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.
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 ways to hold the leaf while it withers, and the choice is usually the weather's to make.
Sun withering (日光萎凋) spreads the leaf outdoors in direct sun and runs about two to three days. It needs two to three clear days in a row to finish clean. The fresh leaf is costly, and a run of cloud or rain during the wither spoils it.
Indoor natural withering (室内自然萎凋) spreads the leaf under cover, out of direct sun, and takes longer for it. The full wither runs about fifty-two to sixty hours.
Indoor heated withering is the answer to the sky. A controlled room held at 38 to 40 degrees Celsius can compress what takes three days in open air into roughly one, and it was built for exactly that, so the work need not wait on the weather. The trade has a name for the split: normal-weather and abnormal-weather initial processing. Fully automated compound withering lines, lit and regulated, have since been developed to remove the weather constraint 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 study 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 wither 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 fall by more than sixty-five percent over processing, 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 methods 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 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 moisture. 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
The room's warmth and the leaf's moisture are the old variables. The light landing on the leaf is now a measured one too.
A controlled study withered leaf under different colors of LED for thirty-six hours, held at 25 degrees Celsius, 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 study 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 an unlit 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
The national standard for white tea is GB/T 22291-2017. It is worth saying exactly what it fixes and what it does not.
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. What the standard leaves open, it leaves to restraint, the same restraint the whole craft is built on.