An egg is probably the best analogy. Undercook it and you have a gag-inducing, cloying mess on the plate. Overcook it and the dryness will transcend it from joyful, rich, subtle food to mere nutrition.
The coffee bean is similar. You can underbrew and you can overbrew but when you hit the sweet spot with decent coffee beans, properly roasted, you will be rewarded with rich, intense complexity. Sweetness and acidity will present themselves, and a full mouth experience is the result. This is regardless of coffee preparation method or technique. We call this proper extraction.
Under-extraction leaves you at the sour end of the flavour spectrum, your coffee watery and unfulfilling. It’s the result of the grind being too coarse, the coffee in contact with the water for too brief a time period or the temperature too low.
Over extraction is the opposite. Too fine a grind, too much contact time between the water and the coffee, or water that is too hot mean bitterness and a kind of dirtiness are your new companions.
Perhaps the easiest way to understand things is to think of the good old coffee plunger. If you used tepid water, you’d expect under-developed flavours? Correct? Well, maybe. Unless. Unless you brewed for longer. You see, all of the factors – grind, time and temperature – are interlinked and play off against each other. Decrease the water temperature and you can compensate with a longer extraction time.
To gain real insight into extraction, I suggest you actually do the following experiment. Use the usual amount of water and coffee (60g of coffee per litre of water is the gold standard). Leave the cold water on the coffee for 18 to 24 hours and then plunge! Try it. Seriously. I promise the sweetest extraction ever!
As an aside, it is also worthwhile to add a little water your coffee in the plunger, stirring to a paste, leaving it to bloom before topping up (I use cold water to do this even with hot extraction!).
The idea I’d most like coffee drinks to embrace is just not to over-extract for best flavours. Stop extraction before the bitterness develops. Water over 90⁰C in long contact periods (over 30 seconds) will always over-extract! In a plunger with water at 80⁰C, great coffee takes about three minutes out of interest. Flavour not bitterness is the goal. In this, the second age of Tim Noakes, who the hell needs sugar anyhow?