Overview
ISO 20938:2008 specifies a reference method for determining moisture content in instant coffee using the Karl Fischer titration. The standard defines the principle, required reagents and apparatus, sample preparation, titration procedure and calculation of moisture (expressed as percentage mass fraction). It is intended for use as a laboratory reference method for accurate moisture analysis in spray‑dried, agglomerated (granules) and freeze‑dried instant coffee.
Key topics and technical requirements
- Method principle: Coulometric/volumetric Karl Fischer titration of water released from a weighed test portion dissolved in a water‑free solvent mixture (formamide–methanol–salicylic acid, “FMS”) and titrated with a pyridine‑free one‑component Karl Fischer reagent (KFR).
- Reagents: Pyridine‑free KFR (titre ≈ 5 mg/ml), low‑water methanol and formamide, salicylic acid, disodium tartrate dihydrate for primary titre standard.
- Solvent mixture (FMS): Formamide:methanol ratio 1:2 by volume; salicylic acid dissolved in methanol before adding formamide. Prepare fresh daily.
- Apparatus: Dry titration vessel (~100 ml) with platinum double electrode, magnetic stirring, piston burette for KFR, and drying tubes/receivers to exclude ambient moisture. Analytical balance with 0.1 mg accuracy.
- Titration and titre determination: Condition receiver with FMS and pre‑titrate KFR; determine KFR titre using disodium tartrate dihydrate or weighed water (25–30 mg) in duplicate. Typical test portion for instant coffee solids: 300–500 mg.
- Safety: Formamide handling precautions are required (protective clothing, splash guards, immediate washing after exposure).
- Precision: Interlaboratory results reported for three sample types:
- Spray dried (mean 4.442 g/100 g): repeatability r = 0.234, reproducibility R = 0.336
- Agglomerate (mean 4.598 g/100 g): r = 0.193, R = 0.329
- Freeze dried (mean 1.722 g/100 g): r = 0.129, R = 0.289
- Overall repeatability and reproducibility limits narrow to ~0.2% and ~0.3% mass fraction respectively.
Applications and users
- Quality control and R&D laboratories in the coffee and beverage industry performing moisture analysis of instant coffee, granules and concentrates.
- Contract testing and certification labs needing a reference method for auditing or method validation.
- Regulatory and standards organizations requiring traceable, reproducible moisture results for specifications, shelf‑life and hygroscopicity assessments.
- Useful for process control (drying, packaging) where accurate moisture content impacts product stability, flowability and rehydration properties.
Related standards
- ISO 4788 - Laboratory glassware - Graduated measuring cylinders (referenced)
- ISO 5725-1 and ISO 5725-2 - Accuracy (trueness and precision) of measurement methods and results (used in precision evaluation)
Keywords: ISO 20938:2008, instant coffee, moisture content, Karl Fischer titration, reference method, FMS solvent, pyridine-free KFR, moisture analysis, coffee quality control.