ISO 5765-2:2002

Overview

ISO 5765-2:2002 (IDF 79-2:2002) defines an enzymatic method for the determination of lactose content in dried milk, dry ice‑mixes (including in the presence of other carbohydrates and reducing substances), and processed cheese. The method targets the galactose moiety of lactose: lactose is enzymatically cleaved and the released galactose is oxidized with concomitant formation of NADH, which is measured spectrophotometrically at 340 nm. This part complements ISO 5765-1 (which uses the glucose moiety) and includes Good Laboratory Practice (GLP) requirements in a normative annex.

Key topics and technical requirements

• Scope: Applicable to all types of dried milk, dry ice‑mixes, and processed cheese for lactose quantification.
• Analytical principle:
  • Deproteination of a test extract to obtain a purified sample.
  • Hydrolysis of lactose with β‑galactosidase to release galactose.
  • Oxidation of galactose by galactose dehydrogenase in the presence of NAD+, producing NADH.
  • Measurement of NADH absorbance at 340 nm; correction for endogenous galactose is required.
• Reagents and consumables: citrate and phosphate buffers, NAD+, β‑galactosidase and β‑galactose dehydrogenase enzyme preparations, lactose standard, zinc sulfate, potassium hexacyanoferrate(II), etc.
• Apparatus: analytical balance, calibrated pipettes and volumetric flasks, filtration equipment, blending device for processed cheese, and a spectrometer capable of 340 nm readings.
• Quality assurance: test blanks, reagent checks, GLP rules (Annex A), and defined precision data (repeatability and reproducibility) are included.
• Limitations noted:
  • Choose part 1 or part 2 depending on sample glucose/galactose levels (high interfering monosaccharide content reduces accuracy).
  • Lactulose (from heat treatment) may be partially measured as lactose by this method.
  • Lactose bound by Maillard reactions may not be determinable.

Applications and users

• Typical applications:
  • Quality control and routine testing in dairy processing (milk powder, infant formula ingredients, ice‑cream mixes).
  • Nutritional labeling and composition verification for processed cheeses and dried dairy ingredients.
  • Regulatory and compliance testing in food control laboratories.
• Who uses it:
  • Food and dairy laboratories, QC analysts, food chemists, standards bodies, and auditors seeking a validated enzymatic assay for lactose determination.

Related standards

• ISO 5765-1: Determination of lactose using the glucose moiety (complementary method).
• IDF 79 series (joint ISO/IDF publication); developed in collaboration with AOAC International.

Keywords: ISO 5765-2, lactose determination, enzymatic method, galactose moiety, dried milk, processed cheese, ice‑mixes, β‑galactosidase, NADH, spectrophotometry 340 nm.