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You leave the tea on the stove to answer the door, and it comes back dark and harsh. That harshness is chemistry, and you can measure it with three cups and a timer.
What makes tea bitter?
Tea leaves hold a family of polyphenols called catechins, usually lumped together as "tannins". Four main catechins make up about 80% of the polyphenols in a green or oolong tea infusion, according to a 2014 study in the Journal of Food and Drug Analysis. Black tea goes through a browning step driven by the leaf's own enzymes, and the same paper lists theaflavins and thearubigins alongside catechins as causes of astringency.
Two catechins matter most for bitterness. In a 2022 sensory study of green tea in npj Science of Food, the main contributors to bitterness were ECG and EGCG, and the amount of EGCG tracked bitterness with a correlation of 0.922. Caffeine is bitter too, and the paper notes earlier work finding that EGCG makes caffeine taste more bitter.
Is it bitterness or astringency?
Over-brewed tea usually gives you both. Bitterness is a taste. Astringency is the dry, rough feeling on your tongue and teeth.
Astringency happens when tannins bind proline-rich proteins in your saliva, the clumps fall out of solution, and your mouth loses its slipperiness, as the Journal of Food and Drug Analysis paper explains. Catechins carrying a gallate group, such as EGCG and ECG, are the more astringent ones.
This helps explain why milk seems to soften strong chai. A 2010 study in Milk Science found that skim milk clearly reduced the astringency of a catechin solution and sharply lowered the amount of free catechin, with casein, the main milk protein, tested as part of the reason.
Why do heat and time make it worse?
Hot water pulls molecules out of the leaf faster, for the same reason colour spreads faster in hot water than cold: warmer molecules move faster. Time decides how far the extraction goes.
The clearest numbers come from a 2015 study of Turkish green tea in the Journal of Food Science and Technology. The team brewed tea at 75, 85 and 95 °C for 1 to 45 minutes, and nine trained tasters scored each cup from 0 to 10.
At 85 °C, taste scored 8.4 at 3 minutes and fell to 1.7 at 45 minutes.
At 95 °C, taste scored 8.3 at 3 minutes, 4.9 at 10 minutes and 1.2 at 30 minutes.
At 75 °C, the fall was gentler: 8.1 at 3 minutes and 4.7 at 30 minutes.
The authors blamed the very low scores for 30- and 45-minute brews at 85 and 95 °C on bitter taste and dark colour. Caffeine at 85 °C peaked at 3 minutes, at 11.36 mg per 100 ml, and did not change significantly after that. Total catechins at 85 °C also peaked at 3 minutes, though some catechins, such as GCG, kept rising as heat changed others into new forms. So the extra harshness of a long brew is unlikely to be extra caffeine, and the study does not pin down which molecules caused it.
That study used green tea in a lidded pot, not CTC black tea boiled with milk, so treat its minutes as a rough guide for your chai, not a rule.
How to test it: three cups in your kitchen
The timings below echo the 2015 brewing study, where the best-scored cups came at 3 to 5 minutes. You need one plain black tea, three identical cups, a timer, and an adult nearby for the boiling water.
Put the same amount of tea in each cup.
Pour the same amount of just-boiled water into all three.
Take the tea out of cup A at 2 minutes, cup B at 5 minutes and cup C at 15 minutes.
Let all three cool for 10 minutes so heat does not fool your tongue. Ask someone to shuffle the cups and mark the bottoms.
Taste each without milk or sugar. Score bitterness and dryness from 0 to 10, and note the colour.
Add a spoon of milk to what is left of cup C and taste again.
Write a prediction first: which cup will be most bitter, and will bitterness and dryness rise together? If they don't, you have separated a taste from a feeling. For a second round, compare just-boiled and slightly cooled water at 5 minutes.
If molecules moving faster in hot water is hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read, and concepts such as diffusion and reaction rates each have their own lesson in the Fermi Concept Library.
How do you brew tea that isn't bitter?
Time the steep. In the 2015 study, taste scored highest at 3 minutes at every temperature.
Take the leaves out. Tea left in the pot keeps brewing.
Go gentler with green tea. In the same study, taste scores at 95 °C fell fastest after 5 minutes.
For chai, keep the boil short once the tea is in.
If a cup is already harsh, a splash of milk softens the dryness.
For more chemistry in your cup, read why lemon makes tea lighter and why tea cups get brown stains.
