Milk Curdling

We've all seen it, lumpy milk poured from an outdated jug or lemon juice added to warm milk. What was once smooth, creamy milk becomes chunky, lumpy, and completely unappetizing. But curdled milk isn't always a bad thing. Although it sometimes signifies spoilage, it can also be a method of producing more delicious food, like cheese. Milk curdles because of a simple chemical reaction that can be set into place for a variety of reasons. Let's take a closer look at some of those reasons.
What is Curdled Milk?
Milk is comprised of several compounds, primarily fat, protein, and sugar. The protein in milk is normally suspended in a colloidal solution, which means that the small protein molecules float around freely and independently. These floating protein molecules refract light and give milk its white appearance. Normally these protein molecules repel each other, allowing them to float about without clumping, but when the pH of their solution changes, they can suddenly attract one another and form clumps. This is exactly what happens when milk curdles. As the pH drops and becomes more acidic, the protein (casein) molecules attract one another and become "curdles" floating in a solution of translucent whey. This clumping reaction happens more swiftly at warmer temperatures than it does at cold temperatures.
Spoiled Milk
All milk, even pasteurized milk, contains bacteria. As bacteria go about happily with their lives, they eat the natural sugars in milk, called lactose. As they digest lactose, a number of byproducts are created, including lactic acid. When the amount of lactic acid in the milk begins to increase, the pH drops and the casein molecules begin to clump. The high levels of lactic acid are also what give spoiled milk its characteristically sour smell.
Milk and Lemon Juice or Vinegar
It's not uncommon for recipes to call for lemon juice or vinegar to be added to milk. In fact, lemon juice and vinegar can be added to milk as a substitute for buttermilk in