Randy Chandler
Cheese is a beloved food product made from milk, cream, acid, and a little chemistry. The process of cheesemaking involves removing water from milk, which concentrates the milk's protein, fat, and other nutrients, increasing its shelf life. This process is facilitated by adding beneficial bacteria to milk, which convert lactose (milk sugar) into lactic acid, and coagulating the milk into curds, which are then pressed and cut into the desired shape. The coagulation process is facilitated by the addition of rennet, a mixture of enzymes that promote the aggregation of casein proteins into micelles, forming the backbone of the cheese structure. The basic steps of cheesemaking are simple, but the endless variables allow for the vast variety of cheeses available worldwide.
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Milk to curds
Milk is composed of water, fat, carbohydrates, proteins, minerals, and vitamins. 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 first step in turning milk into curds involves adding beneficial bacteria to the milk. Bacteria convert lactose (milk sugar) into lactic acid, which lowers the milk's pH, making it more acidic. This process can occur naturally in unpasteurised milk, but in pasteurised milk, acidifying bacteria must be added.
The addition of acid or rennet causes the milk's casein proteins to coagulate and form curds. Casein proteins exist in structures called micelles, which are suspended in the liquid phase of milk. Micelles have negatively charged outer layers, which allow them to remain dispersed in liquid milk. When acid is added, the micelles are broken up, and the negative charges are removed. This allows the casein proteins to join together, forming a network that becomes the curds.
Rennet, which is derived from the stomachs of young mammals, can also be added to the milk to form curds. It contains the enzyme chymosin, which cuts the negatively charged ends on the micelles' surfaces. Without the negative charges, the micelles are no longer repelled by water and begin to stick together, forming a three-dimensional matrix that traps milk-fat molecules. This process results in a stronger and firmer curd.
By heating the milk and acid mixture, the proteins begin to denature or degrade, further facilitating curd formation. As the mixture reaches 85° Celsius, the curds solidify and can be separated from the whey, which is the liquid that remains after milk proteins have solidified.
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Curds to cheese
Milk is composed of water, fat, carbohydrates, proteins, minerals, and vitamins. The process of making cheese involves removing water from milk, which concentrates the fat, protein, and other nutrients, thereby increasing its shelf life.
The first step in making cheese is to add beneficial bacteria to milk. Bacteria convert lactose (milk sugar) into lactic acid, which lowers the milk's pH. This process can occur naturally in unpasteurised milk, but in the case of pasteurised milk, acidifying bacteria must be added. The pH level is an important indicator of the activity of the bacteria and the development of the curd.
The next step is coagulation, where the milk separates into solid curds and liquid whey. This can be achieved by adding an acid, such as vinegar or lemon juice, or by adding rennet, a mixture containing the enzyme chymosin, which is found in the stomachs of young mammals with a milk-based diet. Rennet speeds up coagulation and produces a stronger curd, allowing curdling at a lower acidity. The curds are then salted, shaped, and left to ripen in a controlled environment.
The process of converting milk to cheese involves a series of chemical reactions. The addition of acid or heat causes the denaturation of proteins, which leads to the formation of curds. The milk proteins, casein molecules, aggregate into spheres called micelles. The outer layer of these micelles is negatively charged, allowing them to remain dispersed in liquid milk. When an acid is added, these negative charges are removed, and the casein proteins join together to form curds. Similarly, the chymosin enzyme in rennet cuts the negative charges on the micelles, allowing them to stick together and form a three-dimensional matrix that traps milk-fat molecules.
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Bacteria and enzymes
The process of making cheese involves bacteria and enzymes, which are essential to transforming milk into cheese. Bacteria and enzymes are used to trigger chemical changes in milk, enhancing specific flavours and textures.
Bacteria
Bacteria are used in the cheesemaking process to create chemical responses that affect the flavour, texture, and look of the cheese. Beneficial bacteria are added to milk to coagulate it into a soft white substance called curd. The bacteria replicate and culture the milk at an optimal temperature, after which the milk coagulates and changes from a liquid into a firm, rubbery material. This change takes around one to two hours and is made possible by the casein proteins in milk. As more lactic acid is produced, the milk's pH lowers, indicating that the bacteria are alive and well.
Enzymes
Enzymes are also crucial to the cheesemaking process. The most common enzyme used to make cheese is rennet, which causes the milk to coagulate and is used to make hard cheeses. Rennet is a mixture containing the active enzyme chymosin, which 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 binds with casein (a protein), causing it to coagulate. The chymosin enzyme in rennet cuts the negatively charged ends on the micelles' surfaces, causing them to stick together and form a three-dimensional matrix that traps the milk-fat molecules.
Combined Effect of Bacteria and Enzymes
Both bacteria and enzymes play a crucial role in the ripening or ageing process of cheese. During this process, a second wave of diverse bacteria and fungi (secondary microbiota) grow within the cheese and on its surface, sometimes forming a rind. These microorganisms contribute to enzymatic activities that result in various byproducts that affect the colour, flavour, texture, and look of the cheese. The ripening process allows the bacteria to break down the proteins, altering the flavour and texture of the final cheese.
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Flavour and aroma
The flavour and aroma of cheese are influenced by a multitude of factors, from the type of milk used to the specific ripening conditions employed. Each variety of milk, be it cow, goat, sheep, or buffalo, has unique fatty acid profiles and lactose content, which contribute to the foundation of cheese flavour. The type of animal feed can also affect the aroma of the milk, as it alters the levels of certain plant-derived odor molecules, such as terpenes.
The process of cheesemaking involves introducing specific bacteria and cultures into the milk, initiating a series of biochemical transformations that develop its flavour and aroma. The starter cultures provide the enzymes that degrade proteins (caseins) into peptides and amino acids, which are major precursors for volatile aroma compounds. The conversion of methionine, aromatic, and branched-chain amino acids is particularly crucial in this process.
The ripening process is a crucial stage in the development of cheese flavour and aroma. During this controlled ageing process, a variety of chemical reactions take place, with enzymes breaking down proteins and fats to release flavourful compounds. The specific types of enzymes, along with the duration and temperature of ripening, significantly influence the final flavour profile. For example, cheeses like Gruyère undergo extended ripening, allowing enzymes to break down complex milk proteins.
The perception of cheese flavour results from the combined detection of aroma and taste compounds by their respective sensory receptors. Volatile flavour compounds interact with receptors in the oral cavity, throat, and nasal passages, while non-volatile flavourants interact with taste receptors. The release of flavourants from the cheese matrix must occur at a specific rate and extent to accurately represent the expected cheese flavour character.
The complex flavour and aroma profiles of cheese have been extensively studied, with researchers employing methods such as gas chromatography-olfactometry (GC-O) and sensory analysis to identify key odorants and understand their contribution to the overall sensory experience of cheese.
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Preservation
Cheese is a way of preserving milk for long periods. The process transforms the milk into something completely different, with its own interesting and delicious properties. The main preservatives that give cheese its longevity are salt and acids.
Cheese-making is a long and complex process that uses bacteria, enzymes, and naturally formed acids to solidify milk proteins and fats and preserve them. Milk is inoculated with lactic acid bacteria and rennet. The lactic acid bacteria convert the sugar in milk (lactose) to lactic acid. The rennet, which is derived from the stomachs of young mammals, contains enzymes that modify proteins in milk. Specifically, an enzyme in rennet called rennin converts a common protein in milk called caseinogen into casein, which does not dissolve in water. The chymosin in rennet breaks down the kappa casein on the surface of the micelles, changing them from hydrophilic to hydrophobic. This causes them to aggregate together, trapping fat and water molecules in the developing curd. Further processing of the curd helps remove more water and compress the curd to form a solid cheese.
Once turned into cheese, milk can be stored for months or years. However, cheese is susceptible to contamination by pathogenic and spoilage microorganisms, which can result in a reduced shelf life and risks to consumers' health. This has led to the use of preservatives in the cheese-making process. Traditional preservation methods include freezing, aging, maturation, and salting. However, the use of synthetic preservatives has raised concerns due to their potential health and environmental impacts. As a result, there is a growing demand for natural and chemical preservative-free alternatives.
To address these concerns, researchers are exploring new preservation strategies that prioritize consumer safety and environmental sustainability. This includes the use of plant-derived preservatives, such as natural herbs, and innovative technologies like high-pressure processing (HPP), ultrasonication, cold plasma, and pulse light. High hydrostatic pressures, vacuum and modified atmosphere packaging, edible coatings and films, and intelligent packaging technologies are also being explored to extend the shelf life of cheese products. Some cheeses are also treated with natural preservatives like endolysins, propolis, and mycocins, which have shown promising results.
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Frequently asked questions
The basic steps of 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.
Milk is about 86% water but also contains fat, protein, and other nutrients. The cheesemaking process involves removing water from milk, which concentrates the milk's protein, fat, and other nutrients and increases its shelf life.
Acid is essential to cheesemaking. It helps neutralize negatively charged proteins so that they can clump together into curds. The addition of acid also lowers the pH of milk, which helps indicate the activity of the bacteria and ensures the fresh curd is on the right path to becoming cheese.
