Bushra Griffith
Milk curdling is an essential step in the production of cheese. It involves breaking up an emulsion or colloid into large parts of different compositions through physio-chemical processes like flocculation, creaming, and coalescence. Curdling can occur naturally if cow's milk is left open in a warm environment for a few days, but cheesemakers control curdling by adding coagulants such as rennet, acids (e.g. lemon juice or vinegar), or salts. The type of coagulant and the proportion of acid to rennet used depend on the desired characteristics of the final cheese product. For example, softer, smaller, and fresher cheeses are curdled with a greater proportion of acid to rennet than harder, larger, and longer-aged varieties.
Characteristics and Values of Curdling Milk for Cheese Manufacture
| Characteristics | Values |
|---|---|
| Purpose | To make cheese and tofu |
| Process | Milk is coagulated to transform it into a solid mass or "curd" |
| Coagulation Methods | Enzyme action, acid addition, or acid/heat addition |
| Enzymes | Rennet, rennin, pepsin, chymosin |
| Acids | Lemon juice, vinegar, lactic acid |
| Salts | Magnesium chloride, calcium chloride, gypsum |
| Bacteria | Lactic acid bacteria, thermophiles, mesophiles |
| Function of Bacteria | Convert lactose into lactic acid, remove water from cheese curd, develop cheese texture and flavour |
| Additional Functions of Bacteria | Preservation, flavour development, prevention of bad bacteria growth |
| Curd Formation | Milk proteins (casein) tangle into solid masses or curds |
| Curd Texture | Semi-solid gel or mesh-like structure |
| Whey | Liquid by-product of curdling, contains whey proteins |
| Ripening | Aging process that brings about chemical changes to transform fresh curd into aged cheese |
| Ripening Time | Varies from one month to a year or more, depending on the type of cheese |
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What You'll Learn
Curdling milk is the first step in making cheese
Milk curdling is a crucial step in the process of making cheese. Curdling is the process of breaking an emulsion or colloid into large parts of different compositions through physio-chemical processes. In the context of cheese-making, curdling refers to the transformation of liquid milk into a solid mass or curd. This solid mass is often called a "curd", "gel", or "coagulum".
The curdling process can be initiated by adding enzymes (typically rennet), acids (such as lemon juice or vinegar), or various salts (magnesium chloride, calcium chloride) to the milk. These additives cause the milk proteins (casein) to tangle into solid masses or curds, while the remaining liquid, which contains only whey proteins, is called whey. The curds are then pressed and drained to varying degrees, depending on the desired style of cheese.
The process of curdling milk can also occur naturally. For example, some nursing animals like calves, piglets, or lambs produce the rennin enzyme in their stomachs to help them digest their mother's milk. Additionally, if cows' milk is left open in a warm environment to air for a few days, it will naturally curdle.
The type of curdling agent used depends on the variety of cheese being produced. For example, cheeses such as cheddar, gouda, and queso fresco are produced through rennet coagulation, while cottage cheese, quark, and chèvre are examples of cheeses made through acid coagulation.
The curdling process is essential to the cheese-making procedure as it helps to concentrate the protein and fat in milk, which can then be dried, aged, and packaged as a cheese product. Curdling also plays a role in removing water from the cheese, which increases its storage life.
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Curdling agents include enzymes, acids, and salts
Milk can be curdled using enzymes, acids, or salts. Curdling is the process of breaking an emulsion or colloid into large parts of different compositions through the physio-chemical processes of flocculation, creaming, and coalescence.
Enzymes are proteins that perform a specific chemical reaction. The enzyme rennet, derived from the stomach of calves, is commonly used to curdle milk. It contains the enzyme chymosin, which alters the casein micelle structure, causing milk to curdle. Other enzymes that can be used include proteases, which chop up proteins, causing milk to curdle.
Acids can also be used to curdle milk. This process involves lowering the pH of the milk, causing milk proteins like casein to unwind and unfold, allowing them to interact and clump together. Common acids used include lemon juice, vinegar, and fruit juices like pineapple juice.
Salts such as magnesium chloride, calcium chloride, or gypsum can also be used as curdling agents.
The choice of curdling agent and the specific process depend on the type of cheese being made and the desired characteristics, such as texture and flavour.
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Milk curdles naturally if left in a warm environment
Milk curdling is a common phenomenon that occurs when milk is heated or left open in a warm environment for a few days. Curdling is the process of breaking an emulsion or colloid into large parts of different compositions through the physio-chemical processes of flocculation, creaming, and coalescence. During curdling, the pH of the milk decreases, and it becomes more acidic. This causes independently floating casein molecules to attract one another, forming "curdles" that float in a translucent whey. At warmer temperatures, the clumping reaction occurs more quickly than at colder temperatures.
Milk curdling is undesirable when making sauces or custards but is an essential step in the production of cheese and tofu. In cheese-making, the curds are pressed and dried to form the final cheese product. The curds can also be allowed to stick together, forming a mat.
Milk can be curdled by adding an acidic ingredient, such as lemon juice, orange juice, or vinegar, to hot milk. Milk can also be curdled using heat alone, although this will produce smaller curds. To create larger curds, a combination of heat and an acidic ingredient can be used. The more acid is added, the larger the curds will be and the faster they will form.
There are different types of cheese-making processes, and some do not require the use of bacterial cultures or rennet. For example, queso fresco, queso blanco, and paneer are made by curdling hot milk by adding an acid such as vinegar or fruit juice. This type of fresh cheese is simple to make and has a higher yield because all the whey proteins are included in the cheese.
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Curdling milk increases its acidity and causes milk proteins to tangle
Curdling is a crucial step in the process of transforming milk into cheese. It involves breaking down the emulsion or colloid in milk, resulting in the formation of large parts with different compositions. Curdling can be achieved by adding enzymes, acids, or salts to milk, causing a decrease in pH and an increase in acidity.
The process of curdling milk increases its acidity, which is essential for the cheese-making process. By adding starter cultures, the milk ferments and becomes more acidic. This increase in acidity aids in the curdling process, flavour development, and preservation of the final cheese product. The acidity also helps prevent the growth of undesirable bacteria.
The addition of acids, such as lemon juice or vinegar, directly or through starter cultures, plays a crucial role in curdling. Acid coagulation neutralises the negative charge surrounding casein micelles, allowing them to attract and stick together. This process is particularly effective at the isoelectric point of casein, with a pH of 4.6.
The increased acidity causes the milk proteins, primarily casein, to tangle and form solid masses or curds. Casein micelles, which usually float independently in milk, begin to attract one another due to the increased acidity. This attraction results in the formation of "curdles" that float in a translucent whey. The tangling of milk proteins is a critical step in the transformation of milk into cheese.
The curdling process can be influenced by temperature, with warmer temperatures accelerating the clumping reaction. Additionally, the type of milk used, the bacterial culture, and the coagulating enzyme selected all impact the curdling process and the final characteristics of the cheese. The curds are then pressed, drained, and further processed to create the desired cheese product.
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Curdling milk is a crucial step in transforming liquid milk into solid curd
There are several methods to induce curdling in milk. One common method is to add enzymes, typically in the form of rennet, which is obtained from the fourth stomach of calves. Rennet contains proteolytic enzymes, including rennin and pepsin, which react with the proteins in milk, causing them to coagulate and form curds. The enzyme rennet causes casein to form chains and create a mesh-like network or gel, trapping water and fat. This semisolid gel is the curd.
Another method to induce curdling is by adding acids such as lemon juice, vinegar, or lactic acid. These acids neutralise the negative charge surrounding the casein micelles, allowing them to attract and stick together. This process is commonly used in making cheeses such as cottage cheese, quark, and chèvre.
Curdling can also be achieved through the addition of salts such as magnesium chloride, calcium chloride, or gypsum. Additionally, curdling can occur naturally if cow's milk is left open in a warm environment for a few days, as the milk will sour and produce curds.
The curdling process is essential to separate the curds and whey. The curds are then further processed, drained, and aged to develop the desired flavour and texture characteristics of the final cheese product. The specific techniques and processes used in cheesemaking can vary depending on the intended use, consumption, and characteristics of the desired cheese variety.
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Frequently asked questions
A combination of enzymes, acids, or salts are used to curdle milk for cheese. Common enzymes used are rennet or the lab-created equivalent, rennin. Common acids used are lemon juice or vinegar. Common salts used are magnesium chloride, calcium chloride, or gypsum.
Curdling milk causes it to separate into curds and whey. These curds are then further processed to create the final cheese product.
Curds are coagulated lumps formed by concentrating the protein and fat in milk or a milk product.
Most cheeses use a curdling process, but some examples are cheddar, queso fresco, queso blanco, and paneer.
