Overview
ISO 939:2021 specifies an entrainment distillation method for the determination of moisture content in spices and condiments. The method uses an organic solvent immiscible with water (toluene, unless otherwise specified) to entrain water by azeotropic distillation and collect it in a graduated receiver. The standard is intended to provide a repeatable laboratory procedure to support quality control, shelf-life assessment and regulatory compliance.
Key Topics
- Principle: azeotropic/entrainment distillation using an immiscible solvent to extract and measure water.
- Reagent: toluene (saturated with water then distilled; other solvents may be used where specified by product standards).
- Apparatus: short-necked flask (≥500 ml), reflux condenser, receiver with a graduated tube (0–5 ml, 0.1 ml graduations), analytical balance, heat source and a spiral copper wire for aiding separation.
- Sampling & sample prep: sampling by ISO 948 and preparation of a ground sample by ISO 2825 are referenced and required for representative results.
- Test portion: about 40 g (weighed to 0.01 g) so that collected water volume does not exceed 4.5 ml.
- Expression of results: moisture content w (%) = 100 × V / m, where V is volume of collected water (ml) and m is mass of test portion (g).
Procedure (practical steps)
- Apparatus cleaning: clean with a laboratory-grade detergent, rinse with water and dry thoroughly to avoid adhesion of droplets.
- Sample preparation: prepare ground test sample per ISO 2825.
- Weigh test portion: weigh ~40 g to nearest 0.01 g.
- Charge flask: transfer sample to flask and add ~75 ml toluene to cover the sample; assemble distillation apparatus.
- Fill receiver: pour toluene through the condenser to fill the receiver; use a calcium chloride tube or cotton plug to avoid atmospheric moisture entering the condenser.
- Distill: heat to achieve ~100 drops/min; when most water has distilled, increase to ~200 drops/min until no more water is collected. Periodically wash condenser walls with small toluene portions and use the spiral copper wire to coalesce droplets.
- Equilibration: reflux until the water level in receiver is unchanged for 30 min; then cool/immerse the receiver at room temperature for at least 45 min and read the water volume.
Applications
- Routine quality control of spices and condiments
- Shelf-life and storage condition assessment
- Verification for trade specifications and regulatory testing
Related Standards
- ISO 948 - Spices and condiments: Sampling
- ISO 2825 - Spices and condiments: Preparation of a ground sample for analysis
- ISO 939 Annex A - informative example and detailed illustration of the entrainment distillation apparatus
This guidance highlights practical elements required to implement ISO 939:2021 in laboratory workflows while preserving traceability, repeatability and compliance with referenced sampling/preparation standards.
