536 THE CONTROL OF THE MILK SUPPLY 



the inside is filled with water kept at or near the boiling point. 

 Steam is used in some forms of apparatus instead of water, and the 

 surface may be flat or corrugated, or even a revolving cylinder. 

 But the principle is the same. Care has to be taken that the 

 stream of milk is not too rapid, otherwise it cannot reach the 

 necessary temperature. A combined Lawrence pasteuriser and 

 cooler is constructed on the same basis, which possesses certain 

 advantages. It consists of two machines, the one a scalder and the 

 other a refrigerator. The milk or cream is placed in a receiver 

 whence it flows over the scalder, where it is heated to within a few 

 degrees of the boiling point. The hot or scalded milk or cream is 

 then allowed to run over the refrigerator, and is cooled to within 2° 

 or 3° F. of the water rising within. The heat in the scalder is main- 

 tained by the water in constant circulation heated by a steam jet, so 

 that the temperature is never raised to boiling point, and thus the 

 objectionable flavour of boiled milk is avoided. 



A third form of pasteuriser which was described by H. L. Russell 

 of Wisconsin, consists of a scalder, a water cooler, and an ice cooler. 

 The pasteuriser is heated by hot water in the outside casement. 

 Agitation is obtained by movable rods suspended vertically in the 

 milk, and thus the heat is rapidly diffused through the whole mass. 

 The pasteurised milk is led at once into the coolers without ex- 

 posure to air. The coolers are of two kinds, which may be used 

 separately or conjointly. In one set of cylinders there is cold 

 circulating water, in the other finely crushed ice.^ 



Summary. — Without entering into a long discussion upon the 

 various methods adopted, we may summarise some of the chief 

 essential conditions. It need scarcely be said that the operation 

 must be efficiently conducted, and in such a way as to maintain abso- 

 lute control over the time and temperature. The apparatus should 

 be simple enough to be easily cleaned and sterilised, and economical 

 in use. Arrangements must also be made to protect the milk from 

 re-infection during and after the process. The entire preparation 

 of the milk for market may be summed up in four items : — 



1. Pasteurisation in heating apparatus. 



2. Rapid cooling in water or ice coolers. 



^ Report from Wisconsin Agricultural Expt. Sta., 1896. Many modifica- 

 tions of this system are in vogue in England and Denmark as well as America, 

 especially under the management of large dairy companies. The important 

 points in either of the three methods referred to above, are (i) exact and clean 

 management of details ; and (2) control of the source and handling of the milk 

 to be pasteurised. 



