Overview
ISO 21187:2021 - Milk - Quantitative determination of microbiological quality - Guidance for establishing and verifying a conversion relationship between results of an alternative method and anchor method results - provides practical guidance for converting quantitative microbiological results obtained with an alternative method to the scale of a defined anchor method, and for verifying that conversion. The document focuses on milk and addresses how to make results and regulatory limits comparable across methods so that rapid or novel analytical approaches can be used in place of traditional methods while maintaining traceability and confidence in results.
Key topics and technical requirements
- Scope and purpose: Guidelines for establishing and verifying a conversion relationship for quantitative microbiological testing of milk; conversion can be used both to express alternative-method results on an anchor-method basis and to translate anchor-method limits into alternative-method units.
- Principles: Distinguishes conversion from calibration - conversion aligns different methodological scales rather than attempting to define a “true” microbiological value.
- Factors influencing conversion relationship: Environmental (animal species, bulk-milk storage, seasonal variation, production conditions), sampling and pre-treatment, and analytical (instrument make/model, reagents, high somatic cell counts).
- Test-sample design: Guidance on number, range and representativeness of test samples, pre-treatment, sub-sampling, storage and transport.
- Analysis and statistical treatment: Establishing the conversion relationship (now based on grouped data in the 2021 edition), removal/identification of outliers, linear regression approaches and calculation methods.
- Verification and frequency: Procedures for ongoing verification of the conversion relationship and statistical tests to confirm continued validity.
- Reporting: Required elements for test reports and annexes with worked examples (outlier identification, conversion calculation, verification spreadsheets).
Practical applications
- Enabling adoption of rapid or automated microbiological methods in dairy quality control by translating their output to established regulatory/anchor-method units.
- Supporting method validation and method-comparison studies for milk microbiology.
- Facilitating regulatory compliance, trade, and supplier/laboratory equivalence when different labs use different validated methods.
- Informing decisions about quality limits, acceptance criteria, and corrective actions based on alternative-method results.
Who should use this standard
- Dairy laboratory managers and quality-control personnel
- Method developers and instrument manufacturers
- Regulatory authorities and food safety auditors
- Accreditation bodies and testing laboratories performing microbiological analyses on milk
Related standards
- ISO 8196‑1 / IDF 128‑1 and ISO 8196‑2 / IDF 128‑2 (accuracy, calibration and quality control of alternative milk analysis methods)
- ISO 16140‑1 (microbiology method validation vocabulary)
ISO 21187:2021 helps dairy stakeholders convert, verify and confidently use alternative microbiological methods while preserving comparability and traceability to anchor-method results.
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