Frederic Melton
Cheese is a beloved food product consumed worldwide, with over 1700 varieties produced in the United States alone. It is made through a biological process that transforms milk into a semi-solid, long-lasting product with a distinct flavour and texture. This process involves the addition of bacteria and/or rennet to milk, causing it to coagulate and separate into solid curds and liquid whey. The curds are then processed, salted, and moulded, after which the cheese is left to ripen in a controlled environment. The specific steps and techniques used in cheesemaking vary depending on the type of cheese being produced, resulting in the diverse array of cheeses available today.
How is Cheese Made Biology?
| Characteristics | Values |
|---|---|
| Basic Steps | Adding beneficial bacteria to milk, coagulating the milk into a soft white substance called curd, pressing and cutting curd into the finished cheese shape |
| Milk Composition | 86% water, fat, carbohydrate (mainly lactose), proteins (casein and whey), minerals and vitamins |
| Milk pH | Between 6.6 and 6.7 |
| Coagulation | Milk proteins stick together to form a gel |
| Curdling | Rennet is added to milk to speed up coagulation and produce a stronger curd |
| Rennet Source | Stomachs of young mammals with a milk-based diet |
| Microorganisms | Bacteria, yeast, and filamentous fungi (molds) |
| Role of Microorganisms | Contribute to flavour and texture of the final cheese |
| Processing Steps | Vary depending on the type of cheese |
| Ripening | Cheese is left to age in a controlled environment of temperature and humidity |
| Salt | Added to control acidity, improve texture, and inhibit contaminating microbes |
| Pasteurization | Heating milk to kill bacteria and replaced with standardized starter cultures |
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What You'll Learn
The role of microbes, bacteria, yeasts and moulds
The role of microbes, bacteria, yeasts, and moulds is crucial in the process of cheese-making. Microbes, short for microorganisms, are microscopic living organisms that significantly influence the uniqueness and character of cheese. This includes bacteria, yeasts, and moulds, which work in combination to create the desired outcome.
Bacteria play a vital role in cheese-making, with specific types of bacteria added to milk to initiate the process. For instance, Non-Starter Lactic Acid Bacteria (NSLAB) are naturally present in milk or introduced during cheesemaking. They contribute to the ripening process, and their role in flavour development is still being studied. Another example is Propionibacterium freudenreichii ssp. shermanii, which is responsible for the formation of holes in Swiss cheese by converting lactic acid into carbon dioxide and other acids. Additionally, bacteria like Brevibacterium linens produce distinctive compounds that give surface-ripened cheeses their unique aroma.
Yeasts are also essential in cheese production, especially in artisanal cheeses. They are commonly used in moulded and surface-ripened cheeses and are naturally present in natural rind cheeses. Yeasts contribute to the maturation process and play a role in developing sensory characteristics such as flavour and aroma. Selected yeasts, such as G. Geotrichum, Pichia jadinii, Y. lipolytica, and D. hansenii, have been studied for their impact on flavour development in Raclette cheeses.
Moulds are used in cheese-making to form and consolidate curds, giving the finished cheese its desired shape. In blue cheeses, moulds like Penicillium roqueforti are added during the preliminary steps or introduced through piercing to encourage mould growth by allowing oxygen to reach the cheese. Geotrichum candidum is another mould commonly used in cheese-making, contributing to the "brainy" appearance of some cheeses.
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The coagulation of milk
Milk coagulation is a crucial step in the cheesemaking process. It involves transforming milk into a solid mass by separating it into curds (soft jelly-like substances) and whey (a liquid). The curds are then drained and further processed to release more whey, before being pressed and moulded into blocks of cheese.
Coagulation can be achieved through enzymes (rennets), acid, or heat. Rennet coagulation refers to the addition of enzymes to milk to make it clot. The widely used milk-clotting enzyme is chymosin, which is derived from the stomachs of young mammals with a milk-based diet. It cuts the negatively charged ends on the micelles' surfaces, causing them to stick together and form a three-dimensional matrix that traps milk-fat molecules. Rennet also allows curdling at a lower acidity, which is important for certain types of cheese.
Acid coagulation, on the other hand, can be achieved by adding acid directly or by using starter cultures that produce lactic acid. As the bacteria replicate and culture the milk, the milk's pH lowers, indicating that the bacteria are active and the curd is forming.
Heat also plays a role in coagulation by affecting whey proteins. When heated, whey proteins unravel, exposing "sticky" portions that can bond to each other or to casein proteins.
The choice of coagulation method and specific clotting enzyme is essential for determining the cheese's yield, texture, and flavour. Different enzymes and acids have been used throughout history, including fig latex, wild thistle flowers, and seeds of Carthamus tinctorius and thyme. Today, genetically modified (GM) rennet is the most commonly used coagulant due to its cost-effectiveness.
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Curdling milk with rennet
The process of making cheese involves removing water from milk, which concentrates the milk's protein, fat, and other nutrients, increasing its shelf life. The basic steps in cheesemaking are: adding beneficial bacteria to milk, coagulating the milk into a soft white substance called curd, and pressing and cutting the curd into the finished cheese shape.
Rennet is a mixture containing the active enzyme chymosin, which is found in the stomachs of young mammals with a diet that consists mostly of milk. Calf rennet was the major source of chymosin until the 1960s when it was predicted that the increasing demand for meat and cheese production would lead to a shortage of calves and, therefore, a shortage of rennet. This led to the development of substitutes, including extracts from adult cows and pigs, fungi, and, more recently, microorganisms that have been genetically modified (GM) to yield GM rennet.
The chymosin enzyme in rennet cuts the negatively charged ends on the micelles' surfaces. No longer polar, the micelles are repelled by water and begin sticking together. The micelles form chains, which extend in all directions and interlock into a three-dimensional matrix to trap the milk-fat molecules. The more acidic the milk (the lower the pH), the faster this coagulation occurs and the firmer the curd. Rennet is added to the milk after a starter bacteria, and it speeds up the coagulation of casein and produces a stronger curd. It also allows curdling at a lower acidity, which is important for some types of cheese.
Rennet-formed curds, if done correctly, are just one big structure linked from one end of the pot to the other. They must be physically cut apart. If the curds are drained, the resultant cheese is quite strong because it is linked up with calcium.
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Removing water from milk
Milk is about 86% water, so removing water from milk is a crucial step in cheesemaking. This process involves several steps and concentrates the milk's protein, fat, and other nutrients, increasing its shelf life.
Firstly, bacteria are added to the milk. These bacteria convert lactose into lactic acid, which acts as a preservative, removes water, and develops the cheese's texture. The milk's pH lowers as more lactic acid is produced, indicating that the bacteria are active and replicating. The bacteria also culture the milk, causing it to coagulate and change from a liquid into a firm, rubbery material. This coagulation is facilitated by the casein proteins in milk, which aggregate into spheres called micelles. The outer layer of these micelles is negatively charged, allowing them to remain dispersed in liquid milk. However, when the proteins coagulate, they stick together to form a three-dimensional matrix that traps the milk-fat molecules.
The addition of rennet, a mixture containing the enzyme chymosin, is often used to speed up coagulation and produce a stronger curd. Rennet is derived from the stomachs of young mammals with a milk-based diet. Historically, it was sourced from calf stomachs, but today, it is mostly produced using genetically modified microorganisms as a more cost-effective approach. The chymosin enzyme in rennet cuts the negatively charged ends of the micelles, making them no longer polar. As a result, the micelles are repelled by water and begin to stick together, forming chains that extend in all directions and interlock to trap the milk-fat molecules. The more acidic the milk, the faster the coagulation occurs, and the firmer the resulting curd.
After coagulation, the milk separates into solid curds and liquid whey. The whey is then drained away, and the curds are further processed to release additional trapped whey. The curds are captured, pressed, and moulded into blocks of cheese. Salting is then applied to the curds, which can be achieved through brine (as with Gouda), surface salt (as with feta), or dry salt (as with cheddar). The curds are then shaped and left to ripen in a controlled environment, where microorganisms influence the final flavour and texture of the cheese.
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The ripening process
The duration of the ripening process varies depending on the type of cheese and the desired quality. It can range from a few weeks to several months or even years. For example, Brie typically requires a ripening time of about a month, while sharp cheddar may need a year or more. The speed of the reactions and the overall ripening process are influenced by temperature, humidity, and the moisture content of the cheese.
During ripening, the cheese undergoes three primary reactions: glycolysis, proteolysis, and lipolysis. These reactions are responsible for the development of distinct characteristics in the cheese, such as the blue veins in Roquefort, the holes (or "eyes") in Emmental, and the white mould on Brie. The growth of surface mould and the activity of fungi are carefully controlled during this process to ensure the desired flavour and texture profiles.
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Frequently asked questions
The first step in making cheese is adding beneficial bacteria to milk.
The second step is coagulating the milk into a soft white substance called curd.
The third step is pressing and cutting the curd into the finished cheese shape.
The fourth step is ripening the cheese in a controlled environment.
