Opening the refrigerator to find a gallon of spoiled milk is a rotten way to start the day. But for fresh cheese makers, every day begins with sour milk. Here’s why: 80% of the proteins in milk belong to a family called caseins. Adding acid to milk, like lemon juice or vinegar, makes these invisible proteins visible as a white, chunky solid we call the curds.
In a glass of milk, caseins aggregate into small spheres called micelles. The outside of each protein cluster is negatively charged, causing neighboring spheres to repel each other. Thus, these micelles remain evenly distributed throughout the milk.
Acidic vinegar neutralizes the negative charge on the spheres. With the repulsive force gone, the protein clusters clump together and form an observable solid, the curds. When chefs collect the curds and discard the liquid whey, they have queso fresco. Try it yourself with this recipe.
Stretching the hot curds instead of pressing them into a cake gives you homemade mozzarella cheese. I've tried to make mozzarella using this kit, but it only worked once. That's because the quality of the curds depends on the type of milk that you use.