Overview
ISO 19662:2018 (IDF 238:2018) specifies the acido-butyrometric (Gerber) method for determining the fat content of milk. Published jointly by ISO and the International Dairy Federation (IDF), this standard covers routine laboratory analysis of whole milk and partially skimmed milk, including milks preserved with potassium dichromate or bronopol. It does not apply to formalin-treated milks or milks that have been homogenized.
Keywords: ISO 19662:2018, Gerber method, milk fat determination, acido-butyrometric, butyrometer, dairy laboratories.
Key topics and technical requirements
- Principle: Proteins are dissolved by concentrated sulfuric acid; amyl alcohol assists separation; fat is separated by centrifugation in a butyrometer and read directly from the scale.
- Reagents: Analytical-grade reagents - concentrated sulfuric acid (density ~1.820 g/ml) and a specified amyl alcohol composition/density are required. Purity checks (e.g., colour with acid) are specified.
- Apparatus:
- Milk butyrometer (characteristics in Annex A) with matching stopper (Annex B).
- Pipettes/automatic dispensers capable of delivering precise milk and reagent volumes (e.g., 11.00 ml or 10.75 ml milk; 10.0 ml H2SO4; 1.00 ml amyl alcohol).
- Centrifuge capable of ~350 g (±50 g) at the butyrometer end, with speed guidance provided.
- Water baths at 40 °C ±2 °C (sample prep) and 65 °C ±2 °C (reading).
- Procedure highlights: warm and mix sample, cool to ~20 °C, add acid and alcohol, stopper and shake to dissolve proteins, centrifuge immediately, equilibrate in 65 °C bath, then read fat column (bottom and meniscus) within 10 s to 0.025 g/100 ml accuracy.
- Performance: Method includes instructions for calculation, and gives precision information (repeatability and reproducibility) and guidance for different fat ranges.
Applications and users
- Who uses it: dairy laboratories, food control authorities, dairy processors, quality assurance teams, and testing service laboratories.
- Practical uses:
- Routine determination of milk fat for labeling, payment, and quality control.
- Regulatory compliance testing where the Gerber method is accepted.
- Rapid, cost-effective screening in production and incoming milk testing.
- Limitations: Not suitable for homogenized milk or formalin-preserved samples; consult the standard for permitted preservatives.
Related standards
- ISO 707 | IDF 50 - recommended sampling of milk (sampling is outside the method but referenced).
- The standard notes equivalence (after density correction) with the gravimetric reference method for certain fat ranges; see ISO 19662:2018 for details and annexes on butyrometer and stopper characteristics.
For implementation, consult the full ISO 19662:2018 text for exact procedural steps, annex specifications and precision data.
