Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/395

 equipment of the dairy.]

D A I R Y-F ARMING.

or “scalded layer” capable of protecting the tubercle bacilli, and enabling them to resist a higher temperature

than otherwise would be fatal to them. At a temperature not much above 150° F. milk begins to acquire the cooked flavour which is objectionable to many palates, whilst its “ body ” is so modified as to lessen its suitability for creaming purposes. Three factors really enter into effective pasteurization of milk, namely, (1) the temperature to which the milk is raised, (2) the length of time it is kept at that temperature, (3) the maintenance of a condition of mechanical agitation to prevent the formation of “scalded layer.” Within limits, what a higher temperature will accomplish if maintained for a very short time may be effected by a lower temperature continued over a longer period. The investigation of the problem forms the subject of a paper1 in the th Annual Report of the Wisconsin Agricultural Experiment Station, 1900. The following are the results of the experiments :— 1. An exposure of tuberculous milk in a tightly closed commercial pasteurizer for a period of ten minutes destroyed in every case the tubercle bacillus, as determined by the inoculation of such heated milk into susceptible animals like guinea-pigs. 2. Where milk is exposed under conditions that would enable a pellicle or membrane to form on the surface, the tubercle organism is able to resist the action of heat at 140° F. (60° C.) for considerably longer periods of time. 3. Efficient pasteurization can be more readily accomplished in a closed receptacle such as is most frequently used in the commercial treatment of milk, than where the milk is heated in open bottles or open vats. 1 “ Thermal Death-Point of Tubercle Bacilli, and Relation of same to Commercial Pasteurization of Milk.” By H. L. Russell and E. G. Hastings.

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4. It is recommended, in order thoroughly to pasteurize milk so as to destroy any tubercle bacilli which it may contain, without in any way injuring its creaming properties or consistency, to heat the same in closed pasteurizers for a period of not less than twenty minutes at 140° F. Under these conditions one may be certain that disease bacteria such as the tubercle bacillus will be destroyed without the milk or cream being injured in any way. For over a year this new standard has been in constant use in the Wisconsin University Creamery, and the results, from a purely practical point of view, reported a year earlier by Farrington and Russell,2 have been abundantly confirmed. Dairy engineers have solved the problem as to how large bodies of milk may be pasteurized, the difficulty of raising many hundreds or thousands of gallons of milk up to the required temperature, and maintaining it at that heat for a period of twenty minutes, having been successfully dealt with. The plant usually employed provides for the thorough filtration of the milk as it comes in from the farms, its rapid heating in a closed receiver and under mechanical agitation up to the desired temperature, its maintenance thereat for the requisite time, find finally its sudden reduction to the temperature of cold water through the agency of a refrigerator, to be next noticed. Refrigerators are used for reducing the temperature of milk to that of cold water, whereby its keeping properties

Fig. 20.—Refrigerator and Can. are enhanced. The milk flows down the outside of the metal refrigerator (Fig. 20), which is corrugated in order to provide a larger cooling surface, whilst cold water circulates through the interior of the refrigerator. The conical vessel into which the milk is represented as flowing from the refrigerator in Fig. 20 is absurdly called a 1£ milk-churn, ’’whereas milk-can is a much more appropriate name. For very large quantities of milk, Fi0, 21.—Cylindrical Cooler or Refrigerator. such as flow from a pasteurizing plant, cylindrical refrigerators (Fig. 21), made of tinned copper, are available; the cold water circulates inside, and the milk, flowing down the outside in a very thin sheet, is rapidly cooled from a temperature of 2 IQth Kept. Wis. Agric. Expt. Station, 1899, p. 129.