GEORGE W. PUTNAM 431 



The continuous-flow holder of the long-distance flow type (see Fig. 3) should be 

 checked by flow tests with water at the pasteurizing temperature by adding a chemi- 

 cal, such as starch or chloramine, which can be easily detected to determine that the 

 holder provides a thirty-minute holding period. Slight variations as to holding period 

 or temperature of the milk can be readily corrected in this type when first installed. 

 The holding period can be shortened by the operator to less than thirty minutes when 

 using a steam pump to discharge the milk into the holder by simply opening the 

 steam valve so as to increase the speed of the pump. The remedy is the installation 

 of a piston pump geared to a constant-speed motor which is set at a speed which will 

 fill the holder in approximately thirty-three minutes. In this type of continuous- 

 flow holder the holding period during operation is only slightly less than the calcu- 

 lated theoretical holding period. 



In order to insure a thirty-minute holding period while emptying the holder at 

 the end of a run, it is often necessary to equip the outlet-drain valve with a stop 

 permitting only partial opening in order that the rate of emptying will be slowed 

 down sufficiently to insure that all of the milk is held thirty minutes. 



Temperature defects to be guarded against include the failure of the operator to 

 heat the holder above the pasteurizing temperature before starting. In this case, the 

 first milk to enter gives up part of its heat to the mass of colder metal in the holder 

 tubes until they reach the pasteurizing temperature. Under certain conditions as 

 much as 100 gallons of the first milk will flow from the holder, starting as low as 125° 

 F. and gradually increasing until the pasteurizing temperature is reached. To mini- 

 mize the possibility of this occurring in commercial operation, an air thermometer 

 should be placed on the holder air-chamber and the operator instructed to heat the 

 holder above pasteurizing temperature before starting the milk flow. The recording 

 thermometer charts will show if he fails to do this properly. The temperature of 

 the air surrounding the holder tubes must be maintained high enough throughout 

 the run to prevent any drop in the temperature of the milk in passing through the 

 holder. 



Continuous-flow milk heaters must be used in connection with holders of the pocket 

 and continuous-flow type, and are frequently used with a series of holders of the vat 

 type. Failure to maintain the flow of milk at the pasteurizing temperature for any 

 considerable interval constitutes a defect. In the case of vat- and pocket-holders a 

 small amount of milk at a temperature lower than that required for pasteurization 

 will mix with the main volume of milk so that the temperature of the entire batch 

 may be lowered only a fraction of a degree. When the resulting temperature is ^° or 

 more lower than that required, it should be raised in the case of holders equipped 

 with heating coils or jackets. This is not possible in the case of holders of the vat or 

 pocket type which are merely insulated, since there is no means for adding heat. In 

 the case of long-distance, continuous-flow-type holders, mixing is limited principally 

 to a single tube as the milk enters, so that milk entering the holder at a temperature 

 lower than that required for even brief intervals often does not become mixed with 

 milk at a higher temperature. 



Improved attention and servicing of the steam controllers on heaters are necessary 

 to limit the variation in milk temperature to less than 1°. Automatic shut-off valves 



