Why does vinegar and milk curdle?

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 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.

Answer 2

curdling of milk occurs in different ways with the help of chemistry let's try to understand. The methods of curdling:

ACIDIFICATION:

It may occur thanks to milk ferments that transform lactose in to milk acid, taking the milk to 4.6 pH, or for the addition of acid substances;

RENNET: it is, generally, the sweet curdling that occurs thanks to enzymes. The substance that in the milk gets the transformation is casein, a fundamental protein that in the milk is found in a colloidal solution. The casein is a conjugate protein formed (by beside C, H, O, N, S) also phosphorus (P) in the form of ester phosphoric acid of which a part is found inside every unit of a,b,k- "salificato" casein with calcium and magnesium ions, a part is in the form of tricalcium phospote (apatite) that joins together the units of a, b, k- casein as in the picture. The whole casein "micella" is called "phospho-caseinatum of calcium". The quantity of calcium ions regulates the aggregation's state of "micelle" and the sped of their flocculation: without calcium casein can't coagulate.

ACID CURDLING

In normal conditions milk has a pH of about 6.5-6.7 and at this pH value the casein is without protons, it has a negative charge and therefore the casein micelles are relatively soluble, because they repel each others. In an acid ambient milk coagulates, because casein has its isoelectric point at 4.6 pH, that is at this pH value it has a quantity of positive charges equal to the quantity of negative charges and the positive part of each "micelle" is attracted by the negative part of the others, causing the formation of ionic bonds among the "micelle" working against the dipole-dipole bonds with water, so that the protein precipitates in the form of demineralized casein and in the solution remain soluble calcium salts.

TRADITIONAL CURDLING OF RENNET

The rennet (see above) is a substance extracted by the abomasus of the suckling calf and contains natural "proteolitici" enzymes that change the structure of casein, that "qelifies" with calcium ions whose presence is guaranteed the acidification of milk to use. The fundamental enzyme is the "chimasi" that splits a particolar peptiolic bond, between methionine and phenylanine that is found in the k-casein and that keeps a hydrophyle carbohydrate's fragment. After the removal of this fragmet, the regulation factor in the watery solution is missing and the "micelle", made unstable, in the presence calcium ions, tend to join, forming the curd called "calcium para- caseinatum". Unlike the curd obtained by acidification, this keeps the most part of tricalcium phospate.

Answer 3

Milk curdles due to an increase in acidity, or PH. Casein are naturally negatively charged and repel one another, and evenly dispersing the casein. However, when the acidity is increased by products such as lemons, that charge becomes neutral and the casein clump together.

Answer 4

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 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 electrom 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) proteins away, and the stronger the acid, the stronger the tendency to force proteins 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 proteins 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.

See some microscope pictures here: http://www.magma.ca/~pavel/science/Yogurt.htm

Answer 5

Vinegar is a dilute acetic acid.

In normal conditions milk has a pH of about 6.5-6.7 and at this pH value the casein is without protons, it has a negative charge and therefore the casein micelles are relatively soluble, because they repel each others. In an acid ambient milk coagulates, because casein has its isoelectric point at 4.6 pH, that is at this pH value it has a quantity of positive charges equal to the quantity of negative charges and the positive part of each "micelle" is attracted by the negative part of the others, causing the formation of ionic bonds among the "micelle" working against the dipole-dipole bonds with water, so that the protein precipitates in the form of demineralized casein and in the solution remain soluble calcium salts.

Answer 6

There are a couple of reasons why milk can curdle. It obviously curdles when it sours and goes bad. Milk and egg products can also curdle when they are added to a hot pot and the temperature is too high compared to the temperature of the milk, sour cream, etc. So if you add sour cream or yogurt to a dish, for example, you want to wait until all the cooking has completed, take the dish off the burner, let it cool down towards room temperature, and then add the yogurt, sour cream or milk.

Answer 7

high temperatures, acids, tannins, enzymes, and salts

Answer 8

Milk curdles when heated due to the protein's denaturing and coagulating.

Answer 9

Some Yeast and aspergillus is required.