420 ENGINEERING ASPECTS OF PASTEURIZATION 



of heating coils or a lieating jacket and agitator, held for thirty minutes, and then 

 run over a cooler, (c) In many instances a combination of the two methods is used, 

 the milk being heated to from ioo° to 140° F., by a milk heater, discharged into the 

 vat, brought to the exact pasteurizing temperature by means of the heating coil or 

 jacket, held for thirty minutes, and cooled, (d) In a few instances the milk, after being 

 heated and held by one of the foregoing methods, is cooled in the vat by passing cold 

 water or brine through the coils. This usually reduces the cream volume on the bottled 

 milk. In most cases the milk is pumped or flows by gravity over separate coolers. 

 On the other hand, cream, on account of its greater viscosity, is very often heated, 

 held, and cooled in the same vat. 



The pocket-type pasteurizer consists of a series of small vats built integrally in a 

 single holder unit (see Figs. 2 and 10). The individual vats or pockets are either in- 

 sulated or jacketed and may or may not be equipped with an agitator. Milk is 

 pumped continuously through a heater, brought to the pasteurizing temperature, 

 and discharged into the holder pockets in succession by means of one or more auto- 

 matically operated valves. 



Two circular-type holders on the market are designed to rotate at a constant rate 

 so that the sector pockets will be filled in succession by passing under a stationary 

 milk-inlet pipe. In all types of pocket holders the milk, after a thirty-minute holding 

 period, is discharged from the pockets in succession by automatic means, insuring 

 practically continuous flow of milk to the cooler. 



The continuous-flow pasteurizer consists of a holder through which heated milk 

 flows continuously. One holder used extensively consists of a cylindrical tank with 

 nested baffles in which milk, discharged in at the top, flows down through the holder 

 by gravity and is pumped from the bottom over a cooler. 



A second holder, termed the "long-distance-flow type," consists of a series of 

 metal tubes, 5! or 7 inches in diameter, having a very slight slope, housed in a large 

 boxlike compartment in which the air is heated and maintained at a fairly uniform 

 temperature (see Fig. 3). The milk is heated to the pasteurizing temperature, enters 

 the holder, flows through each of the tubes in succession to the outlet, taking thirty 

 minutes or more for this travel, and is then pumped to the cooler. 



The in-bottle pasteurizer, as its name implies, is an arrangement for pasteurizing 

 milk in the bottle in which it is to be delivered to the consumer. Small outfits con- 

 sist of a combination steam and ice-chest in which cases of bottled milk are placed, 

 using special covers over the bottles, heated with steam to the pasteurizing tem- 

 perature, held for thirty minutes, and then cooled with water and ice. The cases 

 of milk are then removed and the bottles capped with the ordinary paraffined card- 

 board cap. In some instances the bottles are capped with crown caps before pasteur- 

 izing. 



The large, continuous in-bottle machines have traveling carriers for the cases. 

 Bottles are filled with milk, preheated to about 100° F., and capped with a crown cap. 

 The cases of bottled milk travel on the carriers through a succession of water com- 

 partments which first heat the milk to the pasteurizing temperature, hold it for thirty 

 minutes, and then cool the milk. 



The in-bottle method has the disadvantage of being somewhat more expensive 



