Overview
ISO 15323:2002 (also published as FIL 173:2002) specifies a laboratory method for determining the Nitrogen Solubility Index (NSI) of dried milk protein products. NSI is a quantitative measure of the solubility of nitrogen‑containing compounds in protein powders and is expressed as a percentage. The method provides a standardized way to assess protein solubility - an important quality attribute for whey protein concentrates, caseinates and other dairy protein ingredients.
Key topics and technical requirements
- Scope: Applies to dried milk protein products; NSI evaluates solubility of nitrogenous compounds.
- Principle: Prepare an aqueous dispersion, adjust pH to 7.00 ± 0.55, agitate (2 h at 22 °C ± 2 °C), centrifuge, and compare nitrogen content of the total dispersion and the supernatant.
- Nitrogen determination: Use the Kjeldahl method in accordance with ISO 8968-1 or ISO 8968-2 to measure nitrogen in the dispersion and filtered supernatant.
- Apparatus & reagents: Includes analytical balance (0.001 g), pH‑meter (±0.05 pH unit), centrifuge capable of 3 000 g at 22 °C, Whatman No.1 filter (or equivalent), 0.1 mol/L NaOH and HCl for pH adjustment, distilled/deionized water.
- Procedure highlights:
- Weigh sample equivalent to ~1 g protein (~0.16 g N).
- Make dispersion to ~100 mL, adjust pH, agitate 2 hours.
- Centrifuge 35 mL at 3 000 g for 10 minutes, filter supernatant, determine nitrogen.
- Calculate NSI = (nitrogen in supernatant / nitrogen in dispersion) × 100%.
- Results & precision: Express NSI to one decimal place. Annex A reports interlaboratory precision for typical powders (examples):
- Whey protein concentrate: mean NSI ~92.16% (repeatability r = 3.00, reproducibility R = 8.61)
- Coprecipitated protein: mean ~85.31% (r = 1.87, R = 2.58)
- Caseinate: mean ~100.37% (r = 1.87, R = 2.71)
Applications
- Quality control and acceptance testing of dairy protein powders.
- Comparing solubility after processing, drying or heat treatment.
- Product development and formulation where protein solubility affects functionality (e.g., beverages, protein concentrates).
- Supplier specification verification and stability studies.
Who should use this standard
- Dairy industry QC laboratories and analytical chemists
- Ingredient suppliers and manufacturers of whey/caseinate powders
- R&D teams formulating protein‑based foods and beverages
- Third‑party testing labs and regulatory bodies assessing product quality
Related standards
- ISO 8968-1 / ISO 8968-2 (Kjeldahl nitrogen determination)
- ISO 707 (Sampling of milk and dairy products)
- ISO 5725 / FIL 135 (method precision and interlaboratory studies)
- AOAC collaborative publications referenced in the standard
Keywords: ISO 15323, NSI, nitrogen solubility index, dried milk protein products, protein solubility, Kjeldahl, dairy quality control, whey protein, caseinate.
