Overview
ISO 11286:2004 specifies a laboratory method for classifying tea grades by particle size analysis (sieve analysis). The standard defines how to separate a tea sample into size fractions, determine the mass percentage retained on each sieve, and assign a grade based on the distribution (peak and second‑peak) of particle sizes. It is intended to provide a consistent international system to supplement traditional, country‑specific grading nomenclature. Note: the method is not applicable to large leafy tea grades and may not be suitable for blends.
Key topics and technical requirements
- Scope: Classification of tea grades by particle size; excludes large leafy grades; limited suitability for blends.
- Apparatus:
- Sieve shaker capable of ~3 000 vibrations/min, up to 3 mm stroke and 30° angle, timer.
- Test sieves per ISO 3310‑1 (200 mm nominal diameter) with apertures: 2 mm, 1.4 mm, 1 mm, 710 µm, 355 µm, 250 µm, 150 µm and 75 µm, plus base pan (<75 µm) and clamp.
- Sample and procedure:
- Recommended duplicate analyses when sample permits.
- Weigh 100 g ±1 g into top sieve, assemble sieves in descending mesh, run shaker for 10 minutes.
- Quantitatively transfer and weigh material retained on each sieve to 0.01 g accuracy.
- Calculation and classification:
- Compute percentage (by mass) retained per sieve, identify the peak (largest retained) and second largest retained.
- Classify into grades defined by the location of the peak and second peak (e.g., peak in 2 mm, peak in 1 mm with second peak in 710 µm, etc.) and by counts of sieves with >25% or <25% retained as specified in Table 1.
- Subgrades A/B/C are assigned by the percentage passing specific sieves (e.g., for grades 1–6: A = up to 2.0% passing 355 µm; B = >2.0% passing 355 µm; C = >5.0% passing 355 µm).
- Test report must identify the sample, sampling method (if known), method reference (ISO 11286:2004), optional operating details/incidents, and results.
Applications and users
- Useful for tea producers, tea packing and processing plants, quality control laboratories, exporters/importers and customs authorities who need an objective, repeatable method for tea grade classification.
- Supports quality assurance, specification compliance, product labeling, and facilitates international trade by providing a common particle‑size based grading approach.
- Also relevant to R&D teams evaluating processing effects (cutting, rolling, milling) on particle-size distribution.
Related standards
- ISO 3310‑1 - Test sieves: technical requirements (normative reference for sieves).
- ISO 1839 - Tea sampling (recommended sampling procedure referenced in ISO 11286:2004).