Overview
ISO 9232:2003 (IDF 146:2003) defines standardized microbiological tests for the identification of the characteristic yogurt microorganisms: Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. The standard specifies procedures based on morphological, cultural and physiological properties and applies to strains isolated from yogurts in which both characteristic organisms are present and viable. It is intended for routine laboratory identification in dairy microbiology and quality control.
Key topics and requirements
- Scope and principle
- Identification is based on morphology, culture behaviour and biochemical/physiological tests for each organism.
- Culture media and reagents
- Specifies prepared media such as skimmed milk, MRS broth, M17 broth, litmus milk, overlay agar and specialized media for CO2 production and growth in 6.5% NaCl.
- Reagents include methylene blue stain, hydrogen peroxide for catalase test, and pH adjustment solutions.
- Guidance on preparation, sterilization and short-term storage (media stored at 2–4 °C for up to 1 week unless otherwise stated).
- Apparatus
- Typical microbiology equipment plus incubators capable of 10 °C, 15 °C, 37 °C and 45 °C; water baths; pH meter; calibrated pipettes and test tubes.
- Procedure highlights
- Isolation of colonies from enumeration plates (per ISO 7889/IDF 117), subculture to obtain pure cultures and incubation (e.g., 37 °C for 24 h for routine cultures).
- Morphological examination using methylene blue-stained smears and microscopic observation.
- Physiological tests such as catalase reaction and fermentation/CO2 production assays; use of diagnostic kits for fermentation tests is permitted when validated.
- Results and reporting
- Identification criteria and expression of results are provided, with normative annexes containing main attribute tables and a method for lactic acid/lactate enantiomer determination.
Applications and users
ISO 9232:2003 is used by:
- Dairy quality control and microbiology laboratories
- Starter-culture manufacturers and R&D teams
- Food safety and regulatory agencies verifying yogurt composition
- Academic and industrial researchers studying yogurt microbiota
Practical use cases include confirming starter strains in commercial yogurts, validating culture collections, and troubleshooting fermentation or quality issues in yogurt production.
Related standards
- ISO 7889 / IDF 117 - enumeration of characteristic yogurt microorganisms
- ISO 6887-1, ISO 7218, ISO 8261 / IDF 122 - general microbiology sampling and examination rules
- Joint publication with IDF and reference to AOAC collaboration in development
Keywords: ISO 9232:2003, yogurt identification, Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, MRS broth, M17 broth, yogurt microbiology, identification tests.