Emphasis has already been laid on the selection of a proper limit of pasteurizing (p. 114). It should be kept constantly in mind that the thermal death-point of any organism depends not alone on the temperature used, but on the period of exposure. With the lower limits given, 140° F., it is necessary to expose the milk for not less than fifteen minutes. If a higher heat is employed (and the cooked flavor disregarded) the period of exposure may be curtailed.
Chilling the milk. It is very essential in pasteurizing that the heated milk be immediately chilled in order to prevent the germination of the resistant spores, for if germination once occurs, growth can go on at relatively low temperatures.
The following experiments by Marshall[150] are of interest as showing the influence of refrigeration on germination of spores:
Cultures of organisms that had been isolated from pasteurized milk were inoculated into bouillon. One set was left to grow at room temperature, another was pasteurized and allowed to stand at same temperature, while another heated set was kept in a refrigerator. The unheated cultures at room temperature showed evidence of growth in thirty trials in an average of 26 hours; 29 heated cultures at room temperature all developed in an average of 50 hours, while the heated cultures kept in refrigerator showed no growth in 45 days with but four exceptions.
Practically all of the rapid-process machines are provided with especially constructed cooling devices. In some of them, as in the Miller and Farrington, the cooling is effected by passing the milk through two separate coolers that are constructed in the same general way as the heater. With the first cooler, cold running water is employed, the temperature often being lowered in this way to 58° or 60° F. Further lessening of the temperature is secured by an additional ice water or brine cooler which brings the temperature down to 40°-50° F.
In the economical use of ice the ice itself should be applied as closely as possibly to the milk to be cooled, for the larger part of the chilling value of ice comes from the melting of the same. To convert a pound of ice at 32° F. into a pound of water at the same temperature, if we disregard radiation, would require as much heat as would suffice to raise 142 pounds of water one degree F., or one pound of water 142° F. The absorptive capacity of milk for heat (specific heat) is not quite the same as it is with water, being .847 for milk in comparison with 1.0 for water.[151] Hot milk would therefore require somewhat less ice to cool it than would be required by any equal volume of water at the same temperature.
Bottling the product. If the milk has been properly pasteurized, it should, of course, be dispensed in sterilized bottles. Glass bottles with plain pulp caps are best, and these should be thoroughly sterilized in steam before using. The bottling can best be done in a commercial bottling machine. Care must be taken to thoroughly clean this apparatus after use each day. Rubber valves in these machines suffer deterioration rapidly.
