Whey or milk plasma is the liquid remaining after milk has been curdled and strained; it is a by-product of the manufacture of cheese or casein and has several commercial uses. Sweet whey is manufactured during the making of rennet types of hard cheese like Cheddar or Swiss cheese. Acid whey (also known as sour whey) is obtained during the making of acid types of cheese such as cottage cheese. Historically, whey used to be a popular drink in inns and coffee houses. Whey is used in a wide variety of food industries as it has excellent nutritional properties. Whey is also used as a 'functional' food in health, slimming and clinical foods.