Overview
ISO 11213:1995 specifies an enzymatic method for the determination of acetyl content in modified starch (both granular and cold‑water‑soluble). The standard describes procedures to measure total acetyl and free acetyl; bound acetyl is calculated as the difference (total − free). Total acetyl is measured after acid hydrolysis and enzymatic conversion of acetate to acetyl‑CoA, with the coupled enzyme reactions producing NADH whose increase in absorbance is monitored spectrophotometrically. The method is suitable for acetyl contents up to 2% (m/m).
Key topics and technical requirements
- Scope: Applies to modified starch (granular and soluble in cold water); suitable for acetyl contents up to 2% (m/m).
- Principle: Acid hydrolysis releases acetate; acetyl‑CoA synthetase (ACS) converts acetate + ATP + CoA → acetyl‑CoA. Coupled reactions with malate dehydrogenase (MDH) and citrate synthase (CS) produce NADH, measured by absorbance increase.
- Analytical measurement: Monitor NADH formation at 340 nm using a molecular absorption spectrometer; temperature control (about 20–25 °C) and defined cuvette path length are required.
- Sample preparation: Separate procedures for granular vs pregelatinized starch; hydrolysis with dilute HCl in a boiling bath, neutralization, filtration and appropriate dilution before enzymatic assay.
- Calculations: Total and free acetyl contents calculated from absorbance difference and molar extinction coefficient; bound acetyl = total − free.
- Quality and precision: Precision established by an ISO interlaboratory study (ISO 5725 procedures); the standard includes guidance on repeatability and reproducibility and recommends check tests (e.g., using anhydrous sodium acetate).
- Reagents and apparatus: Specifies analytical‑grade reagents and enzymatic preparations (ACS, MDH, CS), buffer conditions (pH control), micropipettes, spectrometer, temperature‑controlled baths and filtration.
Applications and who uses it
- Starch manufacturers - quality control during production of acetylated starches.
- Food and ingredient laboratories - verification of acetyl levels for labeling, functionality and regulatory compliance.
- Pharmaceutical and industrial formulators - characterizing modified starch excipients used in tablets, adhesives, paper coatings, and textiles.
- R&D groups - developing or comparing modified starches with targeted acetylation degrees.
- Accredited testing labs - routine or contract testing where ISO‑compliant methods are required.
Keywords: ISO 11213:1995, modified starch, acetyl content, enzymatic method, acetyl‑CoA, NADH, spectrophotometric assay, acetylated starch, quality control.
Related standards
- ISO 5725 - Precision of test methods (repeatability and reproducibility)
- ISO 3696 - Water for analytical laboratory use
- ISO 1666 - Starch - Determination of moisture content