Why does milk get curdled when curd is added to it?

Answer 1

First you need to understand what milk is, other than a just a white liquid you get from mammary glands. Milk is an emulsion of fat, water, lactose (a type of sugar) and a mixture of proteins (loads of other stuff, but these are what we will talk about).

The small fat globules are surrounded by a skin of phospholipids and proteins, which are chemicals that help them stay as small globules in the mix, rather than clumping together into a big pat of butter. If you beat the milk, that is exactly what happens - and is how you make butter from milk.

The water in the milk contains soluble proteins, which wander around by themselves in the liquid, and proteins which have one end which likes water, and one end which doesn't. Think of a match - a red head on one end, with a tail of wood trailing behind. In order for the "water hating" ends of the protein (the wood of the match) to stay away from the water they are floating in, these proteins arrange themselves into globes called micelles. Kind of like a circle of cows in a field standing in a protective circle, with all of the tails in the center and a circle of heads looking outwards.

The proteins that do this are called caseins, there are four different types and they make up about 80% of the weight of the total protein in the milk. The outer layer of these micelles is make up of a type of casein known as kappa-casein, and the kappa casein reaches out a bit into the surrounding liquid. Under an electron microscope, each ball looks a bit like a little polystyrene ball (like you get in bean bags and the like), with a bit of a lumpy surface. The kappa-casein has a negative charge, and as like charges repel, all of the micelles stay away from each other - which keeps them in solution.

An acid is any chemical which loves to give (positively charged) protons away, and the stronger the acid, the stronger the tendency to force protons onto other chemicals (which usually breaks them down into simpler structures - this is the iconic "fizzing" you see in Hollywood movies involving acids.)

As you add acid to milk, say by pouring in lemon juice, or by letting bacteria turn the milk sugar lactose into lactic acid, more and more positively charged protons are given to the negatively charged kappa-casein, the kappa-casein loses its charge and so the casein micelles begin to clump together. Eventually the clumps become big enough to see - which are the lumps we call curds, and the process is called curdling. Good if you want to collect the curds to make cheese. Bad if you want a smooth drink or sauce!

If you heat the milk up to at least 85 degrees C before you add the acid, then the kappa-casein reacts with the soluble protein (called a whey protein, because it doesn't mind floating around by itself in the watery whey) b-lacto- globulin. The result is a complex which makes the casein micelle surface markedly coarser, so if you now add acid (usually by allowing a "good tasting" bacteria to turn the lactose into lactic acid) the casein micelles clump into an open spongy gel. This sponge soaks up the liquid, and you end up with Yogurt. Yogurt is much more stable in cooking and accepting acid for this reason.

Answer 2

Bacteria in the milk produce acid. This lowers the pH of the milk and disrupts the structure of the proteins in. Because some proteins are hydrophilic (soluble in water) and others are hydrophobic (insoluble in water), the proteins aggregate into clusters where the hydrophobic proteins are found in the core, and the hydrophilic ones are found on the surface. These clusters are called curds.

Answer 3

I was at a Spanish place tonight that had some sort of concoction on display. I asked about it and they told me I should try it. It was very sweet and I asked about it.

They said it was a combination of lemon, milk and sugar. It was lumpy and I figured the lumps must be curds. I don't know if this is what you are looking for. I'm just trying to find out exactly how it's made....

Answer 4

Curdling is a process by which a liquid is transformed to a soft semisolid or solid mass. As the pH of the milk drops and becomes more acidic, the protein, called casein, molecules attract one another and become curdles floating in a solution of translucent whey.

Answer 5

Actually the curd contains bacteria(s) which has the ability to convert milk into curd. So, while adding curd into boiled milk, it becomes curd after sometime(milk must be in less temperature while adding curd, so that the bacterias can breed into multiples).

Answer 6

Proteins in the milk are denaturing or losing their shape and clumping together.

Answer 7

BACTERIA

Answer 8

By shaking it

Answer 9

randomly