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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 cumping 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 arange 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 centre 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 made 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, becasue 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 gell. This sponge soaks up the liquid, and you end up with Yoghurt. Yoghurt is much more stable in cooking and acepting acid for this reason.

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โˆ™ 2011-09-13 23:57:40
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Q: Why does milk curdle?
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Related questions

Is it camels milk that does not curdle?

Yes, Camels milk does not curdle


What makes milk curdle besides acidity and temperature?

Bacteria in the milk can also cause it to curdle.


What kind of milk does not curdle?

breast milk


How do you curdle milk?

By shaking it


Does milk curdle in stomach?

milk doesn't curdle in your stomach because it has minerals and vitamins which your body really need


What milk does not curdle?

Camel's milk it the kind of milk that does not curdle, at least not on its own. Ice cream made from camel's milk will stay good for up to 1.5 years.


Why does milk curdle when ascorbic acid or fruit juice is added?

yes it does. anything that contains acid will make the milk curdle


Will plums curdle milk?

My best guess would be yes because plums are slightly acidic and milk will curdle with acid.


Why does coconut milk curdle when heating?

no


Does tabasco sauce curdle milk?

Tabasco contains quite a bit of vinegar so yes it will curdle milk due to the vinegar.


What is the best way to curdle milk?

One of the best ways to curdle milk is to put lemon juice in it. Or, you put it on the stove and allow it to boil.


Does lactose free milk curdle?

Yes. The lactose is generally replaced by fructose which is still a sugar. See 'Why does milk curdle?' - same reasons.


What is the substance used to curdle milk?

rennet


Can freezing goat's milk cause it to curdle?

no.


Which animal's milk will never curdle?

Camel


What type of milk does not curdle?

camel milk, although it takes longer surely does "curdle" It is a stage that is necessary for the making of cheese, which is one of the best cheeses made..


Why does vinegar separate from milk?

Vinegar makes milk curdle and the nature of vinegar is to not mix.


Why does acetic acid cause milk to curdle?

it wil make milk rouit faster?


Will cold milk curdle in hot water?

No. the water and milk may seperate however


Which animals milk is the only milk that doesnt curdle when boiled?

most probably a camel


Can you get sick by drinking lemon and milk together?

No, but the milk will curdle and it will not taste very good


Why does vinegar separate milk?

Vinegar makes milk curdle and the nature of vinegar is to not mix.


Why does milk not curdle when microwaved?

Good quality milk will not curdle when boiled. It can be used as a test to determine how heat stable the milk is before heat treatment. Ie, if it curdles, don't use it as it would foul the equipment.


Does every milk curdle in the same period of time?

no it all depends because you could freeze it and then it wont curdle for awhile


What does milk turn into if its gone off?

It will curdle and go sour