Overview
ISO 10520:1997 - Native starch: Determination of starch content - Ewers polarimetric method specifies a polarimetric analytical procedure for measuring the starch content of native starches (excluding high-amylose types). It describes sample preparation, hydrolysis, clarification and polarimetric measurement, and reports starch content on a dry-matter basis. The method is not applicable to modified or pregelatinized (water‑soluble) starch.
Key Topics and requirements
- Principle: Two complementary determinations are made:
- Total optical rotation after acid hydrolysis of a starch portion.
- Optical rotation of ethanol‑soluble substances (40% V/V ethanol extraction) after the same hydrolysis.
- Starch content is obtained from the difference between the two optical rotations, corrected for sample masses and dry matter, and referenced to the specific optical rotation of pure starch.
- Critical parameters:
- Hydrolysis: dilute HCl, controlled time and temperature (15 min in boiling water bath) with continuous agitation.
- Clarification: Carrez reagents I (potassium hexacyanoferrate(II)) and II (zinc acetate + acetic acid) to remove turbidity before polarimetry.
- Polarimeter: set to 589.3 nm (sodium D line) using 200 mm tubes.
- Sample prep: grind to pass 0.5 mm if necessary; precise weighings to 0.001 g.
- Specified reagents and apparatus:
- Dilute HCl solutions: c(HCl) = 7.7 mol·L–1 and c(HCl) = 0.309 mol·L–1 (procedural concentrations given).
- 40% (V/V) ethanol (extraction solvent).
- Carrez solutions I and II (composition and preparation provided).
- Analytical balance, volumetric flasks (100 mL), boiling/shaking water bath, polarimeter.
- Calculation inputs: measured optical rotations (total and ethanol‑soluble), sample masses, dry matter content (moisture by ISO 1666), and specific optical rotation values for starch types (examples given for rice, potato, maize, wheat, barley, oat).
Applications and users
- Intended for laboratory determination of starch content in native starch products and raw materials.
- Typical users:
- Starch manufacturers and quality control laboratories.
- Food ingredient producers and cereal processors evaluating starch purity and composition.
- Analytical labs performing regulatory, specification or R&D testing on native starches.
- Not suitable for modified or pregelatinized starch testing or high‑amylose starch without validation.
Performance and related standards
- Precision established by an interlaboratory study (12 labs); repeatability and reproducibility limits are reported in the standard and Annex A.
- Annex A discusses reproducibility relative standard deviation (RSDR) and references Horwitz empirical behavior (RSDR values in the study ranged roughly 1.25–3.24%).
- Related ISO standards referenced: ISO 1666 (moisture by oven drying) and ISO 3696 (water for analytical use).
Keywords: ISO 10520, Ewers polarimetric method, native starch determination, starch content polarimetry, starch analysis, polarimeter 589 nm, Carrez clarification, starch quality control.