AOAC Official Method 955.11 Testing Disinfectants against Salmonella typhi Phenol Coefficient Method
AOAC 955.11 is Applicable for testing disinfectants and sanitizers that are miscible with water or for determining the standard resistance of test bacteria. The 95% confidence limits are ±12%.
1-90 and 1-100 dilutions of the disinfectant are utilized and tested at 5 minutes, 10 minutes, & 15 minutes. Results are expressed in terms of phenol coefficient number, or highest dilution killing the test organism in 10 min but not in 5 min, whichever most accurately reflects germicidal value of disinfectant. The phenol coefficient is the number obtained by dividing the numerical value of the greatest dilution of the disinfectant capable of killing Salmonella typhi in 10 minutes but not in 5 minutes by the greatest dilution of phenol showing the same results.
Test is satisfactory only when phenol control gives one of the following readings:
Phenol 1 – 90: 5 min + or 0, 10 min + or 0, 15 min 0
Phenol 1-100: 5 min +, 10 min +, 15 min + or 0
Phenol resistance is no longer required by the EPA as part of the data required by the agency for registering a disinfectant or sanitizer. However, many disinfectant and sanitizer manufacturers find phenol resistance testing useful in the formulation process as it can provide an estimate of the intrinsic resistance or sensitivity of some bacteria to chemical disinfectants and sanitizers.
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