Pasteurization of Milk 



small surface in proportion to the ca- 

 pacity claimed for it must utilize a 

 heating medium of much higher tem- 

 perature in order to heat this quantity 

 of milk. While such a heater will dis- 

 charge milk of apparent uniform tem- 

 perature, it is actually only an aver- 

 aging temperature and the cream of 

 that part of the milk that is over- 

 heated will not raise in the bottle. 



When possible, the heater should be 

 installed above the holding tanks, so 

 that the milk may flow by gravity. 

 Alilk heaters should have, if any, as 

 few joints or loose parts as possible, 

 for joints and unions are likely to leak 

 and cause loss of milk. A deposit from 

 the milk forms on the heating surface 

 that must be removed every day; there- 

 fore, the use of a device for heating 

 milk that cannot be scrubbed clean 

 should be prohibited. 



The instntment for controlling the 

 temperature of the milk discharged 

 from the heater and the instrument for 

 recording the same are hereinafter re- 

 ferred to. 



(c) Holders. The reliable opera- 

 tion of the holding device of the pas- 

 teurizing apparatus is most important 

 if efficient treatment is to be obtained. 

 In the systems using a vat or series 

 of vats in which the milk stands dur- 

 ing the holding period, a part of the 

 milk may be held much longer than the 

 holding period ; while this is no disad- 

 vantage, it is a condition often over- 

 looked. The foam on the top of the 

 milk in the tank may\ lower in tem- 

 perature, or the draw-ofif valves may 

 leak and allow seepage, of the not 

 fully heated milk. If the heating sur- 

 face of these vats is supplemented by 

 coils, these coils are troublesome to 

 clean, and if the coils rotate, stuffing- 

 boxes are inevitable. In the vat sys- 

 tem it is important to have either suf- 

 ficient insulation of the heat main- 

 tained by circulation of controlled tem- 

 perature water, to prevent the lower- 



ing of the temperature of the milk and 

 the foam. It is also necessary to have 

 a safety outlet in the discharge pipe 

 from the vats that will permit any 

 milk that may leak through the valve 

 before the end of the holding period to 

 run out onto the floor. 



In the system using a series of hold- 

 ing tanks for continuous operation, 

 there should not be less than three 

 tanks to the series and set vertically. 

 They must be insulated except when 

 provided with controlled temperature 

 water jackets. One and one-half 

 inches of well-packed hair felt between 

 metal castings is sufficient insulation. 

 These tanks must be provided with a 

 feeder that will distribute the inflow- 

 ing milk into small streams at right 

 angles to the vertical flow of the milk 

 in the tank and a collector for the out- 

 going milk that will collect the milk 

 through several orifices. The milk 

 from the heater is supplied to the first 

 tank of the series, from the first tank 

 to the second, and so on throughout 

 the whole series. 



The milk content of the series of 

 continuous flow holders should be the 

 same as the maximum hour capacity 

 of the heater; this will give with 50% 

 efficiency a net holding time of thirty 

 minutes. A theoretical holding pe- 

 riod of one hour, and with an efficiency 

 of 75%, will give a net holding time of 

 forty-five minutes, an excess holding of 

 50%, or fifteen minutes over the mini- 

 mum holding time of thirty minutes. 

 An excess holding time only requires 

 an increase in the size of the tanks, 

 AA'hich adds but little to the original 

 cost and the additional cleaning is 

 very small, with no extra cost for op- 

 eration. This excess holding time over 

 the thirty minutes' minimum is a fac- 

 tor of safety in pasteurization that may 

 well justify its general adoption on all 



The drain outlet from the bottom of 

 the continuous flow holders must not 

 be connected -directly to the pipe lead- 



