of milk must be treated in the minimum time. In the first method the milk is heated for a longer period of time, about fifteen minutes at a relatively low temperature from 140°-155° F.; in the other, the milk is exposed to the source of heat only while it is passing rapidly through the apparatus. Naturally, the exposure under such conditions must be made at a considerably higher temperature, usually in the neighborhood of 160° F.
The types of apparatus used in these respective processes naturally varies. Where the heating is prolonged, the apparatus employed is built on the principle of a tank or reservoir in which a given volume of milk may be held at any given temperature for any given period of time.
When the heat is applied for a much shorter period of time, the milk is passed in a continuous stream through the machine. Naturally the capacity of a continuous-flow apparatus is much greater than a machine that operates on the intermittent principle; hence, for large supplies, as in city distribution, this system has a great advantage. The question as to relative efficiency is however one which should be given most careful consideration.