First you need to understand what is "curdled milk". This is the condition of product in which the milk is thickened and stratified - separated into a more dense mass and a fluid called serum. There are several factors that can cause this process.First, the milk may curdle because of the process of souring. The composition of any milk there's a special lactic acid bacteria. If the milk chilled, they are in a kind of dormant. When the product is in the temperature close to the room, the bacteria begin to actively proliferate. In this process, the milk changes its properties - texture and taste. Cause souring is usually improper storage. And this is not always the fault of the consumer - if in a factory or store milk for a long time remained at the wrong temperature, it can turn sour very quickly. To prevent such processes in the milk, in addition to storing in the cold, maybe if pasteurization sterilization. In both cases, the milk is heated to a certain temperature. Pasteurized milk can be stored in the fridge for a few days, and sterilized - even a few months, provided that the original package will not be opened.Second, the milk changes its properties and special effects person. For example, in order to obtain the curd or cheese, milk, heated to boiling together with ferment. As a result of chemical processes are accelerated, and milk can curdle in a few minutes. If you continue to cook it, the density of milk will increase and you will be able to get the cheese. After squeezing excess water, it will be the grains of coagulated milk. However, this product will have its own specific taste, and then sought people while cooking.