There are many varieties of cheese around the world but the process is similar.
After the milk passes quality and purity tests, it is standardized and pasteurized. Standardization makes the milk consistent, adjusting the protein and fat ratio. Pasteurization destroys potentially harmful bacteria in the milk. Then good bacteria or starter culture is added. Starter culture ferment lactose (milk sugar) to produce lactic acid. The type of bacteria depends on the type of cheese to produce. Streptococcus thermophilus and Lactobacillus helveticus, and propionic acid bacteria are common starter cultures in Swiss cheese. Aside from starter culture, few other ingredients may be added.
Coagulation is the first step in turning the milk into solid cheese. Lactic acid from starter cultures is what causes fresh cheese to coagulate. Mature cheese is curdled by adding an enzyme called chymosin, which is present in rennet. The resulting solid mass after coagulation is called “gel”, “curd”, or the “coagulum”. This is cut to allow the whey (liquid) to come out. The size of the cut depends on the type of cheese. Small curds produce drier cheese since more moisture is released. Stirring and heating release more whey.
After cooking, the curd is salted and pressed in a form (Cheddar and Colby), or in a hoop (mozzarella and Swiss cheese). The majority of soft cheeses are not mechanically pressed, whereas the majority of semi-hard to hard cheeses are. Pressing closes the texture, promotes curd fusion, and helps extract more whey.
After pressing, the cheese is ripened. The duration depends on the type of cheese, and the target quality. Typically, cheese ripening ranges from 2 weeks to several years. However, there are cheeses that do not undergo ripening, which include cottage, cream cheese, ricotta, and feta cheese.
Here is what happens during ripening.
HOW IT IS DONE
The cheese is transformed into a delicious cheese during the ripening process over the course of two weeks (for mozzarella) to two or more years by the starter bacteria (still present) and additional bacteria (referred to as the finishing or ripening bacteria) and their corresponding enzymes (Cheddars and Parmesans).
There are also cheeses that are ripened with mold. For example, Camembert and Brie cheese is sprayed with mold onto the surface. Blue cheese is ripened by introducing Penicillium roqueforti internally.
Since microorganisms are involved during ripening, temperature and humidity control is important as it eventually impacts the resulting flavor, texture, and aroma of the cheese. Most cheeses are ripened in cheese-ripening cellars or special storage rooms. Ripening cellars imitate the conditions of a cave. The humidity and temperature are particularly well monitored, depending on the type of cheese.
However, most cheeses are ripened between 46° (8°C) and 60°F (0°C), with a relative humidity of 85–95%. The cellar’s climate is regulated by the air flow, humidity, and temperature in the surrounding area.
Affinage, which translates in French as “end” or “final point,” is the word for ripening. An affineur, cheese tenderer, or finisher may occasionally complete this stage of the cheese-making process. Until the cheese has sufficiently aged to be packaged and sold, the affineur takes care of the cheeses. The affineur periodically rubs, washes, brushes, or sprinkle the surfaces of the cheeses with salt brine and ripening bacteria as they ripen. Every cheese needs to be turned regularly to guarantee even bacterial growth and to avoid cheese shape abnormalities. The cheese’s appearance, aroma, taste, and texture are also evaluated by the affineur to determine when it is ready to be packed for sale.
THE CHEMISTRY OF RIPENING
During the cheese ripening (affinage) process, several chemical and physical changes take place that lead to texture, flavor, aroma, and color development. The changes include the degradation of the following molecules:
- Lactose to lactic acid through fermentation
- Fat by lipase through hydrolysis
- Protein to amino acids by rennin through mild proteolysis.
The bacteria that are present during the cheese-making process of ripening, as well as the type of milk utilized, determine the aromatic, flavorful, distinct molecules that are created. Let’s discuss each further.
Lactose
Lactose is the milk sugar found in all dairy products, including cheese. During ripening, lactose is converted into lactic acid, turning the milk acidic. This is the main reaction that occurs during maturation or ripening. Additionally, the starting culture directly affects flavor development through the generation of metabolites and enzymes. Cheesemakers have two options for starting the fermentation process: either they can buy starter cultures made in factories, or they can rely on the bacteria found in raw milk naturally. Acid and taste development is typically more reproducible when industrial starter cultures are used.
As some of the lactic acid bacteria (LAB) start to die, cells release enzymes that further break down milk proteins, particularly casein, into tiny peptides and amino acids. These dead starter culture cells serve as a vital source of food for non-starter lactic acid bacteria. The starter culture must die in order for the cheese to grow.
Aside from the conversion of lactose to lactic acid, another result of bacterial fermentation is the formation of characteristic holes in some cheeses. Propionibacteria—the propionibacteria shermanii is a significant bacterium in Swiss starting cultures and is well-known for its ability to create holes. Like most bacteria, this one starts out as the starter culture for the cheese and continues to absorb lactic acid during the rennet process and while the cheese is ripening. This process continues throughout the ripening stage, converting the lactic acid into a mixture of propionic and acetic acids as well as carbon dioxide gas, which is what gives Emmentaler cheese its distinctive holes. For more about microbes in cheese: this FAQ page should help.
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Lipids/fats
Cheese’s flavor and texture are greatly influenced by fats. In fact, the same ripening enzymes that break down proteins also break down and chemically alter lipids to create a symphony of aromatic and tasty chemicals. In the curd, lipases (enzymes that break down lipids) convert milk fat’s triacylglycerol fats into free fatty acids.
The fatty acids then go through another chemistry involving oxidation and reduction. The fatty acid is oxidized at the carbon that is “beta” to the carboxylic acid during beta-oxidation. This enables a decarboxylation reaction, which shortens the chain of fatty acids by two carbons and produces a ketone or alcohol. In delta-oxidation, a carbon atom that is four units distant from the carboxylic acid is added to produce a δ-hydroxyacid. These substances are capable of cycling into a lactone.
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An ester can also be created from the fatty acid. This chemistry is important because some short fatty acids possess a taste or aroma that is peppery, sheep-like, or goat-like
Protein (casein)
The primary protein in cheese, casein, is hydrolyzed in a manner and at a rate specific to each variety of cheese. The specificity of the enzymes present is related to the variety of peptides of different compositions that are produced by proteolytic enzymes. Amino acids are produced by further hydrolysis.
The amino acids themselves have a variety of savory and sweet flavors. For instance, the amino acids alanine and tryptophan have sweet and bitter tastes, respectively, while cysteine and methionine, which contain sulfur, give food a “meaty” or “eggy” flavor. Savory recipes frequently contain the amino acid glutamate, also known as MSG, to improve flavor. All of these amino acids are created during casein degradation and have an impact on cheese flavor.
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Many of these amino acids will be further broken down into smaller amines, such as trimethylamine (which tastes fishy), putrescine (which smells like rotting meat), or ammonia, while some amino acids will remain in that molecular state. Despite the fact that these flavors sound awful in this context, a certain cheese with a modest amount of them has a complex, rich, and distinctive flavor.
Since skim milk cheese never develops the full aroma of regular cheese, lipid breakdown is crucial for the formation of cheese aroma.
References
M. Gibson (2018). Food Science and the Culinary Arts. Academic Press.
V. Vaclavik, E. Christian (2014). Essentials of Food Science (4th edition). Springer.
J. DeMan, J. Finley, W. Jeffrey Hurst, C. Y. Lee (2018). Principles of Food Chemistry (4th edition). Springer.
M. Wallert, K. Colabroy, B. Kelly, J. Provost (2016). The Science of Cooking: Understanding the Biology and Chemistry Behind Food and Cooking. John Wiley & Sons, Inc.
