Overview
ISO 11285:2004 - Milk - Determination of lactulose content - Enzymatic method specifies a validated enzymatic laboratory procedure to determine the lactulose content of milk. The standard (also published as IDF 175:2004) defines sample preparation, required reagents and apparatus, the enzymatic reactions used, spectrophotometric measurement and the calculation to report lactulose in mg/kg. This method is widely used in dairy testing for quality control and for the characterization of milk according to heat treatment.
Key topics and technical requirements
- Scope: Quantitative determination of lactulose in milk by an enzymatic/spectrophotometric method.
- Principle: Precipitation of fat and protein (zinc sulfate + potassium hexacyanoferrate(II)), enzymatic hydrolysis of lactose and lactulose with β-D-galactosidase, selective oxidation of glucose with glucose oxidase (GOD) and catalase, then phosphorylation/isomerization of residual glucose and fructose using hexokinase (HK), glucose-6-phosphate dehydrogenase (G6P‑DH) and phosphoglucose isomerase (PGI). NADPH produced is measured spectrophotometrically (absorbance at 340 nm) and related to lactulose content.
- Reagents: β‑galactosidase, glucose oxidase, catalase, HK/G6P‑DH mix, PGI, ATP, NADP, plus buffer systems and precipitating solutions (detailed concentrations specified in the standard).
- Apparatus: Analytical balance, volumetric glassware, incubation (40 °C), filtration equipment and a spectrometer suitable for measurement at 340 nm (cuvette path length 10 mm).
- Procedure highlights: sample dilution; clarification; overnight enzymatic hydrolysis; oxidation of glucose; filtration; blank determination (without β‑galactosidase) to correct for free fructose; stepwise enzymatic reactions in cuvettes and absorbance readings after each reaction stage.
- Calculation: Conversion of NADPH absorbance increase to mg lactulose/kg using the specified molar absorption coefficient and sample/volume parameters.
Applications and users
- Dairy quality-control laboratories monitoring heat treatment effects (lactulose is produced during thermal processing).
- Regulatory and food safety laboratories requiring standardized, traceable lactulose measurements.
- Research labs studying milk processing, shelf life and Maillard-type changes in dairy.
- Industry compliance testing for product labeling and process validation.
Related standards
- IDF 175:2004 (same joint publication)
- ISO 707 (referenced for sampling guidance)
- Joint publication with AOAC International (method coordination noted in the standard)
Keywords: ISO 11285:2004, lactulose determination, milk, enzymatic method, β-galactosidase, glucose oxidase, NADPH, spectrophotometer, heat treatment, dairy quality control.