ISO 17792:2006

Overview

ISO 17792:2006 (IDF 180:2006) specifies a standardized colony-count technique at 25 °C for the enumeration of citrate‑fermenting lactic acid bacteria in milk, milk products and mesophilic starter cultures. The method targets characteristic citrate‑fermenting species (e.g., Lactococcus lactis biovar diacetylactis and Leuconostoc spp.) and is applicable where these microorganisms are present in dairy starter cultures and finished dairy products.

Key Topics and Requirements

• Scope and organisms: Enumeration of mesophilic citrate‑fermenting lactic acid bacteria that form lenticular colonies (about 0.5–1.2 mm) on selective media.
• Colony-count principle: Prepare decimal dilutions, plate on selective media, incubate aerobically at 25 °C, and count characteristic colonies at appropriate dilution levels to calculate counts per gram.
• Selective media: Uses a modified Nickels and Leesment medium with additions such as calcium lactate, calcium citrate suspension, starter culture serum and the chromogenic substrate X‑gal to differentiate species by colony color/zone formation.
• Antibiotic-supplemented variant: A Nickels and Leesment + vancomycin medium is specified for selective enumeration of leuconostocs (incubation 3–5 days).
• Incubation times: Typical aerobic incubation - 72 h on modified medium followed by X‑gal addition and an additional ~24 h; vancomycin plates incubated 3–5 days.
• Confirmation and calculation: Colonies are confirmed by appropriate tests; results are calculated and expressed according to the standard, with precision data and reporting requirements included.
• Safety and quality: The standard notes hazards (X‑gal, vancomycin) and refers to use of analytical‑grade reagents, sterile technique, suitable diluents and water quality.

Applications and Users

Who benefits from ISO 17792:2006:

• Dairy microbiology and quality control laboratories performing routine enumeration of starter culture components.
• Starter-culture manufacturers monitoring citrate‑fermenting strains (L. lactis biovar diacetylactis, Leuconostoc spp.).
• Regulatory, research and product development teams assessing microbial composition in cheeses, fermented milks and related dairy products. Practical uses include quality assurance of starter cultures, troubleshooting fermentation defects, and verifying label claims for culture composition.

Related Standards

• ISO 7218 - General rules for microbiological examinations of food and animal feeding stuffs.
• ISO 8261 / IDF 122 - Guidance for preparation of test samples, initial suspensions and decimal dilutions.
• ISO 565 - Test sieves (used for media preparation steps described).

Keywords: ISO 17792:2006, citrate‑fermenting lactic acid bacteria, milk products, colony‑count technique, Nickels and Leesment medium, X‑gal, vancomycin, dairy starter cultures, enumeration, 25 °C.