Overview
ISO 2173:2003 specifies a refractometric method for determining the soluble solids content of fruit and vegetable products. The method correlates the refractive index of a prepared test solution to a mass fraction of sucrose (expressed as % by mass) and is especially suitable for thick products, products containing suspended matter, and sugar‑rich items. Results are conventionally reported as soluble solids even when other dissolved substances are present (in which case results are approximate).
Key topics and requirements
- Principle: Measure refractive index at the reference temperature (20 °C ± 0.5 °C) and convert to sucrose concentration using tables or direct-read refractometer scales.
- Apparatus:
- Refractometer with refractive index scale (readable to 0.001; adjusted to 1.3330 for distilled water at 20 °C).
- Refractometer with sucrose (mass fraction) scale (readable to 0.10 %; zeroed on distilled water).
- Temperature control for prism assemblies to ±0.5 °C.
- Sample preparation: Procedures provided for:
- Clear liquids (use directly)
- Semi‑thick products (press through gauze)
- Thick products (dilute, boil 2–3 min, filter)
- Frozen products (thaw, remove stones, then follow semi‑thick or thick procedure)
- Dried products (rehydrate with water, heat on water bath ~30 min, filter)
- Corrections and calculations:
- Temperature corrections (formula or table adjustments) when measurements are not at 20 °C.
- Salt correction for added NaCl (linear correction factor).
- Acid correction for highly acidic citrus products (additive table based on total acidity).
- Formulas for converting diluted or sugar‑diluted samples back to original soluble solids (%).
- Quality & reporting:
- Repeatability: two single test results on identical material should differ by ≤ 0.5 g soluble solids per 100 g in most cases.
- Test report must identify sample, sampling method, method reference (ISO 2173), operating details and results.
Applications and users
ISO 2173:2003 is used for routine laboratory and production quality control where soluble solids (sugar content proxy) are critical:
- Fruit and vegetable processing plants (jams, jellies, purees)
- Juice and concentrate producers (when samples contain suspended matter)
- Food control laboratories and regulatory agencies
- R&D and product development teams monitoring sweetness and consistency
Keywords: ISO 2173:2003, soluble solids determination, refractometric method, refractometer, refractive index, mass fraction of sucrose, fruit and vegetable products, quality control, temperature correction.
Related standards
- ISO 2172 - Soluble solids by pycnometric method (clarified juices)
- ISO 750 - Titratable acidity (used for acid corrections)
- EN 12143 and AOAC methods referenced for comparable refractometric procedures and practices.
