Brewing coffee for longer periods or using hotter water extracts larger molecules responsible for tannic and bitter tastes.

“First of all, there's the grind size makes a huge difference, because what you're essentially doing is extracting extractable materials from the solids. In a typical cup of coffee, you're extracting maybe 20% of the original weight of the coffee. It's not that much, except that it's all the good stuff. And the longer you extract, the more you extract, and the larger the molecules you're able to remove. Those larger molecules are the ones that tend to be tannic and astringent, and bitter.”