Withering
How the leaf loses its water and becomes white tea: the sun room and the heated room, what the enzymes do while it waits, how a maker judges it finished, and what light itself has been found to change.
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White tea is withered, then dried. Withering is the first of those two steps and very nearly the whole of the craft. It is measured in hours and days, not minutes.
Oolong and black tea begin the same way, with a wither, but for them it is only a start; rolling, oxidation, and firing follow and do most of the work. In white tea nothing follows. The leaf is left to lose its water in open air, and the drying at the end only sets what the wither has already done. What Is White Tea covers the outline. This is the account of the step itself.
The maker's task is small and exact. It is to arrange the conditions, watch, and wait.
The Room and the Sun
There are three ways4 to hold the leaf while it withers, and the choice is usually the weather's to make.
| Method | Where | How long | What it needs |
|---|---|---|---|
| Sun withering (日光萎凋) | Outdoors, direct sun | About two to three days | Two to three clear days in a row. A run of cloud or rain during the wither spoils costly fresh leaf |
| Indoor natural withering (室内自然萎凋) | Under cover, out of direct sun | About fifty-two to sixty hours | Time, mostly |
| Indoor heated withering | A controlled room at 38 to 40 degrees Celsius (about 100 to 104 degrees Fahrenheit) | Roughly one day | Nothing from the sky. This is the answer to it, and the trade calls the split normal-weather and abnormal-weather initial processing5 |
All three end in the same reading, which is the subject of Judging It Done below. What the table cannot show is where the craft is going: fully automated compound withering lines, lit and regulated throughout, have since been built to take the weather out of the decision altogether.
What the Leaf Does While It Waits
The leaf is not idle. As it loses water, its cell walls break down and release oxidase enzymes, polyphenol oxidase and peroxidase, and a slow oxidation begins and continues through the whole wither.
The register bears this out. In one controlled study1 the total polyphenols measured 33 percent at four hours and fell to 28 percent by ninety-three hours; theaflavins fell from 0.05 percent to 0.036 percent over a comparable stretch, out to ninety-six hours, the oxidation still working near the end. That batch reached its optimum at ninety-six hours.
The chemistry underneath is not one motion but several. A metabolomics study of the wither2 found the free amino acids rising overall as protein breaks down into them, even as theanine itself falls; the catechins and their relatives fall, from weakened synthesis and the ongoing oxidation together; and caffeine, theaflavins, and certain nucleosides rise, the last from RNA breaking down.
Aroma is built the same slow way. As the moisture drops and enzyme activity climbs, the leaf's carotenoids break down, and that loss feeds the aroma compounds that form in their place. Among the key odorants identified in the finished tea are linalool and geraniol, which read as floral, and hexanal, which reads green and grassy. A comparison of methods3 found the account legible in the leaf: sun-withered leaf carried more floral character, higher in linalool and geraniol, while leaf withered in a warmed, air-blown tank at 30 degrees Celsius (about 86 degrees Fahrenheit) carried more grassy character, higher in hexanal. Both were taken down to about ten percent moisture before a shared final dry.
Judging It Done
The wither runs until the leaf holds roughly twenty percent moisture6. The maker reads that point by eye and by hand, not by instrument alone.
The down on the bud turns white. The bud tip and the tender stem curl upward, what the trade calls a cocked tail, and the leaf edge curls down. When the leaf shows this, it is combined and screened for drying.
What Light Does to the Wither
Warmth and the leaf's moisture have always governed the wither. The light landing on the leaf is now a measured variable too.
A controlled study withered leaf under different colors of LED for thirty-six hours7, held at 25 degrees Celsius (about 77 degrees Fahrenheit), give or take two, and 75 percent relative humidity, give or take five, against an incandescent-lit control and across five red-to-blue ratios. A three-to-one red-to-blue ratio gave the lowest bitterness and astringency of any treatment, holding the lowest catechins, caffeine, and gallic acid of the six. A companion study8 found the reason plainly stated: red light raises the activity of polyphenol oxidase, the same enzyme that drives an ordinary wither, quickening the catechin breakdown that lowers bitterness, while blue-dominant light raises soluble sugar substantially over the study's control.
The light is part of the register now, entered like the hours and the moisture before it.
What the Standard Governs, and What It Leaves to the Room
White tea has a national standard, GB/T 22291-20179.
It defines white tea as the product of specified cultivars carried through withering, drying, and sorting, and it sets the four grades by cultivar and plucking standard: silver needle, bai mudan, gong mei, and shou mei. It does not fix a required number of withering hours, and it does not fix a required moisture percentage.
Those are left to the room, the weather, and the maker's judgment, entered and reported, never mandated.