Have you ever poured yourself a cup of milk and instead of a smooth liquid, all you get is clumps? This is usually a sign that the milk has gone bad. And if it smells sour, it probably has. But the physical process of what happened to the milk is called coagulation, which is the mechanism that occurs when proteins in the milk clump. Although you do not necessarily want this in your milk, without coagulation (or curdling) there would be no cheese or yogurt, which is why it is a very important process in the food industry. Wonder how you can make milk curdle—without having it be spoiled? Try this activity to find out!
Humans have turned milk into a multipurpose liquid. By itself, milk is a nutrient-rich beverage. But when you start treating milk in various processes, all kinds of other products can be created such as butter, yogurt, buttermilk and cheese. Milk mostly consists of fat, protein, lactose (a kind of sugar) and water. The milk fat is suspended in the water as fine droplets, which makes it an emulsion. Milk also contains a lot of proteins that, in this case, are mostly whey and casein. Because casein is poorly soluble in water, its proteins build spherical structures called micelles that allow them to stay in suspension as if they were soluble.
With both fat and proteins in suspension, the milk is a white liquid as we know it. The micelle structures, however, can easily be disrupted or changed, and once altered they cannot be reformed. Because the micelle holds the casein protein in suspension, without it the micelles will clump together and the casein comes out of the solution. The result of this process of milk coagulation, or curdling, is a gelatinous material called curd.
The processes for making many other dairy products such as cottage cheese, ricotta, paneer and cream cheese start with milk curdling. This is why cheese producers want the milk to curdle. There are different ways to start milk coagulation. You can do it with acid or heat as well as by letting the milk age long enough or with specific enzymes (which are proteins that perform a specific chemical reaction). Chymosin, for example, is an enzyme that alters the casein micelle structure to make milk curdle. Proteases are other enzymes that disrupt the casein micelle structure by chopping up proteins, causing milk to curdle. In this activity you will try two different methods of making milk curdle—and produce some cheesy results!
- Lemon (fresh)
- Pineapple (fresh)
- Lemon squeezer
- Food grater, juicer or blender
- Cutting board
- Two pieces of cheesecloth or cotton fabric
- 10 small transparent and microwavable cups (that each hold about two ounces)
- Paper towels
- Permanent marker
- Adult helper
- Workspace that can tolerate spills
- Timer (optional)
- Mark four of the small cups with the labels “pineapple juice,” “pineapple juice (heated),” “lemon juice,” and “lemon juice (heated).”
- Take a fresh pineapple and with the help of an adult cut off the rind on a cutting board. Only use about one fifth of the pineapple. Cut the flesh in smaller pieces and grate it. Alternatively, you can use a juicer or blender. Then, place the grated fruit in a piece of cheesecloth and squeeze at least one teaspoon of juice into each cup that you labeled with “pineapple juice.”
- Put the cup that you labeled “pineapple juice (heated)” into the microwave and heat it just long enough to get it boiling (about 10 to 20 seconds). When it starts to boil, carefully take it out of the microwave and let it cool down.
- Take a fresh lemon and use the lemon squeezer to make lemon juice. Add at least one teaspoon of juice into each cup that you labeled with “lemon juice.”
- Again, put the cup labeled “lemon juice (heated)” into the microwave and heat it for 10 to 20 seconds. Once it starts boiling, carefully take it out and let it cool down.
- Label four of the remaining cups “1” to “4.”Fill each of these cups with about one tablespoon of milk.
- Label the last two cups “curd” and “whey.”
- Place the cup that you labeled “1” in front of you. It should contain one tablespoon of milk. How does the milk look? What happens if you gently swirl the milk in the cup? Do you notice anything unusual?
- Use a clean teaspoon to add one teaspoon of the freshly squeezed lemon juice to your milk in cup 1. Swirl the cup slightly. Does the milk change when you add the lemon juice? If yes, does the change occur immediately or after awhile? When you swirl the cup a little bit, what do you observe at the wall of the cup?
- Take your second cup of milk and this time add one teaspoon of the heated lemon juice. Do you see the same reaction happening as before? How does the milk change? Is the reaction as fast as the previous one?
- Use your third cup of milk, and with a clean teaspoon add one teaspoon of pineapple juice to the milk. Observe what is happening for about five minutes. Does the milk curdle with pineapple juice? Is the reaction fast or slow compared with that of the lemon juice?
- To the fourth cup of milk add a teaspoon of heated pineapple juice and swirl the cup slightly. Again, observe the cup for about five minutes. Do you get a similar result again or is it different? If yes, how is it different? What do you think happened? Can you explain your observations?
- Choose the cup with milk that gave you the greatest amount of curd. Then, place your second cheesecloth over the cup labeled “whey” and carefully pour the curdled milk mixture onto the cloth. Fold the cloth over the curdled milk and squeeze the liquid from the mixture into the cup. Do you see a lot of liquid coming out of the cheesecloth? What does the liquid look like? Is it clear, does it have a color or does it still look like milk? What do you think the liquid is?
- Once you have squeezed out all the liquid, open the cheesecloth and scoop the curd into the cup labeled “curd.” How much curd did you get, and what does it look like? Does it remind you of a cheese product? How does it feel if you touch it with your fingers? What parts of the milk do you think are inside the curd?
- If you want (and only if you used clean materials!), you can taste a little bit of the whey and curd. Does it taste similar to milk? Is it sweet, sour, creamy or salty? Does it remind you of a particular food?
- Extra: What other solutions can make milk curdle? Try out different fruit juices or other edible liquids that you find in your kitchen and test how your milk reacts to these. Remember that acidity (how sour something is) and enzymes are good ways to make milk curdle!
- Extra: How much pineapple or lemon juice is necessary to make milk curdle? To find out you could repeat this activity, but this time change the amount of juice that you add to your milk. Is one drop enough or do you need 10? How many drops does it take to make the milk start to curdle?
- Extra: In this activity you tested one kind of milk. Do you think other milks such as fat-free and lactose-free milk or coconut and almond milk will give you the same or similar results? There is only one way to find out: test it!
- Extra: Do you want more cheesy results? Then use this activity to make some real cottage cheese. An example recipe is given below in the "More to explore" section.
Observations and results
Did you see some nice clump formation in your milk? Whereas regular milk looks smooth and white, it changes very fast when you add a teaspoon of lemon juice. It almost immediately gets thicker in consistency, and you see white clumps forming that stick to the cup wall when you swirl the milk slightly. The clumps, or curd, consist of casein proteins that are usually in solution where they form micelle structures. These structures are very fragile, and when you change the conditions of the solution, they can easily break up and form clumps of casein proteins. This can happen if you change the pH, or acidity, of the milk, which means making it sourer. Lemon juice is very acidic, and that is why you see the casein proteins clumping once you add it. Heating the lemon juice does not affect its acidity, which means when you added heated lemon juice to your milk, the exact same reaction should have occurred.
Pineapple juice, on the other hand, is not acidic enough to break the micelle structure of the casein proteins. Your milk still clumps when you add it, however. This time, it is not the acidity but special enzymes within the pineapple that make the milk curdle. The pineapple contains an enzyme extract called bromelain, which contains a protease enzyme that chops up the casein proteins, destroying their micelle structure. You might have noticed the curdling did not happen as quickly with pineapple juice as with lemon juice—the enzymes need some time to activate—but within five minutes the milk should have looked very clumpy. Many enzymes are deactivated when heated. When you put the pineapple juice in the microwave, the enzymes will not work anymore. This is why no milk curdling occurs when you added the heated pineapple juice.
Filtering out the curd through a cheesecloth results in a whitish-yellow solution called whey, which consists of about 94 percent water and four to five percent lactose and whey proteins. The solid part, the curd, looks like cottage cheese—and it actually is! If you want to make it really tasty, look at the recipe below given in the "More to explore" section.
Pour all your solutions including the curdled milk into the sink. You can discard/recycle your cups or wash them with soapy water if you want to reuse them. Wipe down your work area with a wet paper towel.
More to explore
Milk Chemistry, from the Cheese Science Toolkit
Sculpted Science: Turn Milk into Plastic!, from Scientific American
Cottage Cheese Recipe, from The Weekend Artisan
Making Milk Curdle with Pineapple Enzymes, from Science Buddies
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies