Oolong Tea Catechins and Oxidation

Oolong tea contains 80 to 150 mg of catechins per gram of dry leaf, roughly half the green tea figure and three times the red tea figure. Oxidation is stopped partway, so the leaf keeps much of its original catechin load while also carrying the theaflavins that partial oxidation has already produced. That combination of retained and newly formed compounds is what makes oolong chemically distinct rather than a blend of green and red.

The number depends almost entirely on how far the producer let oxidation run. A lightly oxidized Tieguanyin at 20 percent is close to green tea in its catechin content. A heavily oxidized Dongfang Meiren at 65 percent has given up most of its epigallocatechin gallate and carries visible theaflavins instead. Both are oolong, and quoting a single figure for the category hides a threefold spread.

Why the range is so wide

Catechin loss during oxidation is not linear with time. The epigallocatechin gallate fraction falls fastest, because it has the most hydroxyl groups for polyphenol oxidase to attack. Simple catechins such as epicatechin decline more slowly. So a lightly oxidized oolong loses proportionally more EGCG than a heavily oxidized one loses epicatechin, and the total catechin figure conceals a shift in composition that matters more than the total.

The other variable is leaf grade. Oolong is made from mature open leaves, not buds, which is why the catechin content per gram is lower than a bud-only white tea. A coarse summer leaf and a fine spring leaf from the same bush differ by a third in extractable polyphenols before processing even begins.

What partial oxidation creates

Theaflavins appear as soon as oxidation begins, because they form from the condensation of a catechin with a gallocatechin. In oolong they are present at trace to 2 mg per gram, far below the 20 to 30 mg of a red tea, but they are there. A cup of heavily oxidized oolong therefore delivers a small but real theaflavin dose that green tea cannot.

Thearubigins, the larger red-brown polymers, need more oxidation and more time. In oolong they are present at low levels, which is one reason an oolong liquor is amber rather than deep red. The color of the cup is a rough readout of how far the polymerisation went.

A third group is specific to the roasting step, which red and green tea do not have. Pyrazines and Maillard products form when the leaf is baked at 80 to 140 degrees Celsius. They contribute the toasted, nutty notes of a roasted oolong and have no counterpart in either of the other two categories.

How much reaches the cup

The infusion column is lower than a simple division would suggest because gongfu brewing uses a high leaf-to-water ratio and a short steep. Five grams of leaf in 120 ml for thirty seconds extracts perhaps a third of the soluble material, and the second and third infusions extract more. Across six infusions the total extraction approaches 70 percent.

This matters for anyone comparing an oolong to a green tea by dry leaf weight. A gongfu session with seven grams of oolong delivers more total catechins than a Western-style steep of two grams of green tea, even though the green leaf is richer per gram. The brewing method moves the number more than the category does.

The caffeine figure and what drives it

Oolong sits at 20 to 35 mg of caffeine per gram of dry leaf, which is close to green tea and below red. The common claim that oolong is lower in caffeine because it is partially oxidized is wrong in direction: oxidation does not destroy caffeine, and a heavily oxidized oolong has the same caffeine as a lightly oxidized one from the same bush.

What does change caffeine is the leaf part. A bud-heavy oolong carries more caffeine per gram than a coarse leaf oolong, because caffeine concentrates in the young growth. This is why a fine Tieguanyin can feel stronger than a coarse roasted oolong despite the category being the same.

Roasting removes a small amount. A heavy charcoal roast drives off perhaps five percent of the caffeine along with the volatile aromatics, which is not enough to matter but is enough that a roasted oolong measures slightly lower than the unroasted leaf it came from.

Practical reading of a label

• Light oxidation under 25 percent behaves like green tea in the cup, keep the water at 90 Celsius.
• Medium oxidation 30 to 45 percent is the most common commercial range, 95 Celsius.
• Heavy oxidation above 55 percent tolerates boiling water and milk.
• Roasted oolong carries pyrazines, so a lower steep temperature preserves the toast.
• A single catechin number on a package tells you less than the oxidation level does.

The useful question when buying is not how much catechin an oolong has, because no package states it and no producer measures it. The useful question is the oxidation level, which is a settled fact about how the tea was made and predicts the chemistry better than any marketing claim.

What the category does not tell you

The 80 to 150 mg range is honest but coarse. Two oolongs at opposite ends of it are different drinks with different brewing needs, different caffeine delivery and different taste. Treating them as one category is a convenience of the shop shelf, not a chemical fact.

The other limit is absorption. Catechins from a brewed cup reach the blood at low concentrations, and the theaflavins in a heavily oxidized oolong are absorbed poorly. The compounds are present in the drink; how much of them does anything in the body is a separate and much weaker claim.

Reading oxidation from the taste

There is a practical way to estimate where a tea sits on the oxidation curve without a laboratory: taste the finish. A lightly oxidized oolong finishes green and slightly sharp, because the simple catechins are still there. A heavily oxidized one finishes smooth and slightly sweet, because the astringent catechins have been converted.

The middle of the curve is the hardest to read and the most interesting. At around 40 percent, both families are present at once, and the cup shows a green edge and an amber body. That duality is what drinkers describe as complexity, and it is the chemical reason a medium-oxidized oolong can taste like neither of its neighbors.

What happens across infusions

The first infusion extracts mostly surface compounds: caffeine, simple catechins, amino acids. Later infusions pull from deeper in the leaf, where the larger thearubigins and cell-wall bound material sit. This is why the third infusion often tastes fuller than the first, and why the last infusions turn thin and woody.

The practical consequence is that a single infusion is a poor sample of the leaf. Anyone comparing two oolongs by one cup is comparing their first thirty seconds, not their chemistry. Three infusions give a fairer read, and six give most of what the leaf has.

• A green finish suggests light oxidation, under 25 percent.
• An amber body with a green edge suggests the 35 to 45 percent band.
• A smooth sweet finish with no edge suggests over 55 percent.
• The third infusion is usually the fullest, not the first.
• Thin woody late infusions are the leaf running out, not a fault.
• Three infusions is the minimum for a fair comparison.

The soil and altitude matter less than the processing for catechins, but they are not irrelevant. High-altitude bushes grow more slowly, accumulate more amino acids, and produce a leaf that tastes sweeter at the same oxidation level. The catechin figure moves less than the theanine figure, which is one reason a mountain oolong and a lowland one can measure similarly and taste completely different.

Spring and winter picks differ in the same way. A winter pick is lower in catechins and higher in amino acids, which is why winter oolong is prized for its smoothness rather than its strength. A summer pick is coarser and more astringent, and is usually the raw material for roasting, because the roast can convert the harshness into something else.

None of this changes the headline range. An oolong carries 80 to 150 mg of catechins per gram, and the processing decision moves it across most of that range while growing conditions move it across a small part. The buyer reading a label learns more from the oxidation level than from the mountain name.

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