SOP: Total Plate Count (Aerobic)

The aerobic plate count is the broadest hygiene indicator in food microbiology. It will not name the organism, but a rising count tells you the process is drifting long before anyone gets sick. Every micro lab runs this test daily.

Microbiological testing protects consumers from pathogens and tracks hygiene across the plant. Because living organisms are the analyte, aseptic technique and precise incubation conditions are not optional extras; they are the measurement itself. A single lapse in sterility or temperature control invalidates the result.

Purpose

To enumerate aerobic mesophilic bacteria in foods by the pour plate method as a general hygiene and quality indicator.

Scope

Applies to raw materials, in-process samples, and finished products. Not selective; counts all aerobes able to grow on plate count agar at the incubation conditions.

Equipment and Materials

• Plate count agar (PCA), sterilized
• Buffered peptone water or 0.1 percent peptone diluent, 90 mL blanks and 9 mL tubes
• Sterile petri dishes, pipettes, and stomacher bags
• Stomacher or blender
• Incubator at 35 plus or minus 1 C
• Colony counter
• Autoclave

Procedure

1. Aseptically weigh 25 g of sample into a sterile stomacher bag. Add 225 mL of diluent to make the 1:10 homogenate.
2. Stomach for 2 minutes (or blend 1 minute) to release organisms uniformly.
3. Prepare decimal dilutions: transfer 1 mL to 9 mL diluent tubes, vortexing between each, to reach the expected countable range.
4. Pipette 1 mL of appropriate dilutions into duplicate sterile petri dishes.
5. Pour 12 to 15 mL of PCA cooled to 44 to 46 C into each dish. Swirl gently to mix, avoiding bubbles.
6. Let solidify, then invert and incubate at 35 C for 48 plus or minus 2 hours.
7. Count plates with 25 to 250 colonies using the colony counter. Average the duplicates.
8. Include a diluent blank and an air-exposure plate with each batch as controls.

Recording Results

CFU per g equals average colony count times dilution factor. Report as CFU/g, using two significant figures. Plates outside 25 to 250 are reported as estimated counts. Compare against product specifications.

Quality Control

Verify each new lot of culture media with reference strains before use: the target organism must show typical growth and morphology, and a non-target strain must be inhibited or show atypical growth as expected. Include a positive control (reference strain) and a negative control (sterile diluent) with every test batch. Incubate an uninoculated plate from each poured batch as a sterility check. Monitor incubator temperatures twice daily with a calibrated thermometer; do not use incubators found outside tolerance until corrected and re-verified. Track contamination rates in blanks; a rising blank contamination rate signals a systemic problem requiring investigation. Participate in proficiency testing at least annually and investigate any unsatisfactory scores with documented corrective action.

Troubleshooting

• Spreader colonies overrunning the plate: the agar was too wet or incubation too long. Dry plates briefly before use and read at 48 hours sharp.
• No growth on known positive samples: agar too hot at pouring (above 47 C kills organisms). Check the tempering bath.
• High blanks: contaminated diluent or dishes. Re-sterilize and check autoclave performance.
• Uneven distribution: inadequate mixing of the homogenate. Standardize stomaching time.

Safety Notes

• Treat all plates as potentially infectious. Do not open incubated plates outside containment.
• Autoclave all plates and dilutions at 121 C for 20 minutes before disposal.
• Disinfect spills with freshly prepared bleach solution.