We've all seen it; lumpy milk poured from an outdated jug or lemon juice added to warm milk.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.
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; when the pH of their solution changes, however, 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.
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.
Lemon Juice or Vinegar
It's not uncommon for recipes to call for lemon juice or vinegar to be added to milk. Lemon juice and vinegar can be added to milk as a substitute for buttermilk in many recipes. So why does this not cause the milk to curdle? As with many chemical reactions, temperature controls the rate at which the reaction occurs. When adding lemon juice or vinegar to hot milk, it will curdle almost immediately, but adding it to cold milk will not produce a reaction for quite some time.
This is the very same reaction used to create fresh cheeses like ricotta or paneer. Milk is heated to a designated temperature, and then an acid (lemon juice or vinegar) is added. Once the milk curdles, the solid proteins are then strained from the liquid whey and shaped into a round of cheese. In this scenario, curdling has nothing to do with spoilage and is, in fact, very useful.
Coffee or Tea
On occasion, cold milk added to coffee or tea will curdle. This can be alarming as curdled milk is often seen as the same as spoiled milk. In this case, it can be half true. Coffee and tea are both slightly acidic, although usually not enough to curdle fresh milk. When milk is just on the brink of spoilage and bacteria have produced some–but not enough–acid to curdle the cold milk, a little bit of extra acid from the coffee or tea (along with their heat) can tip the scale and cause the milk to curdle. The milk may not be spoiled enough to cause an off odor or flavor; nonetheless, just enough acid and heat in addition to its own can cause curdling.