Same Leaf, Split Into Green and Black, Kept Almost the Same Caffeine

Take one batch of freshly picked leaf. Split it in two. Process one half into green tea and the other into black, the two ends of the oxidation spectrum with oolong sitting between them, and sample both at every stage. Astill and colleagues ran exactly this test in 2001. Catechins in the black-tea half collapsed from 21.34 percent of dry weight to 1.79 percent, a loss of more than ninety percent. Astill names the actor directly: polyphenol oxidase, an enzyme present in fresh leaf, oxidizing the catechins into theaflavins and thearubigins. That reaction is the whole of what oxidation is.

Caffeine did not follow the catechins down

It started at 3.18 percent, rose to about 3.6 to 3.8 percent during withering and rolling on both sides of the split, and held there through full oxidation on the black half and through none at all on the green half. The green half finished at 3.77 percent caffeine, the black half at 3.60, marginally higher on the unoxidized side, not the oxidized one.

Wuyi rock tea, tested by roast level

Liu and colleagues measured caffeine across four cultivars of Wuyi rock tea² (yancha, the heavily oxidized, heavily roasted end of the category) at light, medium, and heavy roast: caffeine ran 3 to 16 percent higher at medium roast than at the light and heavy ends.

A separate 2026 study tracking nine Taiwanese teas through four processing stages reported caffeine changes under 20 percent during fermentation and no association with fermentation intensity, while the same leaves lost as much as 92 percent of their catechins. A study on CTC black tea processing in North East India found caffeine rising most during withering, with a very minimal effect from fermentation.

A Tieguanyin study complicates the picture

A 2022 study out of Anxi, tracking caffeine at every stage of Tieguanyin manufacture from fresh leaf to finished tea, found a real total decline: 47.7 percent for caffeine, 68.5 percent for tea polyphenols, over the whole process. That is a bigger drop than anything the studies above report.

The paper's stage-by-stage figures show caffeine fluctuating widely through tossing and cooling, the alternating zuoqing cycle that opens the leaf's margins to oxidation, dipping slightly at kill-green, the heat that stops oxidation, and then falling rapidly during rolling. That sits in tension with Astill's same-leaf split and the Taiwanese study above, which found caffeine essentially flat through the equivalent stages.

The cultivar decides more than the processing does

If oxidation is not the reliable lever, something else sets the number, and the plant itself is the strongest candidate. A 2022 study comparing Chinese-variety cultivars against a group of Assam-variety cultivars found caffeine at 3.58 to 4.15 percent of dry weight in the Chinese material and as low as 0.05 to 0.55 percent in the Assam group. The authors traced the gap to near-absent expression of TCS1, the gene for tea caffeine synthase. That is a wider caffeine range than any processing study above produces, set by the cultivar under the picker's shears, not by anything the maker does to the leaf afterward.

A study across six Kenyan cultivars found caffeine varying by cultivar, and again by season, running highest in the hot, dry months. A separate survey of thirty tea infusions put oolong's own dry-weight caffeine at 12.36 to 31.66 mg per gram of dry leaf, the widest spread of any of the six categories tested: green, black, white, yellow, and dark included.

The pluck matters too

Layered on top of cultivar is which part of the plant went into the basket. A 2012 study of tea grown in Hawaii reported caffeine and l-theanine both declining as leaf age increased, from bud through the first leaf to the second, while catechins ran the other way, rising with age. A bud-heavy pluck should read higher in caffeine than a coarser one from the same cultivar and the same oxidation level.