Green apples taste tart mainly because they contain more acid and less sugar than ripe ones.

The longer version

Tartness is mostly a measure of acid, and an unripe apple carries a much higher concentration of malic acid than a fully matured one. As the fruit ripens on the tree or in storage, starch converts into sugar and some of the acid is metabolized, so the ratio shifts and the same variety tastes noticeably sweeter.

Firmness follows a similar path, since enzymes gradually loosen the bonds holding cell walls together. Growers select for this balance deliberately, which is why some varieties are picked early for a sharp flavor while others are left to soften and sweeten. Storage temperature controls how fast both changes proceed.

Where this entry sits

Everyday ingredients turn out to be full of quiet chemistry and long history. This collection gathers short, plain explanations of how bread rises, why onions sting, where common spices originate, and how coffee, tea, and chocolate reached the modern kitchen. The full list sits on the Food and Drink page, where every entry is listed in order.