Overview
ISO 9768:1994 is an internationally recognized standard established by the International Organization for Standardization (ISO) that specifies a reliable and standardized method for the determination of water extract in tea. This method involves the extraction of soluble matter from a tea sample by boiling water under reflux conditions, followed by filtration, drying, and weighing of the insoluble residue. The water extract content is calculated as a percentage of dry matter, providing essential quality parameters for tea products worldwide.
This standard ensures uniformity and accuracy in testing, essential for manufacturers, quality control laboratories, and researchers involved in tea production and analysis. It replaces the previous 1990 edition, incorporating technical revisions for improved precision and usability.
Key Topics
- Scope: Defines the procedure to determine the water extract in tea expressed as a percentage by mass on a dry basis.
- Principle: Extraction of soluble material with boiling water under reflux, filtration, drying of insoluble residues, and calculation of water extract.
- Apparatus Required:
- Oven with controlled temperature (103 °C ± 2 °C)
- Borosilicate glass crucibles with defined porosity for filtration
- Reflux condensers and filtration flasks
- Analytical balances precise to 0.001 g
- Test sieves (1.4 mm and 3 mm apertures)
- Sampling and Preparation:
- Use a representative sample that has not been damaged during transport.
- Grind tea to pass through a 3 mm sieve if necessary.
- Dry matter content determination as per ISO 1573 before analysis.
- Testing Procedure:
- Weigh a 2 g test portion of dried tea.
- Reflux with 200 ml distilled water for 1 hour.
- Filter under vacuum, wash with hot distilled water.
- Dry residue at 103 °C for 16 hours.
- Weigh residue and calculate water extract percentage.
- Calculation Formula:
[
\text{Water extract % (dry basis)} = \frac{(m_1 - m_h)}{m_0 \times W} \times 100
]
where:
- (m_0) = mass of test portion
- (m_1) = mass of dried insoluble residue
- (m_h) = mass of crucible
- (W) = dry matter content (%)
- Precision and Accuracy:
- Repeatability limit: 1.0% (dry matter basis)
- Reproducibility limit: 2.5% (m/m) among different laboratories
- Statistical validation confirmed through interlaboratory testing (Annex A)
- Reporting: Test reports must clearly state the method used, sampling details, drying method, test results, and any deviations or incidents affecting results.
Applications
- Quality Control in Tea Production: ISO 9768:1994 provides an essential method for tea manufacturers and quality laboratories to evaluate the extraction efficiency and soluble content of tea, directly influencing flavor and consumer satisfaction.
- Research and Development: Scientific studies on tea composition and processing effects rely on this standardized method for reproducible results in water extract analysis.
- Trade and Compliance: The standardized test allows producers and traders to comply with international quality requirements and ensures uniform evaluation of tea products globally.
- Regulatory and Certification Use: Authorities and certifying bodies use this method to confirm adherence to product standards, supporting labeling and quality assurance claims.
- Agricultural Quality Assessment: Tea growers assess the quality of raw materials, leveraging the water extract determination to gauge leaf maturity and processing suitability.
Related Standards
- ISO 1573:1980 – Tea: Determination of loss in mass at 103 °C
Used to establish the dry matter content prior to water extract testing, crucial for accurate results.
- ISO 1839:1980 – Tea: Sampling
Provides guidelines on proper tea sampling ensuring representativity and consistency.
- ISO 5725:1986 – Accuracy (trueness and precision) of measurement methods and results
Framework for evaluating repeatability and reproducibility of testing methods like ISO 9768.
By integrating ISO 9768:1994 into laboratory practices, stakeholders in the tea industry can achieve precise, repeatable, and globally accepted water extract analysis - a vital parameter for tea quality and consumer satisfaction.
