290 CITY MILK SUPPLY 



sheets of milk over hot surfaces; these are better heated by a hot water 

 jacket than by steam for the temperature is more easily controlled and 

 the milk is less likely to be scorched. 



A pasteurizing unit proper may consist of one or of two parts. In 

 the flash process there is only the heater; in the holder process there is 

 a heater and also a holder or a retarder, while the bottle process accom- 

 plishes the heating and holding in a single machine. The different types 

 of pasteurizers have been grouped according to their mode of operation 

 by Kilbourne and also by Ayres. The following arrangement follows 

 their suggestions. 



Type 1. The Danish heater. In this machine there is a revolving 

 paddle in a milk chamber that has a water jacket which is filled by in- 

 jecting steam from three jets till it condenses and fills the jacket with 

 hot water. Milk is fed through an inlet into the bottom of the chamber 

 and by the centrifugal action of the paddle is thrown against the sides 

 of the hot water jacket, being forced out at the top through the milk 

 outlet pipe in which is a thermometer for taking the temperature of the 

 outgoing pasteurized milk. 



The advantages of this type are its small cost, ease of cleaning and 

 power to lift the milk about 4 ft. to a holder or cooler. 



Type 2. This pasteurizer consists of a cylinder surrounded by a water 

 jacket that is heated by a steam discharge into the water pipe connected 

 thereto. Within the cylinder is a revolving drum to which milk is carried 

 through the milk inlet, and which in revolving, by centrifugal action 

 spreads the milk over its surface in a film, between it and the hot water 

 jacket, finally discharging the milk through the outlet with force to raise 

 it several feet. 



Type 3. This is a machine consisting of a conical surface that is 

 constantly wiped by a tape attached to a revolving frame and that is 

 heated by steam discharging into hot water beneath the cone. Milk is 

 run through the bottom of a reservoir over the top of the cone and flows 

 down its sides, being kept from burning by the action of the tape, into 

 a trough whence it is carried to the holder or cooler. 



Type 4. Machines in which the milk flows with a rotary motion, 

 between two or more water-heated upright cylindrical surfaces. Some 

 of these machines are constructed on the regenerative principle by which 

 the outgoing hot milk is utilized to warm the incoming cold milk, thereby 

 economizing heat. The most satisfactory machines of the fourth type 

 are those that operate with large enough volumes of water to allow the 

 water to be used at low temperatures. 



Type 5. These machines are coils of double tubes, the internal one 

 carrying the milk and the encasing one hot water that is heated by a 

 steam jet introduced to the water before it enters the coil. In this ap- 

 paratus the hot water flows in a direction opposite to that of the milk, 



