302 CITY MILK SUPPLY 



Actual Pasteurization. In actual pasteurization the first thing is for 

 the operator to assure himself that the apparatus is in condition to start 

 the run. It is assumed that the apparatus is so placed that the milk in 

 the course of its passage is not unnecessarily exposed to dust and that at 

 the close of work on the preceding day the whole outfit was thoroughly 

 scrubbed and rinsed. So the inspection preliminary to starting the run 

 should be a close one in which the effort is to make sure that the apparatus 

 is set up properly and is mechanically sound that the run may proceed with- 

 out interruption and to detect any dirt that may lurk in seams, turns and 

 corners. Then, because dust has settled on the machinery since it was 

 scrubbed, every piece is given a final cleansing. The receiving tanks are 

 rinsed with hot water and blown out with steam. The separator and 

 clarifier are rinsed with hot water. If a strainer is used, pains are taken 

 to see that it is in fit condition. Wire strainers are apt to collect cheesy 

 material between the meshes, quartz filters must be carefully washed 

 and sterilized with steam, and cloth filters must be renewed after each 

 run and must be put together carefully. Both the heater and holder 

 are rinsed off with hot water.' Dust that has settled on the cooler is 

 rinsed off with hot water and the cooler afterward steamed. The 

 apparatus is then ready for use; the milk is started through and the proc- 

 ess conducted in accordance with the procedure approved for the 

 machinery in use. The results should be satisfactory but if they are not, 

 samples for bacterial test must be taken of the milk as it enters and as it 

 issues from each machine that composes thep asteurizing unit, for only 

 in this way can the part that is doing faulty work be located. 



Efficiency of Pasteurization. The efficiency of pasteurization is 

 commonly judged by the reduction that is affected in the bacterial con- 

 tent of the raw milk. This is stated either by giving the numbers of bac- 

 teria in both the raw and the pasteurized milk, as for example 10,000,000 

 per cubic centimeter before and 200,000 after pasteurization, or by giving 

 the percentage of reduction accomplished, which is sometimes called the 

 "bacterial efficiency," as for instance 98 per cent. This is sometimes mis- 

 leading, in this way; a bacterial efficiency of 98 per cent, on a milk that 

 contained 10,000,000 bacteria per cubic centimeter would mean that there 

 would remain in the milk 200,000 bacteria per cubic centimeter whereas 

 with the same efficiency on a milk that contained 10 5 000 bacteria per 

 cubic centimeter, but 200 bacteria per cubic centimeter survived. Ayres 

 and Johnson show that the percentage of bacterial reduction has no special 

 meaning since it is influenced by the numbers and kinds of bacteria in the 

 milk pasteurized. As a general rule, when there are many bacteria in 

 the raw milk there will be a high, and when there are few there will be a 

 low, percentage reduction. While the bacterial count is a good index 

 of efficiency, it is open to criticism on the ground that.it makes the assump- 

 tion that a high reduction in the count shows the destruction of pathogenic 



