ISO 3356:2009

Overview

ISO 3356:2009 / IDF 63:2009 specifies a standardized laboratory method for the determination of alkaline phosphatase (ALP) activity in milk. The procedure quantifies ALP activity as the mass of phenol liberated (µg phenol per ml) and is applicable to raw and processed milk, milk powder, buttermilk and powders, whey and whey powder. The method covers activities not less than 1 µg phenol/ml and includes sample preparation, reagent recipes, measurement steps and reporting requirements.

Key topics and technical requirements

• Analytical principle: ALP in the sample hydrolyzes disodium phenylphosphate at pH 10.6 and 37 °C (1 h), releasing phenol which reacts with 2,6-dibromoquinonechloroimide (Gibbs reagent, BQC) to form a blue dye (dibromoindophenol) measured photometrically at 610 nm.
• Reagents and buffers: Detailed formulations provided for barium borate–hydroxide buffer, colour development buffers, disodium phenylphosphate substrate, zinc–copper precipitant, BQC (Gibbs reagent) and phenol standards.
• Apparatus: Photometer (610 nm), thermostatted water bath (37 ± 1 °C), boiling water bath, volumetric glassware, pipettes and filtration equipment.
• Sample preparation: Homogenize milk samples; reconstitute powders (10 g in 90 ml water, ≤35 °C); neutralize acidic samples (pH < 7) with 0.5 mol/L NaOH.
• Calibration and calculation: Prepare a phenol standard curve (0, 2, 5, 10, 20 µg phenol). Convert optical density to µg phenol and express ALP activity as µg phenol per ml (reported to one decimal place). Dilution procedures are specified for high-activity samples.
• Precision: Repeatability and reproducibility limits given (95% level): repeatability difference ≤ 1.5 µg/ml (in ≤5% of cases); reproducibility difference ≤ 3.9 µg/ml (in ≤5% of cases).
• Safety note: The standard warns about hazardous reagents and requires adherence to laboratory safety practices.

Applications and users

Who uses ISO 3356:2009:

• Dairy industry quality-control laboratories
• Regulatory and official food-testing laboratories
• Milk processors and pasteurization validation teams
• Research laboratories working on milk composition and processing

Practical uses:

• Routine ALP testing of milk and dairy powders
• Verification of heat treatment efficacy in processing (ALP is routinely monitored in dairy QC)
• Comparative testing across labs using a harmonized, internationally accepted method
• Support for regulatory compliance, product release and shelf-life studies

Related standards

• ISO 707 / IDF 50 - Sampling of milk (recommended sampling)
• ISO 11816‑1 / IDF 155‑1 and ISO 22160 / IDF 209 - Other methods/units for ALP activity
• ISO 5725‑1 / ISO 5725‑2 - Precision (repeatability and reproducibility) assessment

Keywords: ISO 3356:2009, IDF 63:2009, alkaline phosphatase, milk testing, phenol, photometric assay, dairy quality control, ALP determination, milk powder, whey.