Just Add Acid

• If you have ever let milk sit for too long, or allowed it to go bad, you have probably noticed that chunks form in it. Perhaps you have poured milk into a cup of old coffee, watching it curdle instantly. Milk is made up of many components, including fat, protein and water. By adding an acidic substance such as a citric acid like lemon or orange juice to the milk, you can separate these components.
  
  

Stir and Filter

• Pour one cup of milk into a dish or bowl. Add a 2 teaspoons of lemon juice. Gently stir the mixture. The acid reacts with the milk, causing the protein particles to clump together. You should notice the milk begin to curdle and separate into chunks. Continue to stir the mixture until no more chunks are forming. Place a coffee filter over a jar or other container, and pour the mixture into the filter to separate the water from the curds.
  
  

Turn Up the Heat

• At room temperature, you may notice it takes a little while for the curds to separate. Try adding heat to the mixture and see whether the process speeds up at all. Place a sauce pan on a stove top or hot plate. Turn the stove on low to medium low heat and pour the milk into the pan. Add lemon juice to the milk and stir. You will likely see that the components separate quicker than they did at room temperature. Try varying the temperature of the milk and see if there is a difference in how long the process takes with refrigerated, lukewarm, and heated milk.
  
  

Skim or Whole Milk

• You can also explore the different types of milk. Try repeating the experiment with skim milk, whole milk, and 2 and 1 percent milk. Note any differences in how quickly the milk curdles. Also note the size of the curdles. Whole milk will likely have larger curds due to its higher fat and protein content. Skim milk will have smaller curds. You may notice a difference in the appearance and texture of the curds, as well. These curds, which are groups of proteins, can be pressed together and molded. If left to dry over night, it will become a hard plastic. These curds are also the basis for making cheese.