Overview
ISO/TS 11059:2025 - Milk and milk products - Method for the enumeration of Pseudomonas spp. specifies a standardized colony‑count technique for detecting and enumerating Pseudomonas spp. in milk, milk products and dairy production/handling environmental samples. The method targets both pigmented and non‑pigmented psychrotrophic Pseudomonas (e.g., members of the P. fluorescens group) that are key spoilage organisms in refrigerated dairy products and can indicate post‑pasteurization contamination from equipment or biofilms.
Key topics and technical requirements
- Scope: Applicable to raw and processed milk, milk products and environmental samples in dairy settings.
- Primary method: Colony‑count on selective penicillin and pimaricin agar (PPA) with incubation at 25 °C and examination after 48 h.
- Selective medium function: Nutrient digest (gelatine/casein) with MgCl2/K2SO4 for osmotic balance; penicillin G and pimaricin suppress competing microflora and promote pigment production.
- Dilutions and sampling: Decimal dilutions prepared in accordance with ISO 6887‑1 and ISO 6887‑5. For low limits of determination, examine 1 ml on a single large (140 mm) plate or three 90 mm plates to reach limits of 10 cfu/ml (liquids) or 100 cfu/g (solids). Duplicate plates are recommended when counts are near detection limits.
- Confirmation: Randomly select colonies, subculture on non‑selective medium (e.g., nutrient agar), incubate at 25 °C for 24–48 h, and perform biochemical tests: oxidase reaction and glucose fermentation (confirmation criteria aligned with tests used for presumptive Enterobacteriaceae).
- Reporting and QA: Results expressed per unit (cfu/ml or cfu/g) with recommended quality assurance and media performance testing (Annex B). A flow diagram of the procedure is provided (Annex A).
Practical applications
- Routine quality control in dairy processing plants to monitor raw milk quality and detect post‑pasteurization contamination.
- Environmental monitoring (surfaces, equipment rinse water) to identify biofilm‑related contamination sources.
- Shelf‑life investigations and root‑cause analysis of spoilage (off‑flavour, gelation) linked to heat‑stable proteases/lipases produced by psychrotrophic Pseudomonas.
Who should use this standard
- Dairy microbiologists and laboratory analysts performing microbial enumeration.
- Quality assurance, HACCP and food safety teams in dairy manufacturing.
- Regulatory and testing laboratories tasked with microbiological surveillance of dairy products and environments.
Related standards
Keywords: ISO/TS 11059:2025, Pseudomonas spp., milk and milk products, enumeration, colony‑count, PPA agar, psychrotrophic, dairy microbiology, oxidase test, glucose fermentation.