Green-leaf volatiles

What a drinker notices

• Aroma — an aroma

Cut grass, green bean, fresh-snapped stem. In a young Japanese green tea a measured amount of it is the point and reads as freshness; in a black tea it is a fault and reads as rawness.

How much of it there is matters: The clearest example in tea of the same aroma being a virtue and a defect at different levels. A little is freshness. A lot is grassiness, and grassiness is what buyers reject.

What it is

When any green leaf is torn, an enzyme cascade takes the lipids of its ruptured membranes apart and produces six-carbon aldehydes and alcohols within seconds. That is the smell of mown grass, and it is what a tea leaf smells of the moment it is plucked. Most of tea manufacture is, from this angle, a programme for getting rid of it: withering lets it disperse, fixation halts the enzymes making it, and firing drives off what remains. A finished tea that still smells strongly of it is usually under-withered or under-fired, and the fault has a name in every producing country.

Chemically: Six-carbon aldehydes and alcohols made by enzymes acting on membrane lipids.

Where it occurs in the plant

Not present in the intact leaf. Created on damage, so the amount in a finished tea is entirely a record of how it was handled between plucking and firing.

What processing does to it

In tea, processing is not a modifier applied to a fixed composition — it is what decides the composition. These are the steps that move this compound, and each links to the step itself.

Withering — Lowers it

The wither is partly a dispersal step: hours of airflow carry these volatiles away, and a leaf withered properly has lost most of them before anything else begins.

Kill-green (fixation) — Locks it in place

Fixation denatures the enzymes producing them, so no more are made after this point. What the leaf has at fixation is what it keeps.

Steaming (Japanese fixation) — Lowers it

Japanese steaming is fast and hot enough to shut the cascade down almost immediately, which is why sencha keeps a clean green character rather than a grassy one — the greenness of Japanese tea is not this.

Final firing (hiire) — Lowers it

The final firing drives off what survives, and a tea that smells raw is usually one that has had too little of it. Re-firing an old, flat green tea works partly for this reason.

Fresh-leaf transport and holding — Raises it

The cascade starts at the break, not at the factory. Leaf packed into a bag and driven for hours is bruised, warm and short of air, and it makes these compounds the whole way — which is why transport time and bag depth are manufacturing variables and why a factory close to its fields has an advantage that is not about freshness in the marketing sense.

Drum fixation — Lowers it

A continuous heated cylinder shuts the enzymes down as the leaf passes through, so the dwell time in the drum sets how much of this group survives. Under-fixed leaf leaves the drum still making them, and that is what a persistently grassy Chinese green tea usually is.

Spreading and cooling (tan liang) — Locks it in place

Getting the heat out of the leaf straight after fixation stops the residual cooking that would otherwise keep driving these off — and stops it evenly, across the whole batch, which is the part a hot pile in a corner cannot do.

Cloth-ball rolling — Lowers it

Wrapping leaf in cloth and kneading it repeatedly, with a warming between passes, takes hours and each cycle carries a little more of this group away. That is a real part of why a tightly rolled high-mountain oolong reads as clean and sweet rather than green, at the same oxidation level as a strip-rolled tea that does not.

Steam softening before pressing (zheng ya) — Lowers it

A brief steam before pressing costs a young sheng some of its green top note. It is one of the small differences between a loose maocha and the cake pressed from it, tasted side by side in the same month.

How brewing brings it out

Extraction rate: Fast — among the first things out of the leaf

Highly volatile and out immediately in hot water, then lost quickly to the air. A grassy tea smells worst in the first ten seconds and improves as the cup stands, which is a genuine change rather than acclimatisation.

Across infusions: Almost entirely a first-infusion phenomenon.

Water: Chlorinated tap water is the enemy here rather than hardness. Chlorine reacts readily with the unsaturated aroma compounds and flattens exactly the fresh, cut-grass top note this group provides, which is why the difference between tap and filtered water is most obvious on a young green tea and barely noticeable on an aged shou.

Leaf, ratio and agitation: Volatile, so an open cup loses the top note as it cools while the taste stays put. A covered vessel and a smaller volume drunk quickly keep more of it than a large mug does.

What you can change

• Let a grassy cup stand a minute before drinking
• Rinse the leaf briefly and discard
• Brew cooler, which extracts less of them

Teas this compound defines

• Sencha
• Bancha
• Mizudashi sencha
• Aracha
• Sheng maocha
• Shincha

Read next

• cis-3-Hexenol — Leaf alcohol: the specific compound behind the smell of freshly cut grass, and the one a tea maker is judging when they smell a withering trough.
• Linalool — A major floral aroma compound of tea, largely stored bound to sugar in the leaf and set free by processing.
• Pyrazines — The roasted, nutty and toasted aromas — compounds that do not exist in a tea leaf until heat makes them.
• Leaf lipids — The fraction a drinker never tastes and cannot do without: the raw material every green-leaf aroma is cut from, and the part of the leaf that goes off.