Why temperature decides a cup of green tea
Green tea is unoxidised, so everything in the leaf is still available to the water. Temperature decides which parts of it you get, and in what order.
Amino acids, which give sweetness and body, dissolve readily from about 60 degrees. Catechins, which give astringency and bitterness, need roughly 85 or more to come out quickly.
That gap is why the same leaf can taste sweet or harsh. Brew at 75 and the amino acids lead. Brew at 100 and the catechins overtake them within a minute.
Time interacts with this. A short steep at a high temperature is not the same as a long steep at a low one, because the two compound groups extract at different rates.
The practical consequence is that green tea has the coolest target of any category, and the narrowest margin between right and wrong.
Roasted and pan fired greens lose that fragility during processing, which is why hojicha sits at the opposite end of the scale from gyokuro.
Exact temperatures by green-tea style
Japanese greens sit lowest. Gyokuro is best at 50 to 60 degrees, and sencha at 70. Both taste thin rather than better when brewed hotter.
Chinese greens want more. Longjing and Biluochun sit at 80 to 85, which is hot enough for aroma and cool enough to keep the sweetness.
Anji Bai Cha is a green tea despite the name, and it sits at 80. It is unusually rich in amino acids, which is why a lower temperature suits it.
Roasted greens are the exception. Hojicha and similar are brewed at 95 to 100, because the roasting has already changed the compounds you would otherwise protect.
Bagged green tea is almost always broken leaf, so it extracts faster and sits lower too, around 75 to 80.
When you have no idea which you have, 80 degrees is the safe default. It is too cool for roasted greens and slightly warm for gyokuro, and neither is ruined by it.
Water quality, not just temperature
Water is the other half of the equation, and it is the half most often ignored. Tea is mostly water, and whatever is in that water ends up in the cup.
Total dissolved solids are the measure that matters. Around 50 to 150 parts per million is a sensible range: enough mineral to give body, not enough to flatten the aroma.
Very soft water below about 30 parts per million gives a thin, hollow cup. Very hard water above 200 gives a dull one and leaves scale in the kettle.
Hardness as calcium carbonate is worth knowing separately, because calcium is what mutes aroma. Around 30 to 80 milligrams per litre suits green tea.
Chlorine is the remaining problem. Let tap water stand for a few minutes, or run it through a carbon filter, and most of it dissipates.
Temperature and mineral content are independent variables. Getting one right will not rescue the other.
Practical brewing steps
Boil the water, then let it stand. Two minutes in an open kettle brings it near 90, and five minutes brings it to about 80.
If you have a variable temperature kettle, set it and stop thinking about it. It is the single most useful purchase for a green tea drinker.
Without one, pour between vessels. Each transfer from a hot kettle into a cold cup drops roughly 10 degrees.
Use 3 grams per 100 millilitres for 60 to 90 seconds, then decant. Taste at the earlier end of that range and adjust from there.
Check the spent leaf afterwards. Bright green leaf with a sweet smell means you were in the band. Dull olive leaf with a cooked smell means you were too hot.
Write down what worked once you find it. One line on the packet removes the guesswork next time.
Common mistakes
The most common mistake is using water straight off the boil because that is what the kettle gives you. Two minutes of waiting fixes it.
The second is leaving the leaf in. A mug infuser keeps extracting, and the last third of the cup is brewed far longer than the first.
The third is reboiling the same water repeatedly. Water that has boiled for a while gives a flat cup, so start fresh each time.
The fourth is chasing the numbers rather than the taste. The ranges are a starting point, and a leaf that tastes right at 75 is not wrong.
The fifth is assuming every green tea wants the same thing. A gyokuro and a hojicha are 40 degrees apart, and treating them alike guarantees one will be poor.
When something is wrong, change one variable at a time. Two changes at once and you will not know which one mattered.