ISO 8968-4:2016

Overview - ISO 8968-4:2016 (Milk protein determination, reference method)

ISO 8968-4:2016 (also published jointly as IDF 20-4) is a reference analytical method for the determination of protein and non‑protein nitrogen in liquid milk (whole or skimmed) and for the calculation of true protein content. The standard specifies both a direct and an indirect analytical approach based on precipitation of milk proteins, followed by nitrogen determination using the Kjeldahl principle (see ISO 8968‑1).

Key topics and technical requirements

• Scope: Direct and indirect determination of protein‑nitrogen in liquid milk; expresses results as mass fraction (percentage).
• Principle: Protein is precipitated from a measured test portion with trichloroacetic acid (TCA).
  • Indirect method: measure nitrogen in the TCA filtrate (non‑protein nitrogen, NPN); subtract from previously determined total nitrogen to obtain true protein nitrogen.
  • Direct method: determine nitrogen in the protein precipitate (protein nitrogen) directly.
• Reagents: Analytical‑grade reagents only. Key reagents specified include a TCA solution (15.0 g TCA per 100 ml water) and standardized hydrochloric acid solutions (0.10 mol/L for the direct approach and c(HCl) = 0.01 ± 0.0001 mol/L for the indirect approach). Guidance is given on substitution and dilution practices to preserve method precision.
• Apparatus & sample handling: Standard lab glassware plus specific items such as nitrogen‑free filter paper, 75 mm glass funnels, automatic pipette (10 ml), and a 38–40 °C water bath for sample warm‑up and homogenization. Test portion guidance (e.g., pipette 5.0 ml ± 0.1 ml) and detailed filtration/ rinsing steps are provided.
• Procedure outline: Precipitation with ~12% final TCA concentration, filtration, collection of precipitate or filtrate, and digestion/distillation and titration according to ISO 8968‑1 (Kjeldahl).
• Quality: The standard includes sections on precision, sampling recommendations, and test reporting requirements. A safety warning highlights hazardous materials and laboratory practices.

Applications and users

ISO 8968‑4:2016 is intended for:

• Dairy industry analytical and quality control laboratories verifying milk protein content or NPN.
• Regulatory and compliance testing for nutritional labelling and adulteration checks.
• Research and development in dairy science, product formulation, and nutritional analysis.
• Laboratories seeking a reference method to validate or compare rapid/ routine procedures.

Keywords: ISO 8968-4:2016, milk protein determination, true protein content, protein nitrogen, non-protein nitrogen, trichloroacetic acid, Kjeldahl reference method, dairy quality control.

Related standards

• ISO 8968‑1 | IDF 20‑1 - Kjeldahl principle (total nitrogen determination)
• ISO 8968‑3 | IDF 20‑3 - Block digestion (semi‑micro routine method)

Safety note: Users must implement appropriate lab safety measures for handling acids, TCA and digestion reagents.