Why Does Milk Curdle When it Is Mixed With Vinegar?

Understanding Your Milk

To understand the curdling process, first you have to have a clear picture of milk's makeup. Like other liquids, most of the milk consists of water. Extremely fine globules of fat are mixed into the water, held together in small droplets by their own surface tension. They rise to the top over time, gathering together and forming cream. The remaining solids dissolved in the milk are mostly proteins. Whey proteins defend the milk against bacteria, bind with nutrients to keep them suspended in the milk, and contain many of the milk's enzymes. Casein proteins play less of a structural role in the milk, simply providing high-quality nutrition. It's the casein molecules that react to acidity.

Looking Closely at Casein

Individual casein molecules are tiny, much tinier than the globules of fat in the milk. Whey protein molecules float freely in the milk, but the four different types of casein molecules cluster in loose bundles, called micelles. The micelles, which look like a microscopic pompom, are held together by calcium atoms. The casein clusters hold much of the calcium in milk, which is why cheeses -- mostly casein -- are a good dietary source of calcium. The casein micelles ordinarily repel each other, but acidity changes that.

A Strange Attraction

One of the casein proteins that makes up each micelle has a negative electrical charge. Because those proteins reach out farther from the group than the others, they tend to repel each other and keep the casein clusters apart. When the milk sours naturally, bacteria produce lactic acid that slowly lowers its pH. When it gets down to a pH of about 4.7, the milk's acidity neutralizes that negative charge and the clusters of casein join together to form curds. When you stir vinegar into your milk, you replicate that process at high speed.

Warming To the Idea

Many recipes specify the milk should be warmed, before you add vinegar -- or alternatively, lemon juice -- to curdle it. That's because milk curdles more quickly if it's heated, and requires less acidity. You might have seen that in your own kitchen, when milk that didn't smell sour suddenly curdled when heated. Warming the milk means you don't need to use as much vinegar or lemon juice, and reduces the likelihood of giving your finished baked goods an unwanted flavor or tang.