Clabbered milk is milk which has been allowed to naturally sour, becoming thick, tangy, and very rich. It is often sold in grocery stores, where it is called buttermilk. It is among a family of cultured dairy products which have been consumed for centuries, and in many regions of the world, this sour milk is a very popular drink and cooking ingredient. Many stores sell a pasteurized version, and it is also possible to make this ingredient at home.
The word "clabber" comes from the Irish language, and it means "to thicken." If clabbered milk is allowed to thicken long enough, it becomes clotted cream, a popular spread from scones in many parts of the United Kingdom and Ireland. This cultured milk can also be drunk straight, plain or flavored, and it is especially popular in the American South over ice. Because clabbered milk is more shelf-stable than regular milk, it is often the milk product of choice in areas with spotty or no refrigeration, and for much of the world, fresh milk is a relatively recent delicacy.
Traditionally, clabbered milk is made by allowing raw milk to stand until it has thickened, a process which takes 24-48 hours. The milk is also typically kept warm, encouraging the growth of beneficial bacteria. As it thickens, the acidity of the milk increases, preventing the growth of harmful bacteria and creating a very distinctive tang which many people greatly enjoy. Once the milk has clabbered, it can be refrigerated and then used in an assortment of recipes; it will act as a rising agent, making baked goods lighter and fluffier.
Modern clabbered milk is often made with milk which has been pasteurized. Although this milk will be safer to drink, it may be lacking in beneficial bacteria, because the pasteurization has killed off much of the good bacteria in the milk. This version is often available for sale in stores. You can use it to make your own clabbered milk at home, introducing a small amount to a clean jar as a starter culture, adding pasteurized milk, and then allowing it to ferment.
You can also make a substitute for this ingredient by adding an acid such as vinegar or lemon juice directly to either raw or pasteurized milk, in a ratio of one and one half teaspoons to every cup. While this version will not be as rich and flavorful, it will have the tangy flavor and slightly curdled texture of traditional clabbered milk, and it clabbers much more quickly, which can be handy in a pinch.