Fig. 26. Effect of pasteurizing on germ content of milk. Black square represents bacteria of raw milk; small white square, those remaining after pasteurization.
Bacterial efficiency of continuous-flow pasteurizers. A quantitative determination of the bacteria found in milk and cream when treated in machinery of this class almost always shows a degree of variation in results that is not to be noted in the discontinuous apparatus.
Fig. 27. Reid's Continuous Pasteurizer.
Harding and Rogers[146] have tested the efficiency of one of the Danish type of continuous pasteurizers. These experiments were made at 158°, 176° and 185° F. They found the efficiency of the machine not wholly satisfactory at the lower temperatures. At 158° F. the average of fourteen tests gave 15,300 bacteria per cc., with a maximum to minimum range from 62,790 to 120. Twenty-five examinations at 176° F. showed an average of only 117, with a range from 300 to 20. The results at 185° F. showed practically the same results as noted at 176° F. Considerable trouble was experienced with the "scalding on" of the milk to the walls of the machine when milk of high acidity was used.
Jensen[147] details the results of 139 tests in 1899, made by the Copenhagen Health Commission. In 66 samples from one hundred thousand to one million organisms per cc. were found, and in 22 cases from one to five millions. Nineteen tests showed less than 10,000 per cc.
In a series of tests conducted by the writer[148] on a Miller pasteurizer in commercial operation, an average of 21 tests showed 12,350 bacteria remaining in the milk when the milk was pasteurized from 156°-164° F. The raw milk in these tests ran from 115,000 to about one million organisms per cc.
A recently devised machine of this type (Pasteur) has been tested by Lehmann, who found that it was necessary to heat the milk as high as 176° to 185° F., in order to secure satisfactory results on the bacterial content of the cream.