Curd is milk that is fermented naturally at room temperature, without the addition of a bacterial culture. Heat activates the lactic flora in the milk, producing lactic acid that curdles the milk. Curdled milk separates into two distinct substances - curd and whey. It is stirred or drained before it is eaten.

Although once very popular, curd is now extremely rare because pasteurisation and homogenisation destroy some of the lactic flora required to curdle milk. Sterilized milk and UHT milk cannot be fermented. Since curd is very perishable, it should be stored in the refrigerator and eaten as soon as possible.