30 BULLETIN 420;, U. S. DEPARTMENT OP AGRICULTURE. 



milk was low, the reduction caused by slow cooling was not so marked, 

 although the bacterial count was usually less. 



From the foregoing experiments, conducted on a laboratory and 

 also on a small commercial scale, it seems evident that there is no 

 bacterial increase in pasteurized milk which is cooled slowly; on 

 the contrary, there is often a decrease, especially when there are 

 large numbers of bacteria in the milk before pasteurizing. 



These results are of particular value, for they indicate that it is 

 possible to cool hot pasteurized milk slowly by means of forced-air 

 circulation. From the results of our experiments we believe this 

 is possible without any bacterial increase in the milk, provided the 

 cooling to 50° F. (10° C) does not require a longer period than five 

 hours. This period of time should not be exceeded. It should be 

 remembered that even if a period of three hours is required to cool 

 milk from 145° F. (62.8° C.) to 50° F. (10° C), the temperature is 

 within the growing range of bacteria for only a portion of that 

 period. With the process of cooling by forced-air circulation on a 

 commercial scale milk can be cooled to 50° F. (10° C.) within three 

 hours, which is well within the 5-hour limit, and in order to have a 

 wide margin of safety we recominend that hot-bottled pasteurized 

 milk be cooled within three hours after it is bottled. 



We wish to emphasize the fact that the bacteriological results 

 which have been discussed apply only to the slow cooling of hot 

 pasteurized milk during the process described in this bulletin. Pas- 

 teurized or raw milk must be held, after the cooling process, at low 

 temperatures in order to check bacterial development. 



EFFECT OF THE PROCESS ON THE CREAM LINE AND FLAVOR OF MILK. 



In the consideration of the process of bottling hot pasteurized 

 milk followed by slow cooling, it is of practical importance to know 

 what effect such a process will have on the cream line and flavor of 

 milk. Several laboratory experiments were first made to determine 

 this matter. Milk was pasteurized, and hot 500 c. c. graduated 

 cylinders were filled with hot milk up to the 500 c. c. mark. Together 

 with the cylinder of hot pasteurized milk one cylinder was filled 

 with raw milk and one with pasteurized milk which had been cooled 

 to 50° F. (10° C.) in 15 seconds by running through a coil immersed 

 in brine. The method of cooling the hot-bottled pasteurized milk 

 was varied considerably, as may be seen in Table V. After holding 

 the milk for 24 hours at 45° F. (7.2° C.) the numbers of cubic centi- 

 meters of cream were read off directly from the graduations on the 

 cylinder. This method, of course, gave a very reliable means of 

 determining the effect that heating and cooling had on the cream 

 line ; in fact, it was too accurate, since the marked differences in the 

 cream line seen in the cylinders were not apparent in the bottled milk. 



