Moisture, total ash and water extract are useful green tea control results only when the report identifies the test method, edition, calculation basis, unit, sample and acceptance rule. Moisture indicates mass lost under the stated drying method; total ash is the inorganic residue left after controlled incineration; water extract estimates the material soluble under a defined boiling-water laboratory procedure. None of the three, alone, proves freshness, purity, cup quality or shipment-wide conformity.

Start with the measurement, not the number

Two certificates can show the same parameter name and still be unsuitable for direct comparison. One may use an ISO tea method and report on a dry-matter basis; another may use a different national or in-house method and report the sample as received. The first buyer task is therefore to reconstruct how each number was produced.

The official ISO catalogue checked on 4 August 2026 lists ISO 1573:1980 for loss in mass at 103 degrees C, ISO 1575:1987 for total ash and ISO 9768:1994 for water extract as published standards. ISO 9768 was under systematic review on the check date but remained published; do not accidentally cite its withdrawn 1990 edition.

Keep requirement types separate

  • Trade reference: a grade code or style helps identify the commercial tea but does not define an analytical method or limit.
  • Legal requirement: a mandatory rule in the destination jurisdiction. These three ISO methods are not, by themselves, import laws.
  • Voluntary standard: a technical reference used by agreement or when incorporated elsewhere. China's official standards platform lists GB/T 14456.1-2017 as a current recommended national green tea standard.
  • Buyer specification: the buyer's chosen limits, methods, bases and reporting rules for the intended use.
  • Contract requirement: the obligation accepted by both parties, including sampling, release and remedy. A voluntary method becomes contractually binding when the contract incorporates it clearly.

What moisture actually tells you

ISO 1573 defines a loss-in-mass procedure: a tea test portion is heated at 103 +/- 2 degrees C to constant mass. Read the reported result as the outcome of that method, not as proof that every gram lost was chemically identified as water. Confirm whether the laboratory tested the sample as received, how it protected the sample from humidity before weighing and whether the specification uses an upper limit or a range.

A moisture result can support storage, packing and lot-consistency decisions. It cannot show whether the tea has ever been exposed to moisture, whether every bag is uniform, or whether the cup matches the approved sample. A result below the contractual maximum is one checkpoint, not a complete release decision.

What total ash can and cannot indicate

Under ISO 1575, organic matter is destroyed by heating at 525 +/- 25 degrees C to constant mass, leaving total ash. This residue includes the tea's natural mineral content, so total ash does not mean "dirt" and is not a direct test for one contaminant. A value outside the agreed range is a reason to verify identity, sample preparation, basis and method before drawing a cause.

When total ash needs investigation, choose a test that answers the next question. ISO 1576:1988 separates water-soluble and water-insoluble ash by hot-water extraction. ISO 1577:1987 determines the residue remaining after the total ash is treated with hydrochloric acid. These are different measurands; do not rename total ash as acid-insoluble ash or infer one from the other.

What water extract measures

ISO 9768 uses boiling-water extraction under reflux, followed by filtration, washing, drying and weighing of the hot-water-insoluble residue; water extract is then calculated. It is a controlled analytical extraction, not the yield from a consumer infusion, a tea-bag line or an industrial extractor. It also does not separately quantify polyphenols, caffeine, aroma compounds or cup preference.

Water extract can help compare lots intended for blending, beverage manufacture or extraction when the same validated method and basis are used. A higher number is not automatically a better purchase: the buyer still needs the intended application, sensory target, price level and other specification items. For industrial material, start with the site's green tea raw material for extraction page, then define the factory's own yield and process trials separately.

Normalize the calculation basis before comparing

ISO 1572:1980 covers preparing a ground tea sample of known dry-matter content for determinations that must be expressed relative to dry matter. That basis matters. If a result is reported "as received," moisture remains in the denominator; a dry-basis result removes it mathematically. Never compare the two without conversion and confirmation that the moisture result belongs to the same sample.

For a mass fraction, a common conversion is: dry-basis result = as-received result x 100 / (100 - moisture %). For example, a hypothetical water-extract result of 31.0% as received with 7.0% moisture converts to 33.33% on a dry basis. This example teaches the calculation; it is not a Yunjing lot result or a recommended limit. Keep unrounded values for the calculation and round only as the agreed reporting rule directs.

Use this three-parameter specification template

ParameterWrite into the specificationVerify on the reportDecision checkpoint
Moisture / loss in massMethod and edition; as-received basis; maximum or range; unitSample condition, value, unit, date and methodCompare without changing basis or rounding early
Total ashMethod and edition; dry or as-received basis; agreed rangeBasis, value, unit and any preparation departureInvestigate an exception; do not call it contamination by assumption
Water extractMethod and edition; dry-matter basis; minimum or range; intended useBasis, value, unit and method statusUse with cup or process trial, not as a standalone quality score
All threeLot, sampling plan, lab, precision, uncertainty, decision rule and remedyReport identity, accreditation scope and conformity statementRelease only the represented, unchanged lot

Replace the placeholders with values justified by the applicable product standard, destination requirement and intended use. Our bulk green tea purchase specification guide explains how to connect those values to the approved sample, packing and contract.

Read a COA in seven passes

  1. Match identity: product, supplier lot, package count and sample ID must connect to the shipment under review.
  2. Confirm sampling: record who sampled, where, when, from how many packages and how the portions were sealed.
  3. Verify the method: match the identifier and edition, and check that it is current or intentionally retained by contract.
  4. Match basis and unit: label each value as received or dry basis before comparison.
  5. Review data quality: look for duplicate acceptance, precision, uncertainty, departures and the laboratory's relevant accreditation scope.
  6. Apply the agreed rule: compare unrounded results using the contract's limit, rounding and uncertainty treatment.
  7. Record the action: release, clarify, investigate with a targeted follow-up test, or reject under the stated remedy.

The site's green tea COA checklist provides the wider 12-point document review. Use it with a representative sample and Yunjing Tea's sample-to-shipment quality controls.

Common buyer mistakes

  • Comparing a dry-basis result with an as-received limit.
  • Accepting "ISO method" without the number and edition.
  • Using withdrawn ISO 9768:1990 when the published 1994 edition is intended.
  • Treating total ash as proof of dirt or deliberate adulteration.
  • Treating water extract as cup strength or industrial extraction yield.
  • Converting bases with moisture from a different sample or date.
  • Rounding a borderline result before applying the decision rule.
  • Releasing a shipment when the tested sample is not traceable to the loaded lot.

Practical conclusion

For every result, write one complete line: parameter, method and edition, sample, basis, unit, unrounded value, uncertainty or precision information, limit, decision rule and disposition. If any element is missing, classify the result as "clarify" rather than silently filling the gap. When all three parameters use controlled sampling, comparable methods and the same contractual basis, they become useful lot evidence instead of three isolated numbers.

Send Yunjing Tea your destination, tea type, intended use and draft analytical specification before the lot is sampled or tested.

Sources checked 4 August 2026: ISO 11287:2011, Green tea - Definition and basic requirements; ISO 1572:1980, preparation of a ground sample of known dry matter content; ISO 1573:1980, loss in mass at 103 degrees C; ISO 1575:1987, total ash; ISO 9768:1994, water extract; ISO 1576:1988, water-soluble and water-insoluble ash; ISO 1577:1987, acid-insoluble ash; and SAMR national standards platform, GB/T 14456.1-2017.