Milk coagulation

Types of milk clotting: acid clotting - sour clotting, rennet coagulation - sweet clotting, milk coagulated by combining the two - mixed clotting

Milk clotting with rennet - sweet curdling of milk

Imagine proteins swirling in milk, like protein dandelions. These dandelion-like protein clusters repel each other, so they rarely touch each other. With them, milk fat swirls in the liquid in the form of round drops. The size of the fat droplets (proportionally to the dandelion) is the size of a ball. When rennet enzymes are mixed with milk, it acts on protein dandelions like a breeze that separates small umbrella seeds and releases them freely. Spherical micella clusters are separated and the proteins (seeds) that make up them immediately begin to reconnect. However, due to the changed conditions, it is no longer in the original sphere form, but in the form of chains. These micella chains capture milk fat drops, water and other ingredients of the curds in cages. The speed of the process and later the date of the curds cut determine the initial solidity of the curds. The clotting time indicates the characteristics of the curds.

What does the rennet do? 

The casein-type protein is the base of the curds, its frame structure, the carrier of all other cheese materials. Currently, the most accepting construction model is the micella cluster made up of sub-micelles. This protein forms sets of casein micella floating in separate nodules in milk, repulsed by each other, which are made up of elementary (sub) micelles. (pictured is a set of micellas) The clusters are connected by calcium phosphate bridges. The sub-micelles that make up micella clusters are made up of 15-25 casein molecules. Their core has water-repellent properties (hydrophobe), their surface consists primarily of cappa casein, which provides cohesive force. Properties of the cappa casein molecule: calcium soluble protein with a highly hydrophobeproperty.

The first stage of rennet associated curdling of milk:

The rennet enzyme mixed into milk reduces the cohesiveness of cappa casein, causing the casein micella clusters and nodules to disintegrate.

Phase two:

Loss of charge of casein micelles, decrease in size, and decrease in fluid viscosity make casein micelles suitable for creating new chains. These casein bridge chains capturing water and other ingredients of cheese, while clotting is takes place quickly. The speed of construction of the new micella structure determines the structure of the casein chain that builds the "cage", thus its strength and mechanical solidity. Therefore, in rapidly coavated milk, the densely structured grid structure, which is built with many nodes, closes the water more stablely. A firmer curds is formed, which creates a harder cheese. The rate of coagulation depends on the temperature, pH, calcium ion concentration, calcium phosphate, chymosin concentration. The rennetenzyme has a disruptive effect not only on cappa casein, but also on all other milk proteins, which is an enzymatic effect affecting later the long-term ripening of cheese.

Types of rennets

The most ancient and best-yielding enzymes are obtained from calf fourth stomach chamber (abomasum), these are natural enzymes. Their pure chymosin content is 80-95%, with a pepsin and content of 5-15%. Cheaper and longer-lasting enzymes are produced artificially, with the help of bacteria, by fermentation. Enzymes of plant origin can also be used, which are mostly produced by fermentation of Rhizomucor myehei fungus. Many plants alone contain clotting enzymes. If we put enough of such a plant in the milk, then the milk will sleep. E.g.: a bunch of lady’s bedstraw or some fig leaves can put the milk to sleep. They can be used e.g.: in a canvas bag, slightly crushed.

Factors affecting the quality of the cheese curds and the clotting time:

- the quality and type of milk

- Rennets composition, strength, quantity

- inoculation temperature

- pH of milk

- concentration of calcium ions

- amount of calcium phosphate

The clotting enzymes optimal temperature is 36-38 °C. A different inoculation temperatures are used when making different cheeses, usually in the range of 30-40°C.

Sour curdling: (curdled milk, yoghurt kefir, etc.)

When milk reaches the proper level of acidity, then the casein micella clusters disintegrate, and the elementary micelles begin to create a new structure. This point is called an isoelectric point with a pH of 4,6. Milk is biologically acidified, lactic acid bacteria convert lactose into lactic acid. Acidification is associated with a decrease in negative charges in the micelles. Therefore, the hydration layer and electrostatic repulsion are reduced, as well as calcium become more soluble. At this time, the proteins of casein micelles form a network, forming a chain. A relatively loose frame structure, the support structure of the gel, is created. Curdled milk is a gel with a wee strength and elasticity of almost zero due to the low level of the network structure. It is made up of low-energy hydrophobic bonds, which are mechanically weak. Effect of temperature on curds: micella network of milk curded at lower temperatures binds whey better.

Mixed clotting:

Curds prepared by combining sour and rennet associated clotting. The gel made with the renational enzyme shows strong cohesion, elasticity and porosity, it has a high ability to retain whey. Sleeping milk made with sour curdling is at the other extreme, low whey retention capacity, low mechanical solidity. By combining the two, you can create countless different equilibrium states, which is the way to a diversity of soft cheeses and no-reheating cheeses. By the way, if you want to make a cheese matter with a high dry matter content, then you need to perform a number of operations, such as curds cutting, postheating, pressing, salting and drying. The drier the cheese, the more intensively we have to use them. The method of removal of whey and the acidification of cheese matter are closely related to microbial growth and the development of enzymatic biochemical reactions during the maturation of sugars.

Depending on the method of acidification and dewatering, there are four main cheese classes:- very wet lactic acid (fresh cheese); - rather lactic acid (slightly ripe soft mixture); - Rather ripe (from cheese matter pressed without postheating); - hard, ripe pressed cheese matter, solidified by post-heating, then pressed cheese matter.

Milk clotting defects:

If the milk is difficult to clot, the solidity of the curds is inadequate, this is a mild clotting. Milk may clot more difficultly or not at all for the following reasons:

1.) Milk has a low amount of soluble and insoluble minerals. (feeding errors)

2) Milk from a cow with mastitis mixed with healthy cow’s milk. ( 1 cup infected milk can infect all of the milk mixture)

3.) Milk comes from an "old-milked" cow at the end of lactation.

4.) High antibiotic content in milk.

5.) The inoculation temperature is not adequate.

6.) Heat treatment errors (enzymatic clotting capacity of milk deteriorates due to too high temperatures or too long heat treatment)

7.) Milk stirred, stirred and shaken during clotting

8.) Poor clotting ability of the renation enzyme. (rennet enzymes are very durable, stable, but if stored in a warm place for a long time, their strength may weaken.)