Overview
ISO 8156:2005 (IDF 129:2005) - Dried milk and dried milk products - Determination of insolubility index - specifies a standard laboratory method to assess the solubility of dried milks by measuring the insolubility index (volume of sediment in mL) after reconstitution and centrifugation. The method applies to spray‑dried and roller‑dried whole milk powder, partly skimmed and skimmed milk powder, and is also valid for dried whey, dried buttermilk, dried milk‑based baby food, and products where milk fat is replaced by another fat.
Key topics and technical requirements
- Principle: Reconstitute a defined test portion in water at a controlled temperature, mix with a specified mixer, allow to stand, centrifuge a defined aliquot in a graduated tube, remove supernatant, redisperse, re‑centrifuge and record the sediment volume (insoluble residue) in millilitres.
- Reconstituting temperature: Use 24 °C for products normally reconstituted in cold water (typically spray‑dried) or 50 °C for products intended for warm reconstitution (often roller‑dried). Report results with the temperature, e.g. 0.25 mL (24 °C).
- Water quality: Use distilled, demineralized, or equivalent water.
- Apparatus specifications:
- Mixing jar: 500 mL glass jar (clover‑leaf pattern).
- Mixer and impeller: 16‑bladed stainless steel impeller with defined blade pitch and positioning; impeller operational speed (3 600 ± 100) min⁻¹ and a fixed distance from jar bottom (10 ± 2 mm).
- Thermometer: accuracy ±0.2 °C; water baths controlled to ±0.2 °C.
- Measuring cylinder: 100 mL plastic ±0.5 mL; balance accurate to 0.01 g.
- Reagents: Silicone antifoaming agent (aqueous emulsion ~30%) - test for suitability per procedure.
- Precision: Method precision (repeatability, reproducibility) established via interlaboratory study and considered satisfactory for routine use.
Applications
- Quality control and grading of milk powders and other dried dairy products.
- Routine testing in production laboratories to monitor solubility and detect dry‑heat damage or manufacturing/storage defects.
- Specification compliance for manufacturers, dairy processors, contract laboratories, and regulatory bodies.
Who should use this standard
- Dairy manufacturers and ingredient producers (milk powder, whey powder, baby food).
- Quality assurance / QC laboratories and third‑party testing labs.
- Regulatory agencies and standards bodies specifying product quality criteria.
Related standards
- ISO 8156 is published jointly as IDF 129:2005 and builds on historical ADMI solubility methods; consult other ISO/IDF methods for complementary physical and chemical tests of dried milk products.
Keywords: ISO 8156, insolubility index, dried milk, milk powder, solubility test, spray-dried, roller-dried, reconstitution temperature, dairy quality control.
