Apart from direct consumption as drinking milk, numerous products can be made from it – from butter and yogurt to cheese. They are indispensable from supermarket shelves. But how are these products actually made?
How are dairy products made?
The special thing about milk is that a wide variety of foods can be produced from the same raw milk – usually without many additional ingredients. The crucial factor is how the components of the milk are altered, combined, or separated. This results in different flavors and consistencies (Figure 4). One method is to separate fat and liquid through mechanical action such as stirring. If cream is stirred for a long time, the fat particles stick together and butter is formed. The buttermilk remains as the liquid part. Stirring breaks the protective layer surrounding the fat droplets, causing the fat to clump. Another method is thickening the milk by removing water through heating. This, for example, produces condensed milk. If almost all the water is removed, you get milk powder. To avoid obtaining only a thickened milk product and to also change the flavor, the milk can be acidified. Lactic acid bacteria convert the milk sugar lactose into acid. The milk proteins gradually aggregate, causing the milk to slowly thicken – initially without a clear separation of solid and liquid components. This is how products like yogurt, curdled milk, kefir, or sour cream are made. For many types of cheese, however, the milk is intended to thicken so much that the proteins completely clump together and separate from the liquid part of the milk. This process is called coagulation. The solid part is called curd, and the liquid part is whey. If acidified milk is left to stand for a long time or large amounts of acid are added, such coagulation occurs. However, acid coagulation takes a long time, and the curd and whey taste sour. Therefore, today, rennet (an enzyme mixture) is usually used, which separates the milk faster and makes the cheese milder. In the next section, you will learn in detail how this works. For the rennet to work effectively, slight acidification of the milk by lactic acid bacteria is still necessary. So, today, milk for most types of cheese is first acidified and then separated by rennet. But why do the proteins clump during cheese production?
What role does the protein casein play in the production of dairy products?
To understand why proteins clump in the production of many dairy products, one must look more closely at the milk protein casein. It is not a single protein but a group of several proteins consisting of different parts. The parts have different properties – one part likes to bind with water, the other avoids binding (Figure 5A). When these parts combine in an aqueous environment like milk, they arrange themselves so that the water-loving (hydrophilic) parts face outward and the water-repelling (hydrophobic) parts face inward. This forms tiny particles called micelles (Figure 5A). The outer shell of these micelles is negatively charged, causing the spheres to repel each other – similar to the same poles of a magnet. This keeps them dispersed in the milk, and the milk remains liquid and uniform (Figure 5A). When conditions change – for example, when lactic acid bacteria acidify the milk – the shell loses its negative charge. The micelles no longer repel each other, they stick together, and the milk thickens (Figure 5B). This is how yogurt, which is slightly sour and creamy, is formed. In cheese production, the casein micelles are intended to clump so strongly that they separate from the liquid part of the milk. For this, lactic acid bacteria and the enzyme rennet are used. Enzymes are proteins that have different functions depending on their type. One group is called proteases. These can split proteins by cutting the bonds between the individual building blocks of the proteins. The enzymes contained in rennet belong to this group. They recognize specific structures in casein and cleave it at specific points (Figure 5C). This removes a water-loving part of the protein that normally forms a protective shell on the surface of the micelles. This shell causes the micelles to repel each other and maintain distance. When it disappears, the micelles can more easily touch and connect with each other. They aggregate into large clumps (Figure 5C). This reduces the total surface area of the proteins. Since water is primarily bound to the surface of the proteins, less water can now be held. Some of the water therefore separates from the protein clumps. This creates solid clumps, the curd, and the liquid, called whey, which is then separated. But how does curd turn into cheese?
Do you want to make your own cheese curd at home? Try out how milk reacts with vinegar or kiwi and discover exciting differences!
How does curd turn into cheese?
After coagulation, the curd is cut into small pieces. The smaller the pieces, the more whey can escape, and the firmer the cheese will be later. Then, the curd is pressed to remove more liquid. For hard cheese production, a brine bath follows, which further removes water from the cheese and gives it its typical flavor. Afterward, the cheese is stored for ripening. During this time, bacteria alter the cheese, and for some types of cheese, molds are also added. Different types of cheese with different flavors are created through various lactic acid bacteria, temperatures, and the targeted addition of other bacteria or molds.
Which components of milk are in the cheese and which are in the whey?
During the separation of milk into curd and whey, the components are unevenly distributed into two phases: a solid and a liquid. The solid phase, the curd, is formed by the cross-linking of casein into a kind of matrix. Large portions of milk fat get caught in this network, as well as some minerals. Fat-soluble vitamins are also predominantly enclosed in the curd because they are bound to the milk fat. How much water remains enclosed depends on further processing – this creates different types of cheese. Whey, on the other hand, is the liquid part that drains from this network. It consists mainly of water and primarily contains the substances that cannot be incorporated into the solid matrix. These include, in particular, milk sugar (lactose) and whey proteins, which are too small and too soluble to be retained in the curd. Water-soluble vitamins and some minerals also pass into the liquid. Thus, from the same milk, two very different products are created: a solid, structured cheese and a thin, but still nutritious, whey.
Test your knowledge! Distribute the components of milk into curd and whey. Simply drag the individual components to the correct place.