Overview
ISO 26462:2010 (IDF 214:2010) defines an enzymatic method for determining lactose content in milk and reconstituted milk by measuring the change in pH - a differential pH (pH‑metry) technique. The standard specifies reagents, apparatus, sample preparation, stepwise procedures (blank, calibration and measurement), electrode maintenance, calculation and expression of results (reported in mmol/L), and precision criteria (repeatability and reproducibility).
Key topics and technical requirements
- Principle: Lactose is hydrolysed by β‑galactosidase into glucose and galactose; at pH 7.8, glucokinase phosphorylates glucose, releasing protons. The resulting pH shift is proportional to lactose concentration and measured with a differential pH analyser.
- Reagents & solutions:
- Buffer solution at pH 7.8 ± 0.1.
- Enzyme solutions: glucokinase (target ~290 U/mL ±30 U/mL) and β‑galactosidase (≈1 500 U/mL ±200 U/mL); recommended storage at 4 °C (up to 6 months).
- Lactose standard solution (150 mmol/L) prepared accounting for lactose monohydrate water content.
- Cleaning, regenerating and strong regenerating solutions (safety warning for NaF-containing strong regenerant).
- Apparatus: Differential pH apparatus with peristaltic pumps, mixing chamber and two flow‑through electrodes (E1, E2); water bath (38 °C ±1 °C); micropipettes and class A volumetric flasks.
- Procedure highlights:
- Warm sample to 38 °C, cool to 20 °C before aliquoting.
- Perform blank determinations and calibration checks; ensure electrode offsets and blank ∆pH values meet stated limits.
- Regular electrode cleaning, regeneration and calibration verification are mandated.
- Performance: Standard includes interlaboratory trial data and defines repeatability and reproducibility metrics; results are expressed in mmol/L with conversion guidance.
Applications
- Routine lactose quantification in raw, processed and reconstituted milk for:
- Dairy quality control and product specification.
- Regulatory compliance and labelling (e.g., lactose content declarations).
- Process monitoring in dairy production and product development.
- Alternative to chromatographic methods for labs using enzymatic pH‑metric assays where rapid, routine testing is required.
Who should use this standard
- Dairy testing laboratories and quality assurance teams
- Analytical chemists and method developers in the dairy industry
- Instrument manufacturers (differential pH analysers) and calibration service providers
- Regulatory bodies and accreditation assessors evaluating lactose assay methods
Related standards
- ISO 707|IDF 50 (sampling of milk) - recommended sampling guidance referenced by ISO 26462.
- Annex C of ISO 26462 provides a comparison between pH and HPLC methods for lactose determination.
Keywords: ISO 26462, lactose determination, enzymatic method, differential pH, milk analysis, β‑galactosidase, glucokinase, dairy quality control.
