Enumeration Of Total Aerobic Bacteria In Food Products And Food Ingredients Using 3M^tm Petrifilm^TM Aerobic Count Plates

HPB Method MFHPB-33
February 2001

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1. APPLICATION

This method is applicable for the enumeration of aerobic bacteria in food products and food ingredients. This product may also be used for environmental sampling (see Laboratory Procedure MFLP-41A and MFLP-41B). This revised method replaces MFHPB-33, dated April
1997.

2. PRINCIPLE

Petrifilm plates are a ready-to-use product developed by the 3M Company, St. Paul, MN. Media are coated onto films, so that traditional media preparation is unnecessary, and both labour and time savings are realized. Aerobic bacteria can be enumerated on the Petrifilm Aerobic Count (AC) plates. The method uses bacterial culture plates containing dry medium and a cold-water-soluble gelling agent. One mL samples are added directly to the plates. Pressure, when applied to the plastic spreader placed on the overlay film, spreads the sample over 20 cm². The gelling agent is allowed to solidify, and the plates are then incubated and counted. Validation and collaborative studies have found the Petrifilm AC method to be not significantly different from the traditional method (7.1 - 7.13).

3. DEFINITION OF TERMS

3.1 See Appendix A of Volume 2.

3.2 Petrifilm AC plates contain modified Standard Methods nutrients; a cold-water-soluble gelling agent; and 2, 3, 5-triphenyl tetrazolium chloride (TTC) indicator. Reduction of the TTC will cause the colonies to become red.

3.3 Plastic spreading devices are provided with the Petrifilm plates. The concave side is designed to spread the sample over the growth area.