Frederic Melton
Cheese is a dairy product made from the milk of cows, goats, sheep, or water buffalo. The process of making cheese involves curdling milk and separating the solid curds from the liquid whey. This is done through the process of fermentation, where bacteria feed on milk sugars like lactose and convert them into lactic acid, causing the milk to curdle and solidify. The word cheese itself is derived from the proto-Indo-European root kwat-, which means to ferment or become sour. Different types of bacteria and moulds are used in cheese production to create a wide range of flavours, textures, and forms.
Characteristics and Values of Cheese Fermentation
| Characteristics | Values |
|---|---|
| Definition | Cheese is a type of dairy product with a range of flavors, textures, and forms produced by coagulation of the milk protein casein. |
| Main Ingredient | Milk, usually from cows, goats, sheep, or water buffalo. |
| Other Ingredients | Herbs, spices, hot and sweet peppers, horseradish, and port wine. |
| Basic Steps | Milk is acidified, enzymes or bacterial enzymes are added to cause coagulation, solid curds are separated from liquid whey, and pressed into cheese. |
| Starter Cultures | Groups of bacteria specifically chosen for cheese-making to ensure reliable and consistent acid production. |
| Bacteria Types | Homofermentative (produce only lactic acid) and heterofermentative (produce lactic acid and other compounds like carbon dioxide). |
| Fermentation Temperature | Varies depending on cheese type, e.g., 29-31°C for Cheddar, Stilton, Leicester, and Wensleydale. |
| Role of Bacteria | Convert milk sugar lactose into lactic acid, contributing to flavor, shelf life, and safety of cheese. |
| Curd Handling | Varied based on cheese type; some require curd washing to adjust moisture content and reduce acidity. |
| Salt Addition | Salt is added to some cheeses, like Gouda (soaked in brine) and Feta (surface salt). |
| Ripening | Allows bacteria to grow and continue fermentation, developing flavor and texture. |
| Phages | A concern in cheese-making, as they can infect starter culture bacteria and cause financial losses. |
| Pasteurization | A process to rid milk of pathogens, commonly used in mass cheese production. |
| Cheese Yield | Typically around 10% of the milk used. |
| Classification | Based on moisture content, fat content, curing methods, and production methods. |
| Diversity | Over a thousand types of cheese exist globally. |
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What You'll Learn
Milk is warmed to increase the rate of fermentation
To create cheese, cheesemakers generate conditions that cause milk proteins to coalesce and form a semi-solid. This is achieved by lowering the pH of milk through the process of fermentation, which leads to the coagulation of milk proteins and the formation of a gel. The milk sugar lactose is broken down into lactic acid by a group of bacteria called lactic acid bacteria (LAB).
In traditional cheesemaking, naturally occurring LAB in milk was used, but modern practices often involve inoculating milk with industrial starter cultures of specifically chosen bacteria to ensure consistent acid production. To increase the rate of fermentation, the milk is warmed to the optimal growth temperature of the microbes in the starter culture. This warming process accelerates the conversion of lactose to lactic acid, which is crucial in forming the curds and contributing to the removal of water held within the milk proteins.
The rate at which lactic acid is produced significantly affects the initial chemical characteristics of the cheese, which in turn influences the complex microbial populations that develop during the ripening process. Therefore, warming the milk is an important step in controlling the fermentation process and ultimately influences the characteristics of the final cheese product.
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Citrate fermentation to diacetyl is required for some cheese varieties
The process of cheese-making involves the conversion of milk into a solid food product through fermentation, draining, and pressing. This process is facilitated by microorganisms, specifically bacteria, which transform the milk's proteins and sugars.
One particular type of fermentation that is required for some cheese varieties is citrate fermentation to diacetyl. Diacetyl, or 2,3-butanedione, is a compound that occurs naturally in some fermented foods and beverages, and it is responsible for the characteristic buttery flavor and aroma associated with certain cheeses. While diacetyl can be added artificially to enhance flavor, it is also a natural byproduct of the citrate fermentation process.
Citrate fermentation to diacetyl is particularly important in the production of Dutch-type cheeses, such as Gouda, and blue-veined cheeses. In these varieties, the production of diacetyl through citrate metabolism is prominent, resulting in a distinct flavor profile. Additionally, the carbon dioxide produced during this process contributes to the small eyes in Dutch cheese and the open texture that facilitates mould growth in blue-veined cheeses.
The concentration of diacetyl in cheese is influenced by several factors, including the type of bacteria, temperature, pH level, and milk composition. Lactococcus lactis subsp. diacetylactis and Leuconostoc cremoris are specific bacteria species involved in citrate fermentation, playing a crucial role in determining the final diacetyl concentration. Furthermore, cheeses with shorter aging periods tend to exhibit higher diacetyl levels, as the compound has less time to degrade into other flavor components.
Overall, citrate fermentation to diacetyl is a crucial aspect of cheese-making for certain varieties, contributing to their unique flavor, texture, and characteristics.
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Starter cultures are added to milk to assist with coagulation
Milk coagulation is the first step in cheesemaking, transforming liquid milk into solid curd. This step involves the addition of starter cultures, which are groups of bacteria specifically chosen for use in cheese-making. These bacteria are crucial to the fermentation process, as they ensure reliable and consistent acid production.
The bacteria in starter cultures can be mesophilic, such as strains of Lactococcus lactis, or thermophilic, such as Lactobacillus helveticus and Strep salivarius subsp. Thermophilic. The type of bacteria used depends on the specific cheese being produced and the desired incubation temperature. For example, mesophilic starters are used in cheeses like Cheddar, while thermophilic starters are used in cheeses like Emmental and Parmesan, which require higher incubation temperatures.
The addition of starter cultures also introduces enzymes that aid in coagulation. These enzymes, such as rennet, act on the casein proteins in milk, causing them to aggregate and form the structure of the cheese. The enzymes clip off the κ-casein hairs, allowing the casein micelles to stick together and form a solid mass. This aggregation is crucial in transforming milk into cheese.
Furthermore, the use of starter cultures helps standardize the cheese-making process. By selecting specific bacteria and controlling their population densities, cheese makers can ensure consistent acidification and fermentation rates. This standardization results in more predictable outcomes, improved quality, and the desired characteristics of the final cheese product.
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Rennet is added to cause the casein to precipitate
Cheese is a dairy product that is produced through the coagulation of milk protein casein. It is made up of proteins and fats from milk, usually from cows, buffalo, goats, or sheep. The process of making cheese involves acidifying the milk and then adding rennet, which contains the proteolytic enzyme rennin, to cause the casein to coagulate.
Rennet is a mixture of enzymes that includes chymosin and pepsin. Chymosin is a protease that is responsible for milk curdling as it causes coagulation of casein. It does so by cleaving the Phe105-Met106 bond in κ-casein, which is responsible for preventing milk proteins from aggregating. This cleavage results in the conversion of κ-casein into para-κ-casein and other macropeptides, leading to the precipitation of calcium-insoluble caseins and the formation of a curd.
The addition of rennet causes the casein molecules to fuse with each other, forming a network that coagulates the milk. This network traps the milk fat, resulting in a consolidated curd of milk solids. The solids are then separated and pressed into the final form of cheese.
The use of rennet in cheese-making has some variations. Traditionally, rennet was obtained from the stomachs of young ruminant animals like calves, lambs, and kids. However, in modern times, it is more commonly produced from genetically modified microorganisms or through the fermentation of the fungus Mucor miehei for vegetarian cheese. The retention of rennet enzymes in the final cheese product depends on factors such as the pH of the milk, the type and amount of rennet used, and its ability to withstand cooking temperatures.
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Fermentation by bacteria, added yeasts, or moulds
The process of cheese-making involves a number of steps, including the fermentation of milk by bacteria, added yeasts, or moulds. This process transforms the milk into a solid food with a longer shelf life than milk. The ancient Greeks and Romans were already making cheese, and the word "cheese" itself comes from the proto-Indo-European root "*kwat-*", meaning "to ferment or become sour".
Cheese is made from the milk of various animals, including cows, goats, sheep, and water buffalo. The milk is usually acidified, and enzymes or bacterial enzymes are added to cause the casein to coagulate. This process is called curdling, and the solid curds are then separated from the liquid whey. The curds are then cut into small pieces and heated, which further separates the whey from the curds. The curds are then drained, salted, and formed into mats, which are cut into sections and piled on top of each other. This step, called cheddaring, helps to expel more whey and allows the fermentation to continue until a certain pH level is reached.
The starter organisms or cultures used in most cheese-making processes are crucial and complex. They are typically referred to as mesophilic starters, strains of Lactococcus lactis and its subspecies. Their main function is to ferment the milk sugar lactose into lactic acid, decreasing the pH and contributing to the shelf life, safety, and flavour of the cheese. In the past, traditional cheese-makers relied on naturally occurring lactic acid bacteria (LAB) in milk, but today, it is common to inoculate milk with industrial starter cultures, which are specifically chosen for cheese-making to ensure reliable and consistent acid production.
The type of starter culture used depends on the variety of cheese being made and the production process. For example, thermophilic starters such as Lactobacillus helveticus and Strep salivarius subsp. thermophilic are used in the production of cheeses like Emmental and Parmesan, where a higher incubation temperature is employed. Other starter cultures, such as Leuconostoc cremoris, are used in some cheese varieties like Gouda or blue-veined cheeses to produce carbon dioxide, which creates the small eyes in Dutch cheese or an open texture that facilitates mould growth.
After the whey is removed, the curds undergo further ripening and fermentation by bacteria, added yeasts, or moulds, and enzymatic reactions from added lipases or rennet. This ripening process develops the distinctive characteristics of each cheese, including flavour and texture. The curds are then pressed into finished cheese, and some cheeses have aromatic moulds on the rind, the outer layer, or throughout. The diversity of cheese-making processes and the use of different types of milk result in the wide range of cheeses available today, with over a thousand types of cheese produced worldwide.
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Frequently asked questions
Fermentation is used to convert milk sugar lactose into lactic acid. This process contributes to the formation of curds and the removal of water from milk proteins.
Starter cultures are groups of bacteria specifically chosen for cheese-making. They are used to ensure reliable and consistent acid production.
Artisanal cheese-makers use raw milk and adjust their techniques during production to accommodate changes in milk composition and fermentation rates. In contrast, variations in milk composition are minimised in mass production, and standardised milk and starter activity are used.
Milk is acidified and either rennet or bacterial enzymes are added to cause the milk protein casein to coagulate. The solid curds are then separated from the liquid whey and pressed into cheese. The curds are then ripened through further fermentation, developing the cheese's flavour and texture.
The bacteria used in cheese-making are called lactic acid bacteria (LAB). Some common types include Lactococcus lactis subsp. diacetylactis and Leuconostoc cremoris.
