Why does milk curdle when acid is added?

Question

I know that when you add acid to milk it will curdle but what actually causes the curds to appear?

Answer 1

When the pH of milk is changed, the acidic or basic groups of the proteins will be neutralised. At the pH at which the positive charge on a protein equals exactly the negative charge, the net total charge of the protein is zero. This pH is called the isoelectric point of the protein (pH 4.6 for casein). If an acid is added to milk, or if acid-producing bacteria are allowed to grow in milk, the pH falls. As the pH falls the charge on casein falls and it precipitates. Hence milk curdles as it sours, or the casein precipitates more completely at low pH.
Casein is dispersed in milk in the form of micelles. The micelles are stabilised by the Κ-casein. Caseins are hydrophobic but Κ-casein contains a hydrophilic portion known as the glycomacropeptide and it is this that stabilises the micelles

Answer 2

milk is a colloid to be more specific an emulsion. when acid (ie an electrolyte) is added to it, it coagulates or curdles. It is a property of the colloids.

Answer 3

Becasue acid will denature the proteins present in the milk, causing them to precipitate, meaning, forming the "blobs" we see.

Answer 4

Milk is a base and when you add acid it becomes a neutral.

Answer 5

Butyric acid is formed with acids and butyrate is curd.