ISO 8553:2004

Overview

ISO 8553:2004 (IDF 131:2004) specifies a standardized laboratory method for the enumeration of microorganisms in raw milk using the plate‑loop technique at 30 °C. The method uses a calibrated inoculating loop to transfer a fixed tiny volume of milk onto a Petri dish, pours plate count milk agar over the inoculum, incubates at 30 °C for 72 h, and calculates colony counts to express microorganisms per millilitre of sample.

Key topics and requirements

• Scope: Enumeration of bacteria, yeasts and moulds that form countable colonies under the method conditions for raw milk.
• Inoculum volume: Calibrated wire loop designed to hold 0.001 ml.
• Culture medium (plate count milk agar): example composition per litre - yeast extract 2.5 g, enzymatic digestion of casein 5.0 g, skimmed milk powder 1.0 g, glucose 1.0 g, agar 9–18 g, water 1 000 ml. pH after sterilization: 7.0 ± 0.2 at 25 °C.
• Media handling: Sterilize medium (autoclave 121 °C, 15 min); cool to 44–47 °C before pouring; store at 3 °C ± 2 °C for up to three months if not used immediately.
• Incubation: Aerobic incubation at 30 °C for 72 h ± 3 h; do not stack plates more than six high.
• Counting & calculation: Use plates with 10–300 colonies where possible; multiply the count per plate by 1 000 to obtain microorganisms per millilitre (plate‑loop result).
• Equipment & QC: Platinum inoculating loop, continuous pipette/self‑refilling syringe, silicone tubing, incubator, autoclave, colony counter. Include controls: sterile instrument control, rinsing control (≤2 colonies allowed), and agar control. Loop calibration should be checked by comparing results with ISO 4833 (standard plate count) - averages should differ by no more than ±10% for validation samples.
• Hygiene & asepsis: Specific disinfectant recommendations (sodium hypochlorite 50–100 mg/l active chlorine or 1:1 ethanol/acetone) and procedures for loop sterilization (chemical rather than flame).

Applications

• Routine microbiological quality monitoring of raw milk at collection, receiving and processing points.
• Rapid, low‑volume screening for total viable counts in dairy testing labs.
• Use in quality assurance, regulatory testing, process control and comparative studies where the plate‑loop technique is preferred or historically used.

Who uses this standard

• Dairy laboratory analysts and microbiologists
• Quality assurance and food safety teams in dairies and milk processors
• Third‑party testing laboratories and regulatory bodies assessing raw milk microbial loads

Related standards

• ISO 4833 - Colony count technique (30 °C)
• ISO 7218 - General rules for microbiological examinations
• ISO 8261 | IDF 122 - Sample preparation and dilutions for milk
• ISO/TS 11133‑1 / ‑2 - Culture media preparation and performance testing

Keywords: ISO 8553:2004, plate-loop technique, milk, enumeration of microorganisms, plate count milk agar, 30 °C, raw milk microbiology.