Overview
ISO/TS 27265:2009 (IDF/RM 228:2009) specifies a microbiological colony-count method for the enumeration of specially thermoresistant spores of thermophilic bacteria in dried milk products. The method uses a heat treatment of 106 °C for 30 minutes followed by plating and incubation at 55 °C for 48 hours to count colony-forming units (CFU). Its scope is limited to dried whole milk, skim milk and buttermilk powders destined to be recombined for manufacture of sterilized products (e.g., UHT or retort-treated milk).
Key topics and requirements
- Target organisms: spores that survive 106 °C for 30 min and form colonies aerobically at 55 °C.
- Sample preparation: primary dilution prepared per ISO 6887-5; representative sampling recommended (see ISO 707/IDF 50).
- Heat treatment: samples heated in a pressure vessel or small autoclave; pilot tube monitoring required; time-to-temperature must be ≤ 6 min to reach 105.5 °C, then hold at 106 °C ± 0.5 °C for 30 min.
- Culture medium: plate count skim milk agar (PCSM) with 0.2% starch and bromocresol purple indicator to aid colony visibility.
- Plating & incubation: pour plates using specified volumes and dilutions; incubate aerobically at 55 °C ± 1 °C for 48 h ± 2 h; count all colonies (including pinpoint) and record CFU/g following ISO 7218 calculation guidance.
- Apparatus & controls: details provided for incubator, water baths, digital thermometer, pressure vessel modification and sterility controls; control plates required.
- Interpretation rules: handling of spreading colonies, replicate plate counting (three plates for undiluted), and documentation of counts, including counts outside statistical range.
Applications
- Quality control of dried milk powders intended for recombination into sterilized milk products (UHT, retort).
- Microbiological assurance for ingredient suppliers and dairy processors to assess risk of thermoresistant spore survival through commercial sterilization.
- Regulatory testing and validation of supplier specifications where thermophilic spore loads are a critical parameter.
Who should use this standard
- Dairy laboratories and QA/QC personnel performing microbiological testing on milk powders.
- Ingredient manufacturers supplying powdered milk for sterilized milk production.
- Food safety auditors and regulatory bodies assessing compliance for sterilized dairy product supply chains.
Related standards
- ISO 7218 - General microbiology requirements and guidance for examinations
- ISO 6887-5 - Preparation of test samples, dilutions for milk and milk products
- ISO/TS 11133-1 - Culture media preparation and quality assurance
- ISO 707 / IDF 50 - Recommended sampling of milk and milk products
Keywords: ISO/TS 27265:2009, dried milk, thermoresistant spores, thermophilic bacteria, enumeration, CFU, UHT, plate count skim milk agar.