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Determination of the Aerobic Colony Count in Foods
5. MATERIALS AND SPECIAL EQUIPMENT
The following media and reagents (1-4) are commercially available and are to be prepared and sterilized according to the manufacturer's instructions. See also Appendix G of Volume 2 for the formula of individual media.
1) Plate count agar (PC)
2) Peptone water diluent (0.1%)(PW)
3) 2% sodium citrate (tempered to 450C) (for cheese samples only)
4) Sodium 2,3,5 triphenyltetrazolium chloride (0.1%) (optional)
5) 1N HCl and 1N NaOH
6) pH meter or paper capable of distinguishing to 0.3 to 0.5 pH units within a range of 5.0 to 8.0
7) Stomacher, blender or equivalent
8) Incubator capable of maintaining the growth temperature required for the specific type of aerobic bacteria being enumerated (i.e. for psychrophilic bacteria: 15 - 20oC, for mesophilic bacteria: 30 - 35oC, and for thermophilic bacteria: 55oC) and 45oC waterbath
Note: It is the responsibility of each laboratory to ensure that the temperature of the incubators or waterbaths are maintained at the recommended temperatures. Where 35oC is recommended in the text of the method, the incubator may be 35 +/-1.0oC. Similarly, lower temperatures of 30 or 25oC may be +/- 1.0oC. However, where higher temperatures are recommended, such as 43 or 45.5oC, it is imperative that the incubators or waterbaths be maintained within 0.5oC due to potential lethality of higher temperatures on the microorganism being isolated.
9) Colony counting device (optional)
6. PROCEDURE
Determine which type of aerobic bacteria are being enumerated. Analyze each sample unit individually.
The test shall be carried out in accordance with the following instructions:
6.1 Handling of Sample Units
6.1.1 During storage and transport, the following shall apply: with the exception of shelf-stable products, keep the sample units refrigerated (0-5oC). Sample units of frozen products shall be kept frozen.
6.1.2 Thaw frozen samples in a refrigerator or under time and temperature conditions which prevent microbial growth or death.
6.1.3 Analyze sample units as soon as possible after receipt in the laboratory.
6.2 Preparation of Media
6.2.1 Prepare plate count agar and dispense in appropriate quantities. Sterilize.
6.2.2 Temper prepared melted agar in a waterbath to 45oC ensuring that the water level is 1 cm above the level of the medium in the bottles.
6.2.3 Clean surface of working area with a suitable disinfectant.
6.2.4 Clearly mark the duplicate Petri plates.
