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The science of coffee: how physics can perfect your caffeine hit,

A coffee that’s under-extracted is really sour. And a coffee that’s over-extracted is really bitter. But the two things combined give you a sweet flavour, somewhere in the middle.

So, what causes coffee to be over- or under-extracted? According to David, the size of coffee grinds has a big impact. Larger, coarse particles are more permeable, which means the hot water flows more quickly through them. And if the water flows too quickly, you’re likely to get an under-extracted coffee.