ISO 5765-1:2002

Overview

ISO 5765-1:2002 (IDF 79-1:2002) specifies an enzymatic method utilizing the glucose moiety of lactose for determining lactose content in dried milk, dry ice‑mixes (ice-cream mixes) and processed cheese. The method is based on enzymatic hydrolysis of lactose and spectrophotometric measurement of the resulting glucose-derived NADPH at 340 nm. It is intended for samples that may contain other carbohydrates and reducing substances.

Key topics and technical requirements

• Principle: β‑galactosidase splits lactose into glucose and galactose; hexokinase (HK) + ATP convert glucose to glucose‑6‑phosphate (G6P); G6P dehydrogenase (G6P‑DH) oxidizes G6P to 6‑phosphogluconate while reducing NADP+ to NADPH. NADPH is quantified by absorbance at 340 nm and used to calculate lactose content after correction for native glucose.
• Reagents & buffers: citrate and triethanolamine buffers (specified pH), NADP/ATP solutions, deproteinizing agents and standard solutions (lactose standard c = 0.8 mg/ml; lactose monohydrate dried at 87 °C before use).
• Enzyme activities: specified minimum activities for reliable results (e.g., β‑galactosidase ≥ 60 U/ml; HK ≥ 280 U/ml; G6P‑DH ≥ 140 U/ml).
• Apparatus: routine lab equipment plus a spectrophotometer at 340 nm with 1 cm pathlength cuvettes.
• Procedure elements: sampling, deproteinization, reagent blank, test for procedural integrity, calculation and expression of results.
• Quality & precision: method includes repeatability and reproducibility statements and a normative Annex A on Good Laboratory Practice (GLP) for enzymatic analyses.
• Limitations: not suitable when free glucose or galactose levels are much higher than lactose (choose ISO 5765‑2 if galactose-based method is preferable). Lactulose and Maillard‑bound lactose are not measured by this part.

Applications and users

• Routine quality control in dairy processing (milk powder, ice‑mix powders, processed cheeses)
• Regulatory compliance and nutritional labeling (lactose declaration)
• Laboratory analysis in food testing labs, R&D, contract testing and kit manufacturers
• Use cases: product formulation, raw material acceptance, verification of lactose‑reduced products

Related standards

• ISO 5765-2 / IDF 79-2 - complementary enzymatic method utilizing the galactose moiety of lactose; choice between parts depends on native glucose/galactose levels in samples.

Keywords: ISO 5765-1, lactose determination, enzymatic method, dried milk, ice-mix, processed cheese, β‑galactosidase, hexokinase, glucose‑6‑phosphate dehydrogenase, spectrophotometry 340 nm, dairy quality control.