Water of any temperature will extract
coffee, but hot water extracts faster than cold. Never pour boiling water
on coffee, even on instant, as it brings out a harsh flavor. The optimum
temperature range for coffee preparation is between 92-96°C/197-205°F.
Conversely, if cooler water is used, the brew will be under-extracted.
When coffee is intentionally boiled, as with Turkish brewing, sweetening
offsets any bitterness.
When ground coffee and water are combined, a certain amount of time is
needed for the water to saturate the coffee grounds and extract the
various soluble solids, some of which take longer than others to pass into
the water. For the first few minutes of the brewing cycle, the "blend" of
the flavor compounds in the liquid is changing continuously. If the
contact time is to be limited, the particles must be finer for the water
to penetrate them and extract the flavor compounds. If the contact time is
very long, the grounds should be coarser so as to slow the rate of
extraction. In certain methods of brewing, such as those in which cold
water drips very slowly through coffee grounds for several hours, the
resulting brew will be extremely bitter, which is to be expected with such
over-extraction, but will be offset later by dilution and sweetening.