ISO 15598:1999

Overview

ISO 15598:1999 - "Tea - Determination of crude fibre content" defines a standardized wet‑chemical method for measuring crude fibre in tea. Published by ISO in 1999, the standard specifies sample preparation, reagents, apparatus, step‑by‑step procedure (acid and alkali digestion), drying, ashing and a calculation to express crude fibre as a mass fraction on a dry‑matter basis. Annex A reports interlaboratory precision data.

Key Topics and Requirements

• Scope: Method applies to determination of crude fibre content in tea; expressed as a percent on a dry basis.
• Principle: Successive digestion with boiling sulfuric acid and sodium hydroxide, filtration, washing, drying, weighing and ashing; crude fibre is the mass loss on ashing.
• Reagents: Analytical grade water (ISO 3696 Grade 3), sulfuric acid stock and working solutions (stock ~2.040 mol/L; working ~0.255 mol/L), sodium hydroxide stock and working solutions (stock ~2.504 mol/L; working ~0.313 mol/L), antifoaming octan‑1‑ol, HCl 1%, ethanol (≥95%) and acetone.
• Apparatus: 1 mm laboratory mill, 1 L conical flasks with condensers, Buchner and sintered‑glass crucibles (porosity 100–160 μm), oven at 103 °C ± 2 °C, muffle furnace at 550 °C ± 10 °C, desiccator and precision balance.
• Procedure highlights:
  • Grind sample to pass 1 mm screen.
  • Use 2–3 g test portion; boil with working sulfuric acid for 30 min (critical time), filter and rinse.
  • Reflux with working sodium hydroxide for 30 min, transfer residue to crucible, wash sequentially (water, HCl, water, ethanol, acetone).
  • Dry at 103 °C to constant mass; ash at 550 °C and calculate loss.
• Calculation: Formula provided to convert measured masses to crude fibre percentage on dry matter.
• Precision: Annex A contains interlaboratory results and gives repeatability (r) and reproducibility (R) limits for tested black tea samples; users are advised these values apply to similar matrices.

Applications and Who Uses It

• Quality control laboratories in tea production, blending and packaging.
• Food testing and regulatory labs for composition analysis, product labeling and compliance.
• Research institutions studying tea composition, processing effects and nutrition.
• Certification bodies and suppliers verifying fibre content for product specifications.

Practical benefits include harmonized results across labs, defensible quality data for specifications, and a recognized method for regulatory and commercial use.

Related Standards

• ISO 1573:1980 - Tea - Determination of loss in mass at 103 °C (dry matter).
• ISO 3696 - Water for analytical laboratory use.
• ISO 1839 - Tea - Sampling (recommended sampling method).
• ISO 5725 - Precision of measurement methods (used for interlaboratory evaluation).

Keywords: ISO 15598, crude fibre content, tea analysis, tea testing standard, laboratory method, acid‑alkali digestion, repeatability, reproducibility.