Overview
ISO 5520:1981 specifies laboratory methods for the determination of the alkalinity of total ash and the alkalinity of water‑soluble ash in fruits, vegetables and derived products. The standard defines the analytical principle, required reagents and apparatus, step‑by‑step procedures (incineration, acid treatment, extraction and titration), calculation formulas and reporting requirements. It is intended for routine chemical analysis of foodstuffs where ash alkalinity is a quality or compositional parameter.
Key topics and technical requirements
- Scope and definitions: Alkalinity is expressed in milliequivalents (meq) per 100 g of sample or as an alkalinity index (millilitres of 1 N acid per gram of ash). Definitions reference ash obtained from 100 g of sample.
- Analytical principle:
- Total ash alkalinity: incinerate sample (muffle furnace) at 525 ± 25 °C, treat the residue with an excess of standard 0.1 N sulphuric acid, then back‑titrate with 0.1 N sodium hydroxide using an indicator.
- Water‑soluble ash alkalinity: extract the ash with hot water, neutralize/titrate the aqueous extract directly with 0.1 N sulphuric acid.
- Reagents and indicator: Distilled or equivalent water; 0.1 N H2SO4, 0.1 N NaOH; Tashiro indicator (methylene blue + methyl orange mixture) is specified.
- Apparatus: electric muffle furnace (controlled to 525 ± 25 °C), silica or inert dishes, desiccator, analytical balance and burettes.
- Procedure controls: accurate weighing of test portions (including provision for liquid products by volume), hot‑water extraction, and duplicate determinations.
- Performance: repeatability (same analyst, same sample) limit ≤ 0.1 meq per 100 g (or per 1 g ash); reproducibility (different labs) limit ≤ 0.2 meq.
- Reporting: method used, numerical results, any deviations or operating conditions, and full sample identification.
Practical applications and users
ISO 5520:1981 is useful for:
- Food laboratories performing quality control, compositional analysis or regulatory checks on fruits, vegetables and derived products.
- Manufacturers and ingredient suppliers monitoring mineral/alkaline residues and assessing effects of processing (e.g., blanching, additives).
- Research labs studying ash composition, processing impact or nutritional labeling where ash alkalinity influences product stability or sensory properties.
Keywords: ISO 5520:1981, alkalinity of total ash, water‑soluble ash, fruits and vegetables, ash alkalinity determination, Tashiro indicator, 525 °C muffle furnace, sulphuric acid titration, sodium hydroxide back titration, alkalinity index.
Related standards
- Other ISO methods for ash content and elemental analysis are developed by ISO/TC 34 (Agricultural food products). Refer to relevant ISO food analysis standards for complementary procedures (ash content, moisture, mineral assays).
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