Cheese is a fermented food derived from the milk of various mammals. Since humans began to domesticate milk-producing animals around 10,000 B.C. , they have known about the propensity of milk to separate into curds and whey. As milk sours, it breaks down into curds, lumps of phosphoprotein, and whey, a watery, grey fluid that contains lactose, minerals, vitamins, and traces of fat. It is the curds that are used to make cheese, and practically every culture on Earth has developed its own methods, the only major exceptions being China and the ancient Americas.
The first cheeses were "fresh," that is, not fermented. They consisted solely of salted white curds drained of whey, similar to today's cottage cheese. The next step was to develop ways of accelerating the natural separation process. This was achieved by adding rennet to the milk. Rennet is an enzyme from the stomachs of young ruminants—a ruminant is an animal that chews its food very thoroughly and possesses a complex digestive system with three or four stomach chambers; in the United States, cows are the best known creatures of this kind. Rennet remains the most popular way of "starting" cheese, though other starting agents such as lactic acid and various plant extracts are also used.
By A.D. 100 cheese makers in various countries knew how to press, ripen, and cure fresh cheeses, thereby creating a product that could be stored for long periods. Each country or region developed different types of cheese that reflected local ingredients and conditions. The number of cheeses thus developed is staggering. France, famous for the quality and variety of its cheeses, is home to about 400 commercially available cheeses.
The next significant step to affect the manufacture of cheese occurred in the 1860s, when Louis Pasteur introduced the process that bears his name. Pasteurization entails heating milk to partially sterilize it without altering its basic chemical structure. Because the process destroys dangerous micro-organisms, pasteurized milk is considered more healthful, and most cheese is made from pasteurized milk today.
The first and simplest way of extending the length cheese would keep without spoiling was simply ageing it. Aged cheese was popular from the start because it kept well for domestic use. In the 1300s, the Dutch began to seal cheese intended for export in hard rinds to maintain its freshness, and, in the early 1800s, the Swiss became the first to process cheese. Frustrated by the speed with which their cheese went bad in the days before refrigeration, they developed a method of grinding old cheese, adding filler ingredients, and heating the mixture to produce a sterile, uniform, long-lasting product. Another advantage of processing cheese was that it permitted the makers to recycle edible, second-grade cheeses in a palatable form.
Prior to the twentieth century, most people considered cheese a specialty food, produced in individual households and eaten rarely. However, with the advent of mass production, both the supply of and the demand for cheese have increased. In 1955, 13 percent of milk was made into cheese. By 1984, this percentage had grown to 31 percent, and it continues to increase. Interestingly, though processed cheese is now widely available, it represents only one-third of the cheese being made today. Despite the fact that most cheeses are produced in large factories, a majority are still made using natural methods. In fact, small, "farmhouse" cheese making has made a comeback in recent years. Many Americans now own their own small cheese-making businesses, and their products have become quite popular, particularly among connoisseurs.
Cheese is made from milk, and that milk comes from animals as diverse as cows, sheep, goats, horses, camels, water buffalo, and reindeer. Most cheese makers expedite the curdling process with rennet, lactic acid, or plant extracts, such as the vegetable rennet produced from wild artichokes, fig leaves, safflower, or melon.
In addition to milk and curdling agents, cheeses may contain various ingredients added to enhance flavor and color. The great cheeses of the world may acquire their flavor from the specific bacterial molds with which they have been inoculated, an example being the famous Penicillium roqueforti used to make France's Roquefort and England's Stilton. Cheeses may also be salted or dyed, usually with annatto, an orange coloring made from the pulp of a tropical tree, or carrot juice. They may be washed in brine or covered with ashes. Cheese makers who wish to avoid rennet may encourage the bacterial growth necessary to curdling by a number of odd methods. Some cheeses possess this bacteria because they are made from unpasteurized milk. Other cheeses, however, are reportedly made from milk in which dung or old leather have been dunked; still others acquire their bacteria from being buried in mud.
The unusual texture and flavor of processed cheese are obtained by combining several types of natural cheese and adding salt, milk-fat, cream, whey, water, vegetable oil, and other fillers. Processed cheese will also have preservatives, emulsifiers, gums, gelatin, thickeners, and sweeteners as ingredients. Most processed cheese and some natural cheeses are flavored with such ingredients as paprika, pepper, chives, onions, cumin, car-away seeds, jalapeño peppers, hazelnuts, raisins, mushrooms, sage, and bacon. Cheese can also be smoked to preserve it and give it a distinctive flavor.
Although cheese making is a linear process, it involves many factors. Numerous varieties of cheese exist because ending the simple preparation process at different points can produce different cheeses, as can varying additives or procedures. Cheese making has long been considered a delicate process. Attempts to duplicate the success of an old cheese factory have been known to fail because conditions at a new factory do not favor the growth of the proper bacteria.
Preparing the milk
- 1 Small cheese factories accept either morning milk (which is richer), evening milk, or both. Because it is generally purchased from small dairies which don't pasteurize, this milk contains the bacteria necessary to produce lactic acid, one of the agents that triggers curdling. The cheese makers let the milk sit until enough lactic acid has formed to begin producing the particular type of cheese they're making. Depending on the type of cheese being produced, the cheese makers may then heat the ripening milk. This process differs slightly at large cheese factories, which purchase pasteurized milk and must consequently add a culture of bacteria to produce lactic acid.
Separating the curds from the whey
- 2 The next step is to add animal or vegetable rennet to the milk, furthering its separation into curds and whey. Once formed, the curds are cut both vertically and horizontally with knives. In large factories, huge vats of curdled milk are cut vertically using sharp, multi-bladed, wire knives reminiscent of oven racks. The same machine then agitates the curds and slices them horizontally. If the cutting is done manually, the curds are cut both ways using a large, two-handled knife. Soft cheeses are cut into big chunks, while hard cheeses are cut into tiny chunks. (For cheddar, for instance, the space between the knives is about one-twentieth of an inch [half a centimeter].) After cutting, the curds may be heated to hasten the separation from the whey, but they are more typically left alone. When separation is complete, the whey is drained.
In a typical cheese-making operation, the first step is preparing the milk. Although smaller factories purchase unpasteurized milk that already has the bacteria present to produce lactic acid (necessary for curdling), larger factories purchase pasteurized milk and must add bacteria culture to produce the lactic acid.
Next, the curds must be separated from the whey. Animal or vegetable rennet is added, and then the curds are agitated and cut using large knives. As the whey separates, it is drained. The curds are then pressed into molds, if necessary, to facilitate further moisture drainage, and aged for the proper amount of time. Some cheeses are aged for a month, others for several years.
Pressing the curds
- 3 Moisture must then be removed from the curds, although the amount removed depends on the type of cheese. For some types with high moisture contents, the whey-draining process removes sufficient moisture. Other types require the curds to be cut, heated, and/or filtered to get rid of excess moisture. To make cheddar cheese, for example, cheese makers cheddar, or finely chop, the curd. To make hard, dry cheeses such as parmesan, cheese makers first cheddar and then cook the curd. Regardless, if the curds are to be aged, they are then put into molds. Here, they are pressed to give the proper shape and size. Soft cheeses such as cottage cheese are not aged.
Ageing the cheese
- 4 At this stage the cheese may be inoculated with a flavoring mold, bathed in brine, or wrapped in cloth or hay before being deposited in a place of the proper temperature and humidity to age. Some cheeses are aged for a month, some for up to several years. Ageing sharpens the flavor of the cheese; for example, cheddar aged more than two years is appropriately labeled extra sharp.
Wrapping natural cheese
- 5 Some cheeses may develop a rind naturally, as their surfaces dry. Other rinds may form from the growth of bacteria that has been sprayed on the surface of the cheese. Still other cheeses are washed, and this process encourages bacterial growth. In place of or in addition to rinds, cheeses can be sealed in cloth or wax. For local eating, this may be all the packaging that is necessary. However, large quantities of cheese are packaged for sale in distant countries. Such cheeses may be heavily salted for export (such as Roquefort) or sealed in impermeable plastic or foil.
Making and wrapping processed
- 6 Edible yet inferior cheeses can be saved and made into processed cheese. Cheeses such as Emmental (commonly called Swiss), Gruyere (similar to Swiss), Colby, or cheddar are cut up and very finely ground. After this powder has been mixed with water to form a paste, other ingredients such as salt, fillers, emulsifiers, preservatives, and flavorings are added. The mixture is then heated under controlled conditions. While still warm and soft, the cheese paste is extruded into long ribbons that are sliced. The small sheets of cheese are then put onto a plastic or foil sheet and wrapped by a machine.
Cheese making has never been an easily regulated, scientific process. Quality cheese has always been the sign of an experienced, perhaps even lucky cheese maker insistent upon producing flavorful cheese. Subscribing to analytical tests of cheese characteristics may yield a good cheese, but cheese making has traditionally been a chancy endeavor. Developing a single set of standards for cheese is difficult because each variety of cheese has its own range of characteristics. A cheese that strays from this range will be bad-tasting and inferior. For example, good soft blue cheese will have high moisture and a high pH; cheddar will have neither.
One controversy in the cheese field centers on whether it is necessary to pasteurize the milk that goes into cheese. Pasteurization was promoted because of the persistence of Mycobacterium tuberculosis, a pathogen or disease-causing bacteria that occurs in milk products. The United States allows cheeses that will be aged for over sixty days to be made from unpasteurized milk; however, it requires that many cheeses be made from pasteurized milk. Despite these regulations, it is possible to eat cheeses made from unpasteurized milk to no ill effect. In fact, cheese connoisseurs insist that pasteurizing destroys the natural bacteria necessary for quality cheese manufacture. They claim that modern cheese factories are so clean and sanitary that pasteurization is unnecessary. So far, the result of this controversy has merely been that connoisseurs avoid pasteurized milk cheeses.
Regulations exist so that the consumer can purchase authentic cheeses with ease. France, the preeminent maker of a variety of natural cheeses, began granting certain regions monopolies on the manufacture of certain cheeses. For example, a cheese labeled "Roquefort" is guaranteed to have been ripened in the Combalou caves, and such a guarantee has existed since 1411. Because cheese is made for human consumption, great care is taken to insure that the raw materials are of the highest quality, and cheese intended for export must meet particularly stringent quality control standards.
Because they possess such disparate characteristics, different types of cheese are required to meet different compositional standards. Based on its moisture and fat content, a cheese is labeled soft, semi-soft, hard, or very hard. Having been assigned a category, it must then fall within the range of characteristics considered acceptable for cheeses in that category. For example, cheddar, a hard cheese, can contain no more than 39 percent water and no less than 50 percent fat. In addition to meeting compositional standards, cheese must also meet standards for flavor, aroma, body, texture, color, appearance, and finish. To test a batch of cheese, inspectors core a representative wheel vertically in several places, catching the center, the sides, and in between. The inspector then examines the cheese to detect any inconsistencies in texture, rubs it to determine body (or consistency), smells it, and tastes it. Cheese is usually assigned points for each of these characteristics, with flavor and texture weighing more than color and appearance.
Processed cheese is also subject to legal restrictions and standards. Processed American cheese must contain at least 90 percent real cheese. Products labeled "cheese food" must be 51 percent cheese, and most are 65 percent. Products labeled "cheese spread" must also be 51 percent cheese, the difference being that such foods have more water and gums to make them spreadable. "Cheese product" usually refers to a diet cheese that has more water and less cheese than American cheese, cheese food, or cheese spread, but the specific amount of cheese is not regulated. Similarly, "imitation cheese" is not required to contain a minimum amount of cheese, and cheese is usually not its main ingredient. In general, quality processed cheese should resemble cheese and possess some cheesy flavor, preferably with a "bite" such as sharp cheddar cheese has. The cheese should be smooth and evenly colored; it should also avoid rubberiness and melt in the mouth.
Where To Learn More
Brown, Bob. The Complete Book of Cheese. Gramercy Publishing, 1955.
Carr, Sandy. The Simon and Schuster Pocket Guide to Cheese. Simon and Schuster, 1981.
Kosikowski, Frank. Cheese and Fermented Milk Foods. Cornell University, 1966.
Mills, Sonya. The World Guide to Cheese. Gallery Books, 1988.
Timperley, Carol and Cecilia Norman. A Gourmet's Guide to Cheese. HP Books, 1989.
"American Cheese and 'Cheeses'," Consumer Reports. November, 1990, pp. 728-732.
Birmingham, David. "Gruyere's Cheese-makers," History Today. February, 1991, pp. 21-26.
Raichlen, Steven. "Farmhouse Cheeses," Yankee. February, 1991, pp. 84-92.
Also read article about Cheese from Wikipedia
Chalk Chewing Gum
DiscussionsIt is generally quite straight forward to locate the sources of enzymes; the natural sources of enzymes are where nature needs them. It comes as no surprise that rennet had traditionally been isolated from the fourth stomach of young calves because digestion by suckling calves is nature's primary way of processing cow milk. However, the ways of nature are not always the most economical from man's perspective when a process is adapted to different uses to benefit mankind in ways not originally intended by the nature. Thus enters the study of biochemical engineering.
Chemical engineering is the study of how to make a large quantity of chemicals in an economical fashion. For example, frequently a chemical engineer must devise a process to mass-produce a polymer that is totally different from the one originally used by an organic chemist working with small test tubes and beakers in a laboratory. Biochemical engineering, being a sub-field of chemical engineering, also deals with the same kinds of problems facing chemical engineering, except that the chemicals are not synthetic (manmade) but biological (naturally existing) in nature. Although rennet is naturally excreted from a calf's stomach lining, extracting it from its natural source is not economical. Other proteases can also convert casein to para-casein, but their action does not stop there. They further degrade the curd to soluble subunits. Fortunately, large quantities of rennet of consistent quality can now be produced easier and cheaper in a well controlled environment by microbial fermentation.
A word of caution is in order here. Enzymes as a class of chemicals are not generally considered as dangerous, toxic, nor poisonous; they do not cause skin irritations or burns as acids or bases. Some exceptions are proteases that catalyze the breakdown of protein molecules to amino acids components. Because meat is mainly protein, protease can digest the soft moist sections of the skin. We can all imagine how that is going to feel. Thus, you should exercise the same caution with enzymes as you do with any other chemicals.
- Report your observation of this experiment and discuss the effect of pH on the yield of the curd. Try out other types of processed milk such as nonfat milk and powdered milk. Are they suited for cheese making?
- Taste the cheese you have made. How does it taste compared to cottage cheese? (The product should be edible if the glassware is clean, which is exactly what it should be.)
- In Step 1, why is buttermilk not added to the batch labelled "A"?
- Because its action has long been discovered, rennet does not follow the rules of enzyme nomenclature discussed in class. Its conventional name has already been deep-rooted in the craft of cheese making. Considering the action of rennet, how would you rename the enzyme to conform to the general rules of nomenclature?
- What microorganisms are currently used to manufacture rennet? How is rennet isolated?
- Why is the coloring added before coagulation instead of afterwards?
- Give a list of the chemical components found in milk. In this experiment, you used an enzyme to react selectively with a protein called casein; what other carbohydrates, proteins, and fats are found in milk? List the nutritional information for some popular types of milk and break down each category into individual chemical components. (Example: Nonfat milk has x g/liter of carbohydrates, y% of which is lactose, z% of which is sucrose, etc.)
- What are some of the products other than cheese that are derived mainly from milk? Identify the principle component of milk utilized in each product and point out the use of enzymes if applicable. Choose a specific dairy product and give a description of the processes involved in the production.
- Milk spoilage can be detected visually by the presence of coagulated curd. Identify the chemical mechanisms responsible for coagulation in this case. Are these chemicals/enzymes produced by contaminant microbial actions? Is the curd resulting from spoilage the same as that you have obtained in class? If they are the same, why would you consume cheese but not spoiled milk?
- Comment on ways to improve the experiment.
- Prins, J., Microbial rennet, Process Biochem., May, 1970.
- Robinson, R.K., Dairy Microbiology, Vol. 2, Applied Science Publishers, New Jersey, 1981.
- Richmond, H.D., Dairy Chemistry, A Practical Handbook, Charles Griffin & Co., London, 1930.
- Constituents of milk: Jenness, R. and Patton, S., Principles of Dairy Chemistry, John Wiley, 1959, p3.
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