Overview
ISO 11817:1994 specifies a reference method for determining the moisture content of roasted ground coffee using the Karl Fischer titration. Because of its precision, the method is intended as a laboratory reference for accurate moisture determination in roasted ground coffee samples. The procedure covers sample preparation, solvent extraction under reflux, Karl Fischer titration (bi-amperometric end‑point detection), and result calculation and reporting.
Key topics and requirements
- Scope: Determination of moisture content (expressed as mass %) of roasted ground coffee by Karl Fischer titration; designated as a reference method due to high precision.
- Principle: Water in the coffee is extracted into dehydrated methanol under reflux and titrated with Karl Fischer reagent; the end point is detected electrochemically by a bi‑amperometric system.
- Reagents and materials:
- Karl Fischer reagent (commercial, pyridine‑free, single- or two-component system).
- Methanol with ≤0.01% (m/m) water.
- Molecular sieves (approx. 0.3 nm) or equivalent desiccant for drying glassware and traps.
- Apparatus:
- Reflux assembly (100 ml round-bottom flask, reflux condenser), heating regulated (thyristor controller is recommended).
- Titration vessel with platinum double electrodes and magnetic stirrer; tightness is critical (leakage rate guidance provided).
- Analytical balances (microbalance and ultramicrobalance for reagent titre determination).
- Procedure highlights:
- Condition and dry glassware (oven ~40 °C).
- Extract approx. 3 g sample with 50 ml dehydrated methanol; reflux for 30 min; cool and transfer extract for titration.
- Use aliquots (e.g., 10 ml or 5 ml depending on expected moisture) and perform triplicate titrations.
- Determine reagent titre (ISO 760 referenced) and apply the prescribed calculation to report moisture as % w/w.
- Quality & precision:
- The standard limits precision: repeatability differences typically ≤ 0.14% and reproducibility differences ≤ 0.25% (interlaboratory data provided; example mean ~4.21% with reported standard deviations).
Applications and users
- Quality control and laboratory testing in the coffee industry (roasters, packers).
- Reference and regulatory laboratories validating moisture measurements.
- Research laboratories studying coffee stability, shelf‑life, and processing effects.
- Certification bodies and standardization work requiring a traceable reference method.
Related standards
- ISO 760 - Water determination by Karl Fischer (general method) - referenced for titre determination and general KF principles.
- ISO 5725 - Precision of measurement methods (used for interlaboratory precision evaluation and reporting).
Keywords: ISO 11817:1994, roasted ground coffee, moisture content, Karl Fischer method, reference method, coffee quality control, moisture determination, ISO 760, ISO 5725.
