ISO 5498:1981

Overview

ISO 5498:1981 specifies a general method for the determination of crude fibre content in agricultural food products. Based on the traditional Weende method, the procedure quantifies the fraction of materials that are insoluble and combustible under defined conditions. Results are reported as a percentage by mass, either relative to the product as received or to its dry matter.

Key topics and requirements

• Principle: Sequential acid–alkali digestion - boil the sample with a standard sulphuric acid solution, separate and wash the insoluble residue, then boil with a standard sodium hydroxide solution, separate, wash, dry, weigh and determine the loss in mass on incineration (loss on ignition).
• Reagents: Specified reagent concentrations (H2SO4 c(1/2 H2SO4) = 0.255 ± 0.005 mol/L ≈ 12.5 g/L; NaOH c = 0.313 ± 0.005 mol/L ≈ 12.5 g/L). Distilled or equivalent purity water and analytical-grade solvents are required.
• Sample prep: Grinding (device), sieving (1 mm aperture), and optional preliminary drying. Typical test portion around 3 g (adjustments for special cases noted).
• Extraction / defatting: Annex A details options for removal of fatty substances (solvents such as acetone, ethanol, methanol, propan-2-ol, n‑hexane, light petroleum, diethyl ether) and continuous or wash extraction techniques.
• Separation devices: Annex B describes a range of separating devices and procedures (filter aids, centrifugation, filtration variants) and guidance on their use.
• Operating conditions: Boiling steps conducted at 95–100 °C for 30 ± 1 min each; drying at 130 ± 2 °C; incineration at 550 ± 25 °C to constant mass.
• Quality control: At least two determinations per sample; repeatability limits (difference between two results) - absolute 0.4% for crude fibre