. Bacteriology and mycology of foods. Food. HYGIENIC LABORATORY METHOD 157 matter, and therefore serves as a fairly accurate means of estimat- ing, in the presence of organic matter, the germicidal values of dis- infectants in general. The method of determining the of disinfectants with the addition of organic matter is identical in many respects .with the method in which no organic matter is used, the former differing from the latter principally in the strength of the disinfectant dilutions that have to be prepared and in the prepa- ration and addition of the organic matter to th
. Bacteriology and mycology of foods. Food. HYGIENIC LABORATORY METHOD 157 matter, and therefore serves as a fairly accurate means of estimat- ing, in the presence of organic matter, the germicidal values of dis- infectants in general. The method of determining the of disinfectants with the addition of organic matter is identical in many respects .with the method in which no organic matter is used, the former differing from the latter principally in the strength of the disinfectant dilutions that have to be prepared and in the prepa- ration and addition of the organic matter to the disinfectant dilutions when performing the test. As the method in which no organic matter is added has already been given in detail, further description here will consist only of the differences between the two methods. Dilutions. In making the dilutions of a disinfectant for determining its coefficient in the presence of organic matter, allowance must be made for the further dilution of the disinfectant when the volume of organic matter is added thereto. For instance, if 1 of organic matter is added to 5 of a 1 per cent dilution of a disinfectant the percentage strength of the disin- fectant is proportionately reduced, it then being about per cent. The volume of organic matter that can be added to the disinfectant dilution is, of course, variable, as is also the volume of disinfectant dilution to which it can be added. Consequently, we have decided rather arbitrarily to use the organic matter by adding 1 of it to 4 of the disinfectant dilution contained in a seeding tube. It will be seen that the strength of the disinfectant is reduced 20 per cent by the addition of the organic matter and that the dilutions of the disinfectant must be made accordingly. For example, if it is desired to test a 1 per cent strength of a disinfectant it is necessary to prepare a strength of per cent, 4 of which 1 per cent strength when 1 of the or
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