Lab Quality Indicators for Tea: Moisture, Ash, and Water Extract
To evaluate a pu'er, a tea taster uses eyes, nose, and tongue to assess the leaf, liquor color, aroma, taste, and spent leaf; that's sensory evaluation. A lab uses scales, drying ovens, muffle furnaces, and spectrophotometers to turn the tea's moisture, minerals, and soluble content into numbers; those are the "physicochemical indicators." One soft and one hard, together they make up a complete description of quality.
Which Indicators the National Standard Sets
The national standard for pu'er is GB/T 22111-2008 Product of Geographical Indication: Pu'er Tea, issued on June 17, 2008, and in effect from December 1, 2008. It's still the current standard. Its definitions and geographical indication scope are covered in The National Standard and Geographical Indication for Pu'er.
The standard's physicochemical indicators are set separately for sun-dried tea (raw material), raw tea, ripe loose tea, and ripe compressed tea:
| Item | Sun-dried tea | Raw tea | Ripe loose tea | Ripe compressed tea |
|---|---|---|---|---|
| Moisture (%) | ≤ 10.0 | ≤ 13.0 | ≤ 12.0 | ≤ 12.5 |
| Total ash (%) | ≤ 7.5 | ≤ 7.5 | ≤ 8.0 | ≤ 8.5 |
| Fines (%) | ≤ 0.8 | — | ≤ 0.8 | — |
| Water extract (%) | ≥ 35.0 | ≥ 35.0 | ≥ 28.0 | ≥ 28.0 |
| Crude fiber (%) | ≤ 14.0 | ≤ 14.0 | ≤ 14.0 | ≤ 15.0 |
| Tea polyphenols (%) | ≥ 28.0 | ≥ 28.0 | — | — |
For raw and ripe tea, 10.0% is used as the reference moisture when testing net weight.
A few points for reading the table:
- Raw tea requires at least 28.0% tea polyphenols and at least 35.0% water extract, meaning the material must be rich enough in constituents; ripe tea has no minimum for tea polyphenols, since wodui inherently reduces them greatly.
- Ripe tea's upper limits for total ash and crude fiber are a bit looser than raw tea's, because ripe tea often uses sturdier material and has less soluble content after wodui.
- When testing net weight, the standard converts using a fixed 10.0% "reference moisture," so tea doesn't weigh differently in damp and dry seasons.
Moisture: How Low Should It Be?
Moisture measures the water content of the tea. Ground tea is dried in an oven at around 103°C until its weight stops changing, and the weight lost is calculated.
Moisture is the first checkpoint for storing pu'er. With high moisture, the tea's water activity is high, enzymes and microbes become active easily, aging speeds up, and storage taste or even mold can appear; with very low moisture, transformation is slow. That's also why the humidity of the storage environment matters so much. See The Chemistry of Raw Pu'er Aging and Temperature and Humidity for Storing Pu'er.
Sun-dried maocha has the strictest moisture limit, 10.0%; raw tea and ripe compressed tea have looser limits because they absorb moisture during steaming and pressing and dry more slowly once compressed.
What Is Ash?
Put a tea sample in a muffle furnace at about 525°C and burn it until only a white or grayish-white residue remains: that residue is "total ash," mainly oxides of minerals such as potassium, calcium, magnesium, and phosphorus.
- Normal sources of ash: minerals the tea plant absorbs from the soil. Tender leaves contain less, and older leaves and stems more, so the coarser the material, the higher the ash usually is.
- What high ash may indicate: besides coarse, mature material, it may mean the tea has picked up non-tea contaminants such as soil or dust. So total ash is often used as a reference for "cleanness."
Some standards also break it down into "water-soluble ash" and "acid-insoluble ash": the former is minerals that dissolve into the liquor, while the latter is mainly sand and soil. A high figure calls for attention to the cleanliness of the processing environment.
Water Extract: What Can Be Brewed Out
Water extract measures the total amount of substances the tea can release in boiling water, as a percentage of the dry tea's weight. Roughly, the sample is steeped in boiling water for a set time, filtered, and dried, and the loss in weight of the remaining leaves is measured.
The tea polyphenols, caffeine, amino acids, soluble sugars, and pigments in the liquor all count toward water extract, so it's often seen as an indicator of the liquor's "strength" and "staying power": the higher the figure, the stronger and longer-lasting the liquor usually is.
The minimum is 35.0% for raw tea and 28.0% for ripe tea. Ripe tea's is lower because microbes use and transform some of the soluble substances during wodui. For the composition, see What's in Pu'er.
Crude Fiber and Fines
- Crude fiber: the poorly soluble part left after a tea sample is digested with acid and then alkali, mainly cellulose and lignin. The coarser and more mature the material and the more stems, the higher the crude fiber, so it also reflects how tender the material is.
- Fines: the proportion of fine broken material in loose tea after sieving. A high figure means lots of broken leaf, which tends to cloud the liquor and release flavor too fast. Compressed tea, being a different form, doesn't list this item.
Tea Polyphenols
Tea polyphenols are the collective name for the many phenolic compounds in tea, mainly catechins, usually measured by the "Folin-Ciocalteu method." The standard requires at least 28.0% tea polyphenols for sun-dried tea and raw tea, with no minimum for ripe tea. How tea polyphenols change during wodui and aging is covered in Tea Polyphenols and Catechins.
How to Read a Lab Report
When you get a tea lab report, look at these in order:
- Sample information: product name, raw or ripe, loose or compressed, batch number, to confirm which tea the report is for.
- The standard applied: the report states the basis for its judgment, such as GB/T 22111-2008 or another group or company standard. Different standards have different figures, so compare against the same standard.
- Test methods: each item is usually followed by a method standard number, such as GB 5009.3 for moisture, GB/T 8305 for water extract, and GB/T 8313 for tea polyphenols.
- Measured values and limits: next to each measured value is the standard's upper or lower limit and a judgment of "conforms" or "does not conform."
- Other items: besides physicochemical indicators, reports often list pesticide residues, heavy metals, microbes, and so on, judged by local food regulations.
Remember that passing the physicochemical indicators only means the tea meets basic quality requirements; it doesn't mean it tastes good. For flavor, you still have to go back to sensory evaluation.
FAQ
What physicochemical indicators does the national standard for pu'er set? GB/T 22111-2008 sets moisture, total ash, fines, water extract, crude fiber, and tea polyphenols, with separate figures for sun-dried tea, raw tea, ripe loose tea, and ripe compressed tea.
How low should pu'er's moisture be? Under GB/T 22111-2008: below 10.0% for sun-dried tea, 13.0% for raw tea, 12.0% for ripe loose tea, and 12.5% for ripe compressed tea.
What is ash? The mineral residue left after tea is burned in a muffle furnace, mainly potassium, calcium, magnesium, and phosphorus; coarser material has more ash, and contamination with soil also raises it.
What does high water extract mean? That boiling water can draw out a lot of soluble material, so the liquor is usually stronger and longer-lasting. The standard requires at least 35.0% for raw tea and 28.0% for ripe tea.