Extraction: The Secret to Making Mind-Blowing Coffee
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"The very coarse grind should yield an under-extracted brew. The grounds are so coarse that the water won’t be able to extract enough good-tasting coffee stuff from them, resulting in a brew that’s sour and grassy, with a sensation reminiscent of underripe fruit. (If you’re using a French press and want to get the full, exaggerated effect of under-extracted coffee, try brewing the very coarse-ground coffee for half the usual time. The coffee will be totally gross.)"
The very fine grind should deliver an over-extracted cup. Because the grounds are so fine, the water will have no trouble extracting way too much caffeine and other pungent compounds. The result will be bitter and astringent. (If you’re using a French press and want to get the full, exaggerated effect of over-extracted coffee, try brewing the very fine-ground coffee for twice the usual time. The coffee will be totally gross.)
If your in-between grind is on target, the coffee will be properly extracted. That is, it won’t be sour and it won’t be bitter. If your brew went really well, it will have a natural sweetness.
Source: Extraction: The Secret to Making Mind-Blowing Coffee | FWx