Withering

White tea is made in two steps, withering and drying. Withering 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. White tea's leaf is left to lose its water, and the drying at the end only sets what the wither has already done. What Is White Tea covers the outline.

The Room and the Sun

There are three ways⁴ to hold the leaf while it withers.

In Fuding, LED-lit automated compound withering lines have been built so that production no longer depends on the weather.

What the Leaf Does

As the leaf loses water, its cells grow more permeable and its oxidase enzymes, polyphenol oxidase and peroxidase, grow more active, and a slow oxidation continues through the wither.

Two studies have measured it, and on one compound, theaflavins, they disagree. An Indonesian study¹ sun-withered the buds of an Assam-variety clone by day and kept them indoors at night for four days, measuring by colour-reaction assays. Total polyphenols went from 33 percent at four hours to 28 percent at ninety-three, a downward tendency the authors note was not statistically significant. Theaflavins fell from 0.05 percent to 0.036 percent while thearubigins rose, which the authors read as theaflavins oxidising further into thearubigins as the wither went on. They judged ninety-six hours the optimum.

A Chinese metabolomics study of white tea processing², profiling 99 compounds, 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. Caffeine, theaflavins, and certain nucleosides rise, the last from RNA breaking down. Its rising theaflavins and the Indonesian study's falling ones come from different leaf measured by different methods. The Indonesian figures are a small fraction of the 1 to 2 percent typical of black tea¹.

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 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.

Judging It Done

The wither runs until the leaf holds roughly twenty percent moisture⁵.

Before then, 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, the screens are combined and the wither continues.

What Light Does to the Wither

A controlled study withered leaf under different colors of LED for thirty-six hours⁶, 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 second study⁷ found blue-dominant light raises soluble sugar substantially over the study's control.

What the Standard Governs, and What It Leaves to the Room

White tea has a national standard, GB/T 22291-2017⁸.

It defines white tea as the bud, leaf and tender stem of the tea plant carried through withering, drying, and sorting, and it divides white tea into four products by cultivar and raw material: silver needle, bai mudan, gong mei, and shou mei. It does not fix a required number of withering hours.