Tea made by boiling leaves in milk tastes and behaves differently from tea steeped in water with milk added at the end. The gap is chemistry rather than preference.
Milk proteins bind the astringent compounds
The drying grip of strong black tea comes from polyphenols, loosely grouped as tannins, which latch onto proteins and pull them together. On the tongue, that reads as astringency.
Milk arrives carrying casein, a protein those polyphenols bind readily. The compounds are still in the cup, but they are already occupied before they reach you.
Which is why a brew strong enough to be harsh in plain water can taste rounded once milk is in it. The strength has not dropped; the delivery has changed.
Boiling in milk extracts a different set
Water pulls tea compounds out at different rates. The gentler aromatics come early, and the bitter fractions accelerate as heat and time increase.
Milk is a suspension of fat and protein in water, so it conducts heat differently and offers dissolving room that plain water does not. Fat-soluble aromatics have somewhere to go.
A milk boil therefore recovers a broader spread of compounds than a short water steep, and holds the sharpest of them in a medium that mutes their edges.
Fat carries aroma water leaves behind
A large share of what we call flavour is aroma, and many aroma molecules dissolve far better in fat than in water. Skim the fat and the smell goes with it.
Whole milk gives those molecules a carrier, which is why milk tea often smells fuller across the room than a black brew of the same leaf.
The same fat slows evaporation, so the aroma releases gradually as the cup cools rather than escaping in the first minute after pouring.
The leaf is bred for the method
Leaf processed by crushing and tearing produces small particles with enormous surface area, which give up colour and strength quickly under a rolling boil.
Whole-leaf processing keeps the leaf structure largely intact, so extraction is slower and more layered, and a hard boil simply overwhelms it.
Households that boil their tea gravitate to the crushed grades for a practical reason: those leaves survive a treatment that would flatten a delicate one.
Plant milks change the outcome
Most plant milks contain little or no casein, so the protein that softened the tannins is absent and astringency returns even though the cup looks the same.
Several also carry starch or added stabilisers, which thicken the body without binding polyphenols, giving a heavier drink that still tastes sharp.
Adjusting for that usually means brewing weaker or shorter rather than pouring more of the substitute, since volume alone does not replace what casein was doing.