Overview
ISO/TS 17193:2011 (IDF/RM 208:2011) is a technical specification that defines a photometric reference method for the determination of lactoperoxidase activity in milk when activity exceeds 50 U/l. The method uses the ABTS reagent and measures the enzymatic oxidation product photometrically at 420 nm under controlled conditions (pH 6.0, 25 °C). It is published jointly by ISO and IDF and intended for laboratories performing enzyme-activity testing in dairy matrices.
Key topics and requirements
- Scope: Quantification of lactoperoxidase (LPO) activity in milk > 50 U/l.
- Principle: In the presence of LPO, ABTS is oxidized to its radical cation (ABTS•+); the rate of increase in absorbance at 420 nm is proportional to enzyme activity.
- Controlled conditions: pH 6.0 buffer, temperature 25 °C ± 0.5 °C, measurement at 420 nm.
- Reagents: pH 6 buffer, freshly prepared hydrogen peroxide solution, and a 2 mmol/l ABTS reagent prepared daily (ABTS diammonium salt + H2O2 in buffer).
- Apparatus: Spectrophotometer with temperature-controlled cuvette holder, plastic cuvettes (1.0 cm pathlength recommended to avoid adsorption losses), calibrated pipettes, volumetric flasks, and temperature bath.
- Procedure highlights:
- Dilute milk according to expected activity (e.g., 1→5, 1→10, 1→25 depending on range).
- Add 0.05 ml diluted sample to 2.00 ml ABTS reagent in a cuvette pre-equilibrated at 25 °C.
- Measure absorbance changes and calculate differential absorbance per minute.
- Calculation: Activity (U/l) calculated from the change in absorbance using specified constants (total volume, sample volume, absorptivity ε = 43.2 µmol−1 cm−1, cuvette path length 1.0 cm, stoichiometric coefficient 2 and dilution factor).
- Reporting: Results expressed to the nearest whole number; reports must reference ISO/TS 17193 and include sampling and test details.
- Quality note: No full interlaboratory precision study was available at publication; document issued as a Technical Specification.
Applications
- Routine dairy quality control to monitor LPO activity as an indicator of raw milk quality and antimicrobial system status.
- Research and development in dairy science studying enzyme activity, milk stability, and processing effects (e.g., UHT treatment - specific dilution guidance given).
- Regulatory and analytical laboratories requiring a reference photometric method for validation, method comparison, or internal quality assurance.
Who should use this standard
- Dairy testing laboratories and quality assurance teams
- Food technologists and dairy scientists
- Regulatory testing bodies and method validation units
- Instrument manufacturers and laboratory service providers producing validated enzymatic assay offerings
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