The brewing time of coffee is determined almost entirely by the grind setting, and has almost nothing to do with concentration or brew method. A fine grind requires fast brewing times, a coarse grinds slower ones. This is true whether you are dong espresso, drip or steep; whether you are brewing single strength like regular drip, or octo-strength like modern espresso.

The grinds themselves reach an extraction equilibrium, i.e. it is an asymptotic process. Unfortunately, you can't just let the grinds sit extra long, and let the extraction reach its proper limit. The acidity of the brew also hydrolizes the long chain molecules and leaches very bitter compounds into the cup (think instant coffee taste).

This creates a bit of a gotcha in practical brewing. An under-extracted brew contains the fast dissolving acids and maillard compounds, which have powerful flavors. The caramels and other sweet dessert flavors, that buffer the acids and bitters dissolve more slowly. A fully extracted brew has all the caramels and will be more of a crowd-pleaser than an under-extracted one; although for some coffees, the extra clarity of a slightly under-extracted brew is preferable to afficionados. But let it brew too long, and the crowd pleaser turns into gas station coffee.

One way to get fault tolerant coffee is to use very poorly insulated brewing equipment. If the brew temperature is dropping fast, the leaching won't happen (think of toddy coffee). However, this can also flatten the brew, as is the case with many of the carefully prepared Chemex, Hario and other craft drip coffee I drink.

I've been sniffing my steeping coffees as the time to decant it draws near, since I believe any food prep process is better done with sensory feedback than blind. It may be that the increasingly funky smell of the crust can be used as some kind of readiness indicator; but I'm not sure yet.

endlesscycles wrote:Irr.: The coffee would overextract regardless of time if too much fresh water washed the solids out.

You cannot overextract coffee in neutral PH water, you just extract all the solubles, getting to the par extraction. The evil, penetrating bitter flavors of overextracted brews like instant coffee come from the acids in the coffee breaking down normally insoluble compounds and putting these breakdown products into the brew.

This is why the run out from a lungo espresso tastes so mild, while the 2nd half of an ultra ristretto pull can be quite evil tasting.