Overview
ISO 21528-2:2017 - "Microbiology of the food chain - Horizontal method for the detection and enumeration of Enterobacteriaceae - Part 2: Colony-count technique" - defines a standardized colony-count procedure for enumerating Enterobacteriaceae in foods, animal feeds and relevant environmental samples. The method is intended for cases where counts are expected to be greater than 100 per ml or g; if lower counts are expected, the MPN approach of ISO 21528-1 is generally used. The standard includes procedures for sample preparation, plating on selective medium, confirmation of presumptive colonies and calculation of results.
Key topics and technical requirements
- Scope & applicability: Products for human consumption, animal feed, and environmental samples related to primary production, food production and handling.
- Principle: Prepare initial suspension and decimal dilutions, plate on violet red bile glucose (VRBG) agar, overlay with the same medium, incubate and enumerate confirmed colonies.
- Incubation: Typically 37 °C for 24 h (30 °C can be chosen when psychrotrophic Enterobacteriaceae are of interest); 37 °C is the default in the document.
- Confirmation tests: Presumptive colonies are subcultured onto non‑selective medium and confirmed by glucose fermentation and oxidase-negative reaction, consistent with the Enterobacteriaceae definition in the standard.
- Counting & calculation: Report number of confirmed typical colonies per ml or g of sample; calculation method is specified for expression of results.
- Equipment & media: Lists required laboratory equipment (incubators, water baths, pipettes, homogenizers) and points to Annex A for culture media composition and Annex B for method performance characteristics.
- Quality & safety: Emphasizes use in properly equipped labs under skilled personnel and includes requirements for quality assurance and test reporting.
Practical applications and users
ISO 21528-2 is used by:
- Commercial and public food microbiology laboratories for routine hygiene testing.
- Quality assurance/QA-QC teams in food and feed manufacturing.
- Regulatory authorities conducting compliance testing and surveillance.
- Environmental monitoring programs in primary production and food-handling facilities.
Practical uses include assessing process hygiene, verifying sanitation, and supporting risk-based food safety programs by quantifying Enterobacteriaceae as hygiene indicators.
Related standards
Keywords: ISO 21528-2:2017, Enterobacteriaceae, colony-count technique, VRBG agar, food microbiology, enumeration, food safety, hygiene indicator.