Overview
ISO 6632:1981 specifies a laboratory method for the determination of volatile acidity in fruits, vegetables and derived products. The procedure uses steam distillation to entrain volatile acids (principally acetic and propionic acids), followed by volumetric titration of the distillate with standard sodium hydroxide in the presence of phenolphthalein. Results are reported as milliequivalents or as grams of acetic acid per 100 ml or 100 g.
Keywords: ISO 6632, volatile acidity, fruits and vegetables, steam distillation, titration, sodium hydroxide, phenolphthalein, tartaric acid, food testing.
Key topics and technical requirements
- Sample scope: Applicable to fresh products, products preserved without chemical preservatives, and products containing sulphur dioxide (with or without sorbic, benzoic or formic acids).
- Principle: Acidify the test portion with tartaric acid, steam-distil the volatile acids, collect the distillate and titrate with 0.1 mol/L NaOH using phenolphthalein as indicator.
- Apparatus: Requires a CO2‑free steam generator (~1 500 ml), a bubbler to hold the test portion, a fractionating column (spiral stainless steel packing), a West-type condenser and standard volumetric glassware and balance.
- Reagents: Distilled water, tartaric acid, lime water for the steam generator, standard NaOH solution (prepared and checked immediately before use), phenolphthalein indicator.
- Sample preparation: Liquid, viscous, solid and frozen product procedures are defined (e.g., use of a mechanical mill for homogenization; typical test portions: 20 ml for liquids, ~10 g for solids).
- Special cases: Procedures to identify and correct for sulphur dioxide and sorbic acid in the distillate (iodine titration for SO2; colorimetric or UV methods for sorbic acid) are included.
- Quality and precision: Two determinations are made and the arithmetic mean reported. Repeatability and reproducibility limits are specified (e.g., reproducibility difference ≤ 0.5 meq/100 ml or 30 mg acetic acid/100 ml).
Applications and users
- Food testing laboratories conducting routine quality control and compositional analysis of fruit and vegetable products.
- Manufacturers of juices, jams, canned fruit/vegetables and derived products monitoring fermentation, spoilage or processing-related acidity.
- Regulatory agencies and inspection bodies requiring standardized, traceable methods for volatile acidity reporting.
- Researchers studying fermentation, spoilage, or preservation effects in horticultural products.
Related standards
- Laboratory volumetry and glassware standards referenced for pipettes and burettes (ISO/R 835, ISO/R 385) and general ISO food analytical standards. Consult national and more recent ISO or AOAC methods for updated procedures and equivalence.
This summary helps you understand ISO 6632’s steam‑distillation/titration approach to measuring volatile acidity and where it fits into food quality testing and laboratory workflows.