~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. [Illustration: 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 deg., 176 deg. and 185 deg. F. They found the efficiency of the machine not wholly satisfactory at the lower temperatures. At 158 deg. 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 deg. F. showed an average of only 117, with a range from 300 to 20. The results at 185 deg. F. showed practically the same results as noted at 176 deg. 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 deg.-164 deg. 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 deg. to 185 deg. F., in order to secure satisfactory results on the bacterial content of the cream. The writer tested Reid's pasteurizer at 155 deg. to 165 deg. F. with the following results: in some cases as many as 40 per cent. of the bacteria survived, which number in some cases exceeded 2,000,000 bacteria per cc. ~Pasteurizing details.~ While the pasteurizing process is exceedingly simple, yet, in order to secure the best results, certain conditions must be rigidly observed in the treatment before and after the heating process. It is important to select the best possible milk for pasteurizing, for if the milk has not been milked under clean conditions, it is likely to be rich in the spore-bearing bacteria. Old milk, or milk that has not been kept at a low temperature, is much richer in germ-life than perfectly fresh or thoroughly chilled milk. The true standard for selecting milk for pasteurization should be to determine the actual number of bacterial _spores_ that are able to resist the heating process, but this method is impracticable under commercial conditions.