Why milk is curdled when curd is added to it

The isoelectric point of casein is 4. Since milk's pH is 6. The caseins have a very uneven distribution of charges along the molecule while in the major whey proteins, the charges are more evenly distributed.

This accounts for one of the major properties of the caseins in milk, their amphiphilic nature. Charged regions are, in general, water loving or hydrophilic and the uncharged regions along the molecule are more hydrophobic or water hating.

Since these regions are unevenly spread along the molecule, these regions are more exposed. In proteins with even distributions, the hydrophilic and hydrophobic regions basically cancel each other out.

It is due to the amphiphilic nature of the casein molecules that one of the most important properties of casein emerges, the casein micelle. Most of the caseins in milk are found in intricate loose aggregations called micelles. First calcium phosphate is found in high concentrations in milk and within the casein micelle itself. The concentration is such that small aggregations of the phosphate form. Part of the phosphate is relatively free to pass into and out of the micelle and some of the phosphate is strongly bound to the protein.

This bound phosphate is in tiny aggregations of undissolved colloidal calcium phosphate. These tend to cement the micelle together stabilizing its structure. The other stabilizing effect is due to a property of K-casein. K-casein has within its structure a carbohydrate group which is esterified to threonine in the protein. This carbohydrate group contains some negatively charged groups.

The result is that K-casein is made more hydrophilic at one part of the molecule. In the micelle, K-casein and to some extent Bcasein self-locates as close to, or extending into the water interface of the micelle.

This stabilizes the micelle by preventing aggregation of the micelles by steric repulsion by preventing close approach between casein micelles For K-Casein the peptide bond between the th and th amino acid is vulnerable to hydrolysis by proteolytic enzymes microbial coagulator or rennet.

This is the mechanism for curd formation by removing the carbohydrate containing portion of the protein and thus reducing steric stability. Losing the protection of the K-casein allows the casein micelles to aggregate and form a coherent curd. In order to understand how chymosin coagulates milk, one needs to know something about milk proteins. The majority of milk protein is casein and there are four major types of casein molecules: alpha-s1, alpha-s2, beta and kappa.

The alpha and beta caseins are hydrophobic proteins that are readily precipitated by calcium - the normal calcium concentration in milk is far in excess of that required to precipitate these proteins. However, kappa casein is a distinctly different molecule - it is not calcium-precipitable. As the caseins are secreted, they self-associate into aggregates called micelles in which the alpha and beta caseins are kept from precipitating by their interactions with kappa casein.

In essence, kappa casein normally keeps the majority of milk protein soluble and prevents it from spontaneously coagulating. Chymosin proteolytically cuts and inactivates kappa casein, converting it into para-kappa-casein and a smaller protein called macropeptide. Para-kappa-casein does not have the ability to stabilize the micellar structure and the calcium-insoluble caseins precipitate, forming a curd.

Casein molecules can also be separated from the whey by precipitation of the casein with acid similar to what happens in the stomach when milk is consumed or by disrupting the micellar structure by partial hydrolysis of the protein molecules with a proteolytic enzyme. In the stomach of the young of many species is an enzyme called rennin which specifically hydrolyzes part of the casein micelle resulting in formation of a curd.

This is because coffee and tea contain just enough acidity to tip the pH of milk to the point of curdling. The effect is most often seen in milk that is close to going sour or when adding milk to very hot coffee or tea, since the high temperature can coagulate casein.

The same reaction produces buttermilk, cheese, and yogurt. Adding lemon juice or vinegar to fresh milk is an easy way to make homemade buttermilk.

It would be, if you added the acidic ingredient to hot milk. However, adding acid to cold milk allows casein to coagulate more slowly. Rather than forming clumps, the chemical reaction simply thickens the liquid. The ingredient also affects the flavor of the buttermilk, adding a tangy note.

Yogurt and cheese are slightly more complicated because you usually control the type of bacteria the bacterial culture used to make a product with a pleasing flavor and texture. However, fresh cheeses, such as ricotta, is very simply made by heating milk, adding an acidic ingredient, and straining the curd. 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.

Cleanup Pour all your solutions including the curdled milk into the sink. Wipe down your work area with a wet paper towel. This activity brought to you in partnership with Science Buddies.

Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Discover World-Changing Science. Key concepts Chemistry Food Science Proteins Enzymes Introduction Have you ever poured yourself a cup of milk and instead of a smooth liquid, all you get is clumps?

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. 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. Once it starts boiling, carefully take it out and let it cool down. 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?