Basil turns black in the refrigerator because its leaves are sensitive to cold temperatures.

The longer version

Basil evolved in warm climates, so its leaves are poorly equipped for cold. Low temperatures damage cell membranes, and the contents leak and oxidize, producing dark pigments in the tissue. The injury often looks worst on the thin areas between veins, where cells are most exposed.

Chilling injury also makes the leaf collapse and lose aroma, since the volatile oils that give the herb its scent are held in fragile surface structures. The same sensitivity explains why many tropical fruits and vegetables develop pitting or dark patches after storage in a cold environment. Warmth and humidity delay the damage considerably.

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.