GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 061 of 500
Plate-count worksheet. Enter the sample, dilution and duplicate plate counts to get the aerobic plate count in CFU per gram.
Try sample values — fills the form with the worked example below.
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CFU/g
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Log₁₀ CFU/g
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Detection limit (CFU/g)
Equation: CFU/g = mean count ÷ (V × d), where d = [sample ÷ (sample + diluent)] × further dilution; V = volume plated (mL); d = total dilution factor. Log₁₀ CFU/g = log₁₀(CFU/g). Detection limit = 1 ÷ (V × d).
Method: Aerobic plate count, ISO 4833-1 / FDA BAM Chapter 3. Plate count agar, 30 °C for 72 hours. Countable range 30 to 300 colonies per plate.
Assumptions: Each colony grew from one viable unit; the sample was mixed evenly; the dilution series was prepared correctly; both plates were incubated the same way.
Limitations: Counts below 30 or above 300 colonies are estimates, not reportable counts; clumps and chains count as one colony; injured cells may not grow on the medium used.
Rounding: CFU/g shown as a whole number; log₁₀ to 2 decimal places; intermediates unrounded. In a laboratory report, give CFU/g to 2 significant figures.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Aerobic plate count, ISO 4833-1 / FDA BAM Chapter 3. Plate count agar, 30 °C for 72 hours. Countable range 30 to 300 colonies per plate.
Assumptions: Each colony grew from one viable unit; the sample was mixed evenly; the dilution series was prepared correctly; both plates were incubated the same way.
Limitations: Counts below 30 or above 300 colonies are estimates, not reportable counts; clumps and chains count as one colony; injured cells may not grow on the medium used.
Rounding: CFU/g shown as a whole number; log₁₀ to 2 decimal places; intermediates unrounded. In a laboratory report, give CFU/g to 2 significant figures.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: 10 g sample in 90 mL diluent with a further 10⁻³ step, 1 mL plated, duplicate counts 245 and 255. Total dilution = 0.1 × 0.001 = 0.0001; mean count 250; CFU/g = 250 ÷ 0.0001 = 2,500,000 CFU/g, log₁₀ = 6.40. Both counts are inside the countable range.
Enter values above to see results.
Typical aerobic plate counts: pasteurised milk under 20,000 CFU/mL; ready-to-eat foods are often specified under 100,000 CFU/g — check the specification for the product. Tool 061 of 500 — GIFSQ
