How water temperature and grind size interact in pour-over

The same beans, brewed at a different temperature or ground a little coarser, can taste like two entirely different coffees. Temperature and grind are not two knobs to be set independently; they have to be matched to each other before a brew becomes repeatable.

What temperature changes

Hotter water extracts faster, drawing out more acidity and aroma — but past a point it also pulls harsh and astringent flavours along with them. Light roasts usually need the higher end (around 92 to 96°C) to open up fully. Dark roasts give up their solubles much more readily, and at high temperatures they simply turn bitter, so somewhere around 85 to 90°C suits them better.

How grind size fits in

A finer grind means more surface area in contact with the water and therefore faster extraction; a coarser grind does the reverse. Run hot water through a fine grind and the two accelerating factors compound, which is a reliable way to over-extract into bitterness and astringency. Go the other way — cool water through a coarse grind — and you under-extract, leaving something thin and sour.

Deciding what to change

If the cup is bitter and drying, reach first for a coarser grind or a lower temperature. If it is sour and thin, do the opposite: grind finer or raise the temperature. Change one variable at a time, or you will not be able to tell which one actually moved the flavour.