Overview
ISO 7889:2003 - Yogurt - Enumeration of characteristic microorganisms - Colony-count technique at 37 °C specifies a validated microbiological method to enumerate the two characteristic yogurt bacteria, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, using a colony‑count plating technique at 37 °C. The standard applies when both organisms are present and viable and defines media, incubation conditions, dilutions, counting, confirmation and calculation procedures to express results as colony‑forming units per gram.
Key topics and requirements
- Scope and principle: Decimal dilutions of yogurt are plated and incubated on selective media; counts on plates at appropriate dilutions are used to calculate organisms per gram.
- Culture media: Acidified MRS for L. delbrueckii subsp. bulgaricus and complete M17 (with added lactose) for S. thermophilus. Media preparation and pH targets after sterilization are specified (acidified MRS pH 5.4 ± 0.1; M17 pH 6.8 ± 0.1).
- Incubation conditions:
- Acidified MRS: anaerobic incubation at 37 °C ± 1 °C for 72 h (for L. delbrueckii subsp. bulgaricus).
- M17: aerobic incubation at 37 °C ± 1 °C for 48 h (for S. thermophilus).
- Sampling and dilutions: Follow ISO general rules for preparation of test samples and decimal dilutions; inoculate plates from appropriate dilutions to obtain countable colonies.
- Counting and confirmation: Colony morphology and confirmation tests are required to distinguish the two species; counts are converted to CFU/g using standard calculation procedures.
- Apparatus and quality: Specifies incubators (including anaerobic jars), pH‑meter accuracy, water baths, blenders/stomachers, and colony‑counting equipment. Prepared media storage (2–4 °C up to one week) and reagent quality are addressed.
- Precision and reporting: Includes guidance on repeatability, expression of results and required information in the test report.
Applications and users
ISO 7889:2003 is used by:
- Dairy and yogurt manufacturers for quality control of starter viability and product specifications.
- Microbiological testing laboratories and contract QC labs performing yogurt analysis.
- Regulatory authorities assessing compliance with compositional or labeling requirements related to live cultures.
- Research groups studying starter performance and shelf‑life of fermented dairy products.
Practical benefits include standardized colony‑count procedures, reproducible enumeration of L. bulgaricus and S. thermophilus, and confidence in inter‑laboratory comparisons.
Related standards
- ISO 6887‑1 - Preparation of test samples, initial suspension and decimal dilutions
- ISO 7218 - General rules for microbiological examinations
- ISO 8261 / IDF 122 - Guidance for preparation of test samples and dilutions for milk and milk products
Keywords: ISO 7889:2003, yogurt, enumeration, colony-count technique, Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, MRS, M17, microbiological examination, 37 °C, CFU/g.