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Cover of FAQ: Microbes Make the Cheese

FAQ: Microbes Make the Cheese

Report on an American Academy of Microbiology Colloquium held in Washington, DC, in June 2014
Washington (DC): American Society for Microbiology; .

Cheese, a traditional food incorporated into many cuisines, is used as an ingredient in cooking or consumed directly as an appetizer or dessert, often with wine or other suitable beverages. Great numbers of cheese varieties are produced, reflecting in part the versatility of the microorganisms used in cheese-making that this FAQ report will describe. Cheese is one of the few foods we eat that contains extraordinarily high numbers of living, metabolizing microbes, leading some participants to say, “Cheese is alive!” The broad groups of cheese-making microbes include many varieties of bacteria, yeast, and filamentous fungi (molds).

This report focuses on the microbiology of “natural” cheeses, those made directly from milk, including hard and soft varieties such as Cheddar, Mozzarella, and Camembert. Pasteurized process cheese, the other broad category of cheese, is made by blending natural cheeses with emulsifying agents, preservatives, thickeners, flavorings, and seasonings. “American cheese” is perhaps the classic example of a process cheese, notwithstanding recent examples of American artisanal cheese-making and changing tastes among consumers of those cheeses.

The United States (US) produces more than 1700 different cheeses, and cheese-making is an economic backbone for many states, serving as a major source of revenue (1). A 2014 report on cheese-making from the United States Department of Agriculture’s (USDA) National Agriculture Statistics Services noted that Wisconsin, California, Idaho, New Mexico, and New York are the top five cheese-producing states (2). During the past decade, cheese production in the US has increased by more than one million metric tons, according to a recent report from the United States Dairy Export Council (USDEC) (3).

Front Matter

The American Academy of Microbiology (Academy) is the honorific branch of the American Society for Microbiology (ASM), a non-profit scientific society with nearly 40,000 members. Fellows of the Academy have been elected by their peers in recognition of their outstanding contributions to the field of microbiology. Through its colloquium program, the Academy draws on the expertise of these fellows to address critical issues in microbiology.

FAQ reports are based on the deliberations of 15-20 expert scientists who gather for a day to develop science-based answers to questions the public might have about topics in microbiology. The reports are reviewed by all participants, and every effort is made to ensure that the information is accurate and complete. The contents reflect the views of the participants and are not intended to reflect official positions of the Academy or ASM. The Academy thanks the efforts of previous staff, Leah Gibbons and Shannon Greene, Ph.D., for their assistance on this project, and of current staff, Chelsie Geyer, Ph.D., for her work on the final product.

Contents of the report may be distributed further so long as the authorship of the Academy is acknowledged and this disclaimer is included.

BOARD OF GOVERNORS, AMERICAN ACADEMY OF MICROBIOLOGY

Michele S. Swanson, Ph.D., Chair University of Michigan

Martin J. Blaser, M.D.New York University

Donald A. Bryant, Ph.D.Pennsylvania State University

Terence Dermody, M.D.Vanderbilt University

Gerry Fink, Ph.D.Massachusetts Institute of Technology

Stan Fields, Ph.D.University of Washington

Susan Gottesman, Ph.D.National Institutes of Health

James M. Hughes, M.D.Emory University

Steven Lindow, Ph.D.University of California, Berkeley

Margaret McFall-Ngai, Ph.D.University of Wisconsin-Madison

Mary Ann Moran, Ph.D.University of Georgia

Graham C. Walker, Ph.D.Massachusetts Institute of Technology

COLLOQUIUM STEERING COMMITTEE

Tom Beresford, Ph.D.Teagasc Food Research Centre

Catherine Donnelly, Ph.D.Vermont University

Rachel Dutton, Ph.D.Harvard University

James Steele, Ph.D.University of Wisconsin-Madison

PARTICIPANTS

John Barlow, D.V.M., Ph.D.University of Vermont

Jeff Broadbent, Ph.D.Utah State University

Dennis D’Amico, Ph.D.University of Connecticut

MaryAnne Drake, Ph.D.North Carolina State University

Mark Johnson, Ph.D.University of Wisconsin-Madison

Mateo Kehler Cellars at Jasper Hill

Paul Kindstedt, Ph.D.University of Vermont

Sylvie Lortal, Ph.D.INRA-Agrocampus Ouest

Noella Marcellino, Ph.D.Abbey of Regina Laudis

Donald McMahon, Ph.D.Western Dairy Center – Utah State University

Heather Paxson, Ph.D.Massachusetts Institute of Technology

Dennis Romero, Ph.D.DuPont Culture Development – Nutrition & Health

Ben Wolfe, Ph.D.Harvard FAS Center for Systems Biology

ACADEMY STAFF

Marina Moses, DrPH Director

Chelsie Geyer, Ph.D.Colloquium Postdoctoral Fellow

Dylan Richmond Colloquium and Public Outreach Program Assistant

Erin Seglem Board of Governors Program Assistant

FAQ: Microbes Make the Cheese

Cheese, a traditional food incorporated into many cuisines, is used as an ingredient in cooking or consumed directly as an appetizer or dessert, often with wine or other suitable beverages. Great numbers of cheese varieties are produced, reflecting in part the versatility of the microorganisms used in cheese-making that this FAQ report will describe. Cheese is one of the few foods we eat that contains extraordinarily high numbers of living, metabolizing microbes, leading some participants to say, “Cheese is alive!” The broad groups of cheese-making microbes include many varieties of bacteria, yeast, and filamentous fungi (molds).

This report focuses on the microbiology of “natural” cheeses, those made directly from milk, including hard and soft varieties such as Cheddar, Mozzarella, and Camembert. Pasteurized process cheese, the other broad category of cheese, is made by blending natural cheeses with emulsifying agents, preservatives, thickeners, flavorings, and seasonings. “American cheese” is perhaps the classic example of a process cheese, notwithstanding recent examples of American artisanal cheese-making and changing tastes among consumers of those cheeses.

The United States (US) produces more than 1700 different cheeses, and cheese-making is an economic backbone for many states, serving as a major source of revenue (1). A 2014 report on cheese-making from the United States Department of Agriculture’s (USDA) National Agriculture Statistics Services noted that Wisconsin, California, Idaho, New Mexico, and New York are the top five cheese-producing states (2). During the past decade, cheese production in the US has increased by more than one million metric tons, according to a recent report from the United States Dairy Export Council (USDEC) (3).

Image makethecheese.June.2014.fig01

Image retrieved from: p 2) James McKelvey

1. What is cheese and how is it made?

Simply defined, cheese is a food produced by removing water from milk, yielding concentrated milk proteins, fats, and other nutrients and compounds conferring flavor and aroma. Although critics of cheese point to it being high in fat, cholesterol, and salt, this food also provides many nutritional benefits. Beyond enticing the palate, cheese serves as a rich source of vitamins, minerals, and protein.

With only a few essential ingredients, including milk, microorganisms, salt, and enzymes, it is possible to create thousands of cheese varieties. All cheese-making starts with milk from one of several types of ruminant animals, mainly cows, goats, sheep, and water buffaloes. The key components of milk for cheese-making are the sugar lactose, fat, and the milk proteins known as caseins. In their native state, milk proteins repulse one another, keeping them suspended in liquid. To create cheese, cheese-makers generate conditions that overcome these forces, leading the proteins to coalesce and the milk to form a semi-solid. This conversion step requires the milk to become more acidic, that is, for its pH to be lowered through the process of fermentation, followed by the coagulation of milk proteins to form a gel. This process is much the same as what happens during beer-and wine-making. Thus, fermentation is a means of preserving food stuffs while also profoundly changing them in ways that depend on the metabolism of microbes. Microbial fermentation is in large part an enzyme-dependent process during which protein catalysts made by the microbes break down complex substances—in this case, the proteins and sugars within milk—into simpler products that typically can be stored for longer periods than the original substance. Fermentation thus preserves milk as cheese, making it available for consumption months or years later, long after the unfermented milk would have spoiled. Fresh cheeses may be consumed immediately following the manufacturing process, whereas aged cheeses are stored for 12 months or more, during which time they undergo ripening to develop the distinctive flavor, smell, and texture of the particular cheese (Figure 1).

Figure 1.

Figure 1.

Basic steps of cheese-making

The acid produced during fermentation helps to form curds, also called a gel…

Image makethecheese.June.2014.fig03

Image retrieved from: p 4) goldenagecheese.com;

In cheese-making, not only do the milk proteins change, but so do its sugars. In particular, the milk sugar lactose is broken down into lactic acid by fermentation that depends on a group of bacteria referred to as lactic acid bacteria (LAB). Although traditional cheese-makers relied on naturally occurring LAB in milk, it is now common to inoculate milk with industrial starter cultures, defined groups of bacteria that are specifically chosen for use in cheese-making (See p 12 for description) and that help to ensure reliable and consistent acid production. To increase the rate of fermentation, the milk is warmed to the optimal growth temperature of microbes in the starter culture.

The acid produced during fermentation helps to form curds, also called a gel, and contributes to syneresis of the curd, removal of water held within the milk proteins.

Another significant step in transforming milk into cheese is converting liquid milk into a gel, consisting of a network of casein proteins whose formation depends on acidification of the milk and subsequent coagulation of those proteins. Depending on the cheese variety, three mechanisms to coagulate those proteins are drawn on to produce three very different families of cheese. The products of those three mechanisms are known as acid coagulated, acid-heat coagulated, and rennet coagulated cheeses, respectively. Acid coagulation occurs when bacteria in the warm milk (70 – 90°F) convert lactose to lactic acid, which is allowed to accumulate to high concentrations. This accumulated lactic acid lowers the milk pH to the point where the repulsive forces originally present in the milk proteins are overcome, allowing these proteins to bind to one another, forming a gel. In contrast, acid-heat coagulation yields less lactic acid from lactose but heats the mixture to a much higher temperature (around 185°F). This combination also serves to overcome the repulsive forces originally present in the milk proteins, allowing the milk proteins to gel. Finally, rennet coagulation depends on the action of these specialized enzymes that, when added to warm milk (around 90°F), can also remove the repulsive forces of the milk proteins, again resulting in gel formation.

Those three coagulation mechanisms produce gel-structures with strikingly different characteristics that then give rise to distinct families of cheese. Acid coagulated cheeses such as Cottage cheese and Cream cheese, and acid-heat coagulated cheeses such as Ricotta and Paneer characteristically have very high moisture content, making them vulnerable to spoilage and thus difficult to ripen for extended periods. Consequently, cheeses in these families are usually consumed fresh, with only a few examples of ripened acid and acid-heat coagulated cheeses. In contrast, rennet coagulation results in a gel structure with much lower moisture content, opening the door to almost unlimited possibilities for long-term ripening of such cheeses.

…curds may be knitted and pressed together to form cheese wheels or blocks of various sizes and shapes…

Image makethecheese.June.2014.fig04

Image retrieved from: p 5) Imen McDonnell “Farmette”;

Once the gel reaches a required consistency, it is cut into small pieces referred to as curds, another step that facilitates the expulsion of moisture, referred to as whey, from the curds. Curds are separated from the whey via draining, a step in which the curds may be knitted and pressed together to form cheese wheels or blocks of various sizes and shapes according to the cheese variety being manufactured. Salt, either in crystals or in a brine solution is applied either to the curds or to the more fully formed whole cheese, respectively. This salt inhibits further growth of the starter culture, prevents growth of pathogens and spoilage microbes that are salt sensitive, selects for the growth of desirable microbes, and influences enzymes that break down protein and fat in ways that impart particular flavors and textures.

During the ripening process, a second wave of diverse bacteria and fungi (secondary microbiota) grow within the cheese and on its surface, sometimes forming a rind, the exterior outer layer of a ripened cheese. Microorganisms that are part of the secondary microbiota contribute enzymatic activities that affect the color, flavor, texture, and other important characteristics, such as the holes in Swiss cheese, to the ripening cheese. Cheese was traditionally ripened in caves or cellars because they provided consistently cool and humid environments suitable for ripening (Figure 2A and 2B). In modern cheese-making plants, special ripening rooms are designed where temperature and humidity can be even more tightly controlled, leading to greater consistency in product quality (Figure 2C) (4). The labor-intensive process of “natural” ripening can include rubbing the cheese periodically with salt, wine, beer, fruit juices, or liqueurs to enhance flavor. Depending on the cheese variety, the ripening process is allowed to continue for extended periods, referred to as the aging process. Young cheeses that are not aged are known as fresh cheeses, some of which are spreadable. Hard cheeses are typically aged, at minimum, for 6 months to one year. The longer a cheese is aged, the sharper its flavor becomes. For example, there are mild, sharp, and extra sharp Cheddars. Mild Cheddar is aged for 2-3 months; sharp Cheddar is aged for approximately 8-12 months; and extra sharp Cheddar is aged anywhere from 18 months up to 5 years (5).

Figure 2.. A.

Figure 2.

A.) The entrance to a traditional cheese cave in France (part of the caves of St. Julien in the Auvergne) used to age St. Nectaire cheese. The cheese caves are hundreds of years old and dug out of volcanic rock. B.) Cheese ripening in the cave. C.) A (more...)

Image makethecheese.June.2014.fig06

Image retrieved from: p 6) Roma 2011, Timbres Italy, Italia 2011, Postzegels Italy

2. How did cheese-making get started and are the original techniques still used today?

According to the ASM publication, Cheese and Microbes (2014), the word “cheese” and words associated with cheese-making have ancient origins (1). The ancient Greeks called the wicker cheese basket used for draining whey from the curds a formos, which became forma to the Romans for “formed or molded.” The English word “cheese” is derived from the West Germanic reconstruct kasjus via the Old English cyse and Latin caseus. Their roots are in the proto-Indo-European reconstruct kwat-, to ferment or become sour (1). Both branches of the etymology of the word “cheese” capture its essence; through the processes of fermentation, draining, and pressing, liquid milk becomes a solid food.

Image

Figure

Cheese was originally made to preserve and conserve the nutrients in milk…

Over the course of history, humans have developed creative ways to preserve foods. Cheese was originally made to preserve and conserve the nutrients in milk, typically held for consumption when milk production was reduced during some seasons. With consumption of fermented milk products such as cheese, it is easy to understand why cheese is described as “one of the primary symbols of mankind’s passage into civilization” by Kamber and Terzi (2008) (6,7). This nutritious product evolved over 9000 years, and continues to provide us with a safe and flavorsome variety of cheese types. Today, cheese is not necessarily made to preserve milk but rather to produce the wide diversity of foods that people enjoy in various culinary, cultural, and snacking situations. The transformation from grass to milk to cheese continues to intrigue people, as reflected in the comment from American writer and editor Clifton Fadiman, who calls cheese “milk’s leap toward immortality” (8).

There is no conclusive evidence pointing to when or where cheese-making originated. A common tale has it that some 9000 years ago, nomads stored milk from sheep and cows in pouches that were made from the stomachs of the animals. Unbeknownst to the nomads, those stomach linings contained rennet, which caused the milk to coagulate, and contaminating microbes that were naturally present in the milk converted lactose into lactic acid, thereby acidifying the product. The people of the time tasted this substance, realized it was safe to eat, and this new food began its development. There is no evidence, however, to support this account. What we do know is that Neolithic humans in Anatolia (modern Turkey) began to harvest and store milk as early as 7000 BC through analysis of fragments of pottery from that time that contain organic residues that serve as telltale molecular finger-prints for the milk that was once stored in those pots. Many of these pottery fragments contain traces of milk fat or its residues, indicating that a concentrated form of milk, most likely a simple form of cheese, was placed in those pots 9000 years ago. Some of these early cheese-makers from Turkey migrated to central Europe sometime during the next 1000 years, bringing with them their dairy animals and cheese-making expertise. By 5500 BC, these Neolithic farmers developed specialized ceramic sieves to separate curds from whey, and, here again, some of these sieves contained the telltale molecular fingerprints of milk fat. From around 3000 BC, when written language was perfected by Sumerians of southern Mesopotamia, some recovered examples of those early texts contain references to cheese. Indeed, clay tablets recovered from southern Mesopotamia, dated somewhat later, contained references to nearly 20 different types of cheese. Additionally, evidence recovered from Egyptian tombs dating back to 3000 BC indicates that cheese was then being made in this region as well (1).

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Traditional cheese-making methods that are used today in different parts of the world.

Figure 3.. Traditional methodologies used in cheese-making.

Figure 3.

Traditional methodologies used in cheese-making. (A) Gourds in Africa (B) Stinging nettles in the Pyrenees, (C) Ceramic pots in the Mediterranean and Middle East (D) Traditional wooden tools (curd cutter and barrel) used in making farmstead St. Nectaire (more...)

The production of cheese in America began in the 17th century when the English Puritans brought dairy cows and their cheese-making craft with them to the New England colonies. Kindstedt (from the book, American Farmstead Cheese) states that the American cheese-making industry began in the Massachusetts Colony by Puritans from East Anglia, England, who arrived in North America from 1629-1640 (9). Many cheeses are known by their place of origin such as Cheddar and Cheshire, which took the name of the village and county, respectively, in England where the cheeses were first produced; Gruyère cheese was named after the town Gruyères in western Switzerland; Roquefort cheese was named after the small town of Roquefort-sur-Soulzon in France; and Muenster cheese (from the Latin Monasterium) was first created in a monastery (10). We owe many thanks to cheese-makers of earlier times for their creativity and expertise. They overcame cultural and environmental constraints to create the variety of cheeses we enjoy today.

To meet worldwide demand, most cheese is now produced in large mechanized plants using advanced process control systems. Nonetheless, the basic principles underlying cheese-making remain unchanged and include: (1) removal of water from milk, (2) break-down of milk proteins (caseins) and fat, (3) addition of salt, and (4) an optional ripening period (1). Each of these steps differs, depending upon the cheese that is being created, but they all influence the final texture, aroma, and flavor. Although cheese-making is an ancient art, modern cheese production exploits our knowledge of many branches of science, including protein chemistry, organic chemistry, molecular genetics, enzymology, and microbiology, to assist in the production of consistent high quality products. Processing milk into cheese makes it possible to transform a perishable liquid into a food that has a prolonged shelf life and can be transported readily around the globe.

2A. How is cheese-making different at artisanal and mass production scales?

Both artisanal and mass production cheese-making methods use the same basic steps (See p 2-4). In both cases, commercial suppliers of starter cultures employ quality control measures to ensure that the bacteria introduced to the cheese-making process has consistent activity. Artisanal cheese-making typically uses minimally processed milk from livestock in a closed herd along with traditional utensils and hands-on methods. Meanwhile, mass-produced cheese is made from pooled milk from many herds and in automated, mechanized plants. Artisanal cheese-makers can adjust their techniques during production runs to accommodate changes in milk composition and fermentation rates, while variations in milk composition are minimized prior to industrial-scale production runs. Because milk and starter activity are standardized, there is no need during such runs to adjust for differences in milk composition. Consequently, the time scale for mass-produced cheese-making tends to be shorter compared to artisanal cheese-making. For example, artisanal Cheddar-making is approximately a 6-hour process while, for mass-produced Cheddar, processing takes only 3.5 hours.

During the past 15 years, interest in artisanal cheese-making has been renewed in the US and globally. Small cheese-making farms are returning to the landscape and helping to stimulate local economies while furthering time-honored traditions. In 2012, there were approximately 800 artisan cheese producers, nearly double the number of artisan cheese producers in the US in 2007 (1, 14, 15, 16). Artisanal cheese-making has the potential to revitalize farms, provide new jobs, and develop new cheese varieties with unique flavors for consumers to experience (1, 16). Artisanal cheeses are value added products that provide small dairy farmers with a viable source of income beyond what they can expect from selling milk.

Both artisanal and mass production cheese-making methods use the same basic steps

Figure

Both artisanal and mass production cheese-making methods use the same basic steps.

3. What does microbiology have to do with cheese?

Louis Pasteur (1822-1895), the French microbiolo-gist and chemist, was central in demonstrating the existence of microbes and that microorganisms are responsible for many common phenomena associated with food, including the. fermentations that produce beer, wine, and cheese. He also showed that these microscopic organisms could be inactivated by heating liquids such as milk followed by rapid cooling, a process that later became known as pasteurization. Pasteur’s work was crucial in showing the importance of microbes to our daily lives and that by learning more about these microbes we could exert greater control over our environment and also endeavors such as medicine and trade in perishable foods. His studies on using heat to inactivate microbes revolutionized large-scale cheese-making, providing producers with a process for ridding milk of pathogens and unwanted spoilage organisms.

Without microbes, we would not be able to enjoy many wonderful and varied foods and drinks, including chocolate, tea, kimchi, bread, beer, wine, and cheese. Microorganisms transform the natural sugars and proteins in foods such as milk, vegetables, and grains into very different foods and drinks that are even more complex and interesting in flavor and taste.

Competition and cooperation among microbes (also known as microbial ecology) play a significant role in the conversion of milk to cheese. Successions of various bacterial species are required to produce all ripened cheeses. Some cheeses are produced by an orderly succession of many different microbes, a process that is like the passing of a baton between successive runners during a relay race (Figure 4). In most cheese varieties, the cheese microbiota is initially dominated by the starter culture; in some of the more complex cheese varieties, the starter microbiota is subsequently dominated by yeasts, and then surface mold/bacteria microbiota take over to facilitate ripening and aging of the cheese. Yeasts are known for their neutralizing activity and are commonly applied to the outside of washed rind cheeses to neutralize the surface pH, enabling bacteria such as Brevibacterium linens to grow there.

Figure 4.. Overview of the microbial succession of a natural rind cheese.

Figure 4.

Overview of the microbial succession of a natural rind cheese.

3A. What is the role of the starter culture?

The term “starter culture” is derived from the fact that these bacteria “start” the fermentation process required for cheese manufacture. The starter culture is a selectively characterized group of bacteria that are intentionally added to the collected milk. Their primary purpose is to convert lactose in milk into lactic acid. In addition, the starter culture contributes directly to flavor development through production of enzymes and metabolites. Cheese-makers can purchase industrially produced starter cultures, or they can rely on the microbes naturally present in raw milk to initiate fermentation. Reliance on industrial starter cultures usually results in more reproducible acid and flavor development.

The added starter cultures dominate the cheese microbiota, establishing conditions that select for the next microorganisms that will be capable of thriving in the changing cheese matrix. The starter culture changes the cheese microenvironment, affecting a variety of factors, including pH, redox potential, levels of organic acids such as lactate and acetate, and other nutrients. As some of the lactic acid bacteria (LAB) within the starter culture begin to die, the cells release enzymes that further break down milk proteins, mainly casein, to small peptides and amino acids. In fact, these dead starter culture cells and debris are an important food source for subsequent generations of microbes, referred to as non-starter lactic acid bacteria (NSLAB). If the starter culture is not dying, then the cheese is not growing. Precisely which organisms will comprise the second and future microbial ecologies will depend on conditions such as salt concentration and nutrients present, and which microorganisms are present (either intentionally added or contaminants) in the evolving cheese matrix.

Starter cultures are classified as either mesophilic, which means temperate-loving, or thermophilic, meaning heat-loving, based on their respective optimal temperatures for growth and acid production. These organisms, in turn, dictate the temperatures used in the cheese-making process (See p 22). Overall, there is a great variety of starter cultures to facilitate the making of a wide selection of cheeses (Figure 5) (17).

Figure 5.. Colored scanning electron micrographs of representative cheese starter (A-C) and nonstarter (D-F) lactic acid bacteria (LAB).

Figure 5.

Colored scanning electron micrographs of representative cheese starter (A-C) and nonstarter (D-F) lactic acid bacteria (LAB). Species shown include Lactobacillus helveticus (A), Lactobacillus delbrueckii subspecies bulgaricus (B), Lactococcus lactis ( (more...)

3B. How do microbial communities shape the development of a cheese as it ages?

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Why do some cheeses melt and other cheeses do not?

Figure 6.. Different cheeses have different melting properties as evidenced by the grilled cheese sandwich (A) versus the clumpy Paneer cheese (B).

Figure 6.

Different cheeses have different melting properties as evidenced by the grilled cheese sandwich (A) versus the clumpy Paneer cheese (B). Acid production influences how much calcium is bound to casein, via phosphate groups. The more acid that accumulates, (more...)

Cheese is created by orderly successions of microbial communities that produce compounds responsible for cheese flavor. For some cheese varieties such as Romano, enzymes are added to create a specific flavor. For centuries, traditional cheese-makers relied on microbes that occur naturally in milk and the cheese-making environment to produce their cheeses. Today, cheese-makers typically add specially prepared starter cultures and defined secondary cultures to their cheese mixtures. Regardless, cheese-makers strive to create controlled, reproducible environments and ecosystems for the microbes to produce the flavor compounds desired for the type of cheese being manufactured.

The metabolic activities of the cheese microbiota change conditions (e.g. microbial nutrients and concentration of lactate) within the cheese matrix. These changes allow other microorganisms to thrive and, eventually, to dominate the cheese microbiota. This process is referred to as microbial succession and an example is the development of the characteristic appearance of Swiss cheese (Figure 7). The starter culture used in Swiss cheese-making reduces the pH to a target value that is on the high end of the pH range for rennet coagulated cheeses. The salt level for such cheese is also deliberately kept low. These conditions (high pH and low salt) and adjustments made by the cheese-maker during ripening set up a milieu ideal for the addition of a secondary culture containing the specific bacterial species, Propioni-bacterium freundenreichii. In this environment, the P. freundenreichii bacteria convert some of the lactic acid produced by the starter culture into propionic acid, which contributes to the unique flavor of Swiss-type cheeses, along with acetic acid, and carbon dioxide gas (CO2). This gas collects at weak spots in the cheese matrix, building up pressure until holes form (1, 18). If for some reason the P. freundenreichii fail to grow in the cheese and these “eye” holes do not form, the cheese is referred to as being “blind.”

Figure 7.. Swiss cheese and its characteristic holes created by the action of Propionibacterium freundenreichii bacteria.

Figure 7.

Swiss cheese and its characteristic holes created by the action of Propionibacterium freundenreichii bacteria. P. freundenreichii grows during ripening at 24°C and ferments lactate to acetate, propionate, and CO2. The propionate and acetate provide (more...)

3C. How do microbes affect the flavor, aroma, texture, and color of different kinds of cheese?

Milk composition, steps and techniques used during the process, and the varieties of microbes being used dictate the flavor, aroma, texture, and color of the final cheese product. The microbes are particularly important because their metabolic activity produces hundreds of compounds from the protein and fat components in milk that affect the flavor, aroma, texture, and color of cheese.

FLAVOR

Flavor is perceived by the mouth and, more specifically, receptors on the tongue. The complex flavor characteristics of a cheese typically reflect the activities of several or many strains and species of microorganisms. For example, the white surface mold of a bloomy rind cheese such as Brie or Camembert is due to a complex ecosystem involving at least four different microbes, Penicillium camemberti, Geotrichum candidum, Kluyveromyces lactis, and Debaryomyces hansenii (Figure 8). These microbes impart the flavors and aromas that are characteristic of bloomy rind cheeses. One of them, G. candidum stimulates sulfur flavor, reduces bitterness, and influences the thickness and texture of the rind. The white, bloomy appearance of the rind arises mainly from the activities of P. camemberti (18, 19). The overall flavor is produced as a result of the combined metabolic activities of these microbial species.

Figure 8.. Classic, French style Brie.

Figure 8.

Classic, French style Brie. This cheese possesses a thin, bloomy rind with a gooey middle core. Many flavors such as roasted nuts and cauliflower are elicited by this cheese during peak ripeness.

AROMA

The distinct aromas of cheese, detected in the nose, are due mainly to a special set of volatile organic compounds, or VOCs, that result from microbial metabolic activities and enzymatic reactions during cheese-making and ripening (18). For example, the distinctive aroma of Cheddar cheese is due to more than 500 different compounds produced by the microbes that are used in producing this cheese. Cheeses elicit a smorgasbord of aromas that range from lightly aromatic to bold and strongly overpowering. The smell of a cheese depends on microbial and enzymatic activities changing the casein and fats in milk and also on the microbiota associated with a cheese variety further refining those aromas as the cheese ripens. In general, the more extensively the casein and milk fat are broken down, the stronger the aroma of the final cheese product. The smell of a cheese is also a direct result of the ripening process and length of time the cheese is given to age. Depending on the type of cheese being made, ripening can range from a few months to several years. The smell of a cheese is a key factor in enjoying its flavor. Reflecting the importance of those aromas as well as their complexity, cheese vendors and connoisseurs depend on a broad variety of descriptive terms to describe the smells of cheese (Figure 9). See the list on page 19 (Table 1) for smell descriptors paired with cheese examples.

Table 1.. Various aromas that cheese can elicit.

Table 1.

Various aromas that cheese can elicit.

The smell of a cheese is a key factor in enjoying its flavor.

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Figure

SENSE OF SMELL

Figure 9.. Common descriptors of cheese divided into six families and displayed in a flavor wheel (not all flavors/aromas are displayed).

Figure 9.

Common descriptors of cheese divided into six families and displayed in a flavor wheel (not all flavors/aromas are displayed). These characteristics are a result of the complex ecosystems created among the microbes (bacteria, yeast, and filamentous fungi) (more...)

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CIGC (Comité Interprofessionnel du Gruyère de Comté), the professional organization based in Poligny, France is responsible for Gruyere de Comte, France's largest AOC (controlled designation of origin) cheese in terms of production. (more...)

COLOR

Cheese can range in color from white to ivory, yellow, golden, orange, gray, and blue-streaked. The color of cheese may be due to added colorants or natural pigments such as carotene that come from the grasses that the milk-producing animal eats. Additionally, the microbes involved in the ripening process can affect the color of a cheese, producing pigments as part of the microbial interactions with ingredients in the cheese (Figure 10A). The creation of blue cheese, for example, requires human intervention during the aging process (Figure 10B). Thus, the blue veins of this cheese are due to the growth of a mold, known as Penicillium roqueforti, that will not grow without oxygen. Thus, cheese-makers use long needles to pierce wheels of the ripening cheese, allowing oxygen into the interior of the cheese and enabling P. roqueforti to grow in those channels. Meanwhile, the orange-tinted bacterium Brevibacterium linens gives the color and aroma to the rinds of Limburger and other surface ripened cheeses (1).

Figure 10.. A) Oma, a wash rind cheese with a thin orange rind and occasional white flora.

Figure 10.

A) Oma, a wash rind cheese with a thin orange rind and occasional white flora. The orange-whitish rind is produced by Brevibacterium linens and the yeast-like fungus Geotrichum candidum. B) Baley Hazen Blue cheese, highlighting the blue-green veins. Cheese-makers (more...)

Furthermore, non-toxic bleaching/coloring additives such as titanium dioxide, hydrogen peroxide, benzoyl peroxide, and annatto can be added to milk to alter the color or otherwise improve the appearance of some cheeses. Depending upon the season and breed of cow, the milk produced contains a variable amount of carotene content that can result in Mozzarella cheese appearing yellow. However, when titanium dioxide is added to cow’s milk, the resulting Mozzarella is whiter, making it look more like water buffalo Mozzarella. Water buffalo milk is naturally white because it contains little or no carotene (20).

TEXTURE

The texture of a cheese depends on the initial process during which the milk acidifies, the duration of ripening, and the changing moisture content of the cheese. These factors control how much calcium will be retained in the cheese and the extent to which the milk proteins break down. Typically, the older the cheese, the softer or creamier its texture will become, although this varies and depends on the cheese type. For example, Camembert is ripened for a relatively short time but is soft and creamy, while Parmesan is ripened for two years and is very hard and brittle. The amount of moisture remaining in a cheese determines whether it will be creamy, smooth, firm, or hard. Harder cheeses contain less moisture in comparison to softer cheeses. Therefore, natural cheeses can be categorized according to their degree of hardness or softness (Table 2).

Table 2.. Five categories of cheese based on texture.

Table 2.

Five categories of cheese based on texture.

Typically, the older the cheese, the softer or creamier its texture will become…

A cheese’s texture can also be affected by crystal formation. The ratio of salt to moisture influences the final pH that can cause crystals to form on and within the cheese (21) (Figure 11). Crunchy crystals can form in aged, hard cheeses. In some cheeses, such as Comté and Parmigiano-Reggiano, this characteristic can be regarded as desirable. However, gritty crystals are considered a costly defect in other cheeses such as Cheddar.

Figure 11.. Electron micrograph of Bayley Hazen Blue showing crystals of calcium phosphate interspersed with Penicillium spores.

Figure 11.

Electron micrograph of Bayley Hazen Blue showing crystals of calcium phosphate interspersed with Penicillium spores.

3D. Are certain types of microbes associated with specific kinds of cheese?

The identity of most cheese types is based on the microbial communities that make the cheese. Bacterial species that withstand temperatures as high as 132°F during fermentation are referred to as thermophilic. Thermophilic species such as Streptococcus thermophiles, Lactobacillus helveticus, and Lactobacillus delbrueckii ssp. are associated with Swiss- and Italian-type cheeses. Bacterial species can also be classified as mesophilic because these microorganisms ferment lactose only at 105°F or less. Examples of mesophilic species include Lactococcus lactis and Leuconostoc mesenteroides, both of which aid in Cheddar and Gouda production (1, 22).

The identity of most cheese types is based on the microbial communities that make the cheese

Figure

The identity of most cheese types is based on the microbial communities that make the cheese.

4. Where do all of these microbes come from and how are they introduced during the cheese-making process?

Microbes associated with cheese production can be introduced deliberately (i.e., added as a starter culture) or unintentionally (i.e., by people, animals such as insects and rodents, and production equipment) (Figure 12). Not surprisingly, virtually all the starter cultures used in cheese manufacture are species that were originally isolated from cheese or milk. Microorganisms that can inadvertently enter the cheese-making process can come from the soil, feces, bedding materials, and feed (i.e., silage). These microbes can attach to the milking animals’ teats and enter into the milk upon collection. There are also environmental microbes from the cheese house and aging facilities that can settle on the cheese as the product is being made and as it ages. In fact, some aging facilities develop their own desirable microbial communities symbiotically with the cheese types they produce. Even with thorough sanitation practices, however, some unwanted microorganisms (e.g., NSLAB) can remain on the surfaces of milking and processing equipment and can enter into the product during manufacturing.

Figure 12.. Migration and introduction of microbes throughout cheese production.

Figure 12.

Migration and introduction of microbes throughout cheese production.

5. Are the microbes in cheese good for me?

Humans have been consuming cheese microbes for centuries. Each piece of cheese that we eat may contain as many as 1010 microbes (that amounts to 10,000,000,000 or 10 billion microbes, more microbes than there are people on Earth) (Figure 13). Although the microbes found in cheeses are considered safe to eat, these organisms are considered neither particularly healthful nor harmful. Nevertheless, cheese, like other foods, can become contaminated with other microorganisms, some of which may be harmful for consumers, if it is improperly made or stored. Additionally, consuming certain cheeses can be risky for immunocompromised individuals and for pregnant women when pathogens such as Listeria are present (See p 27 for more information).

Figure 13.. Analysis (electron micrograph) of a cheese rind highlight the incredible number of microbes that are involved in its formation.

Figure 13.

Analysis (electron micrograph) of a cheese rind highlight the incredible number of microbes that are involved in its formation. There are nearly 10 billion cells per gram of cheese rind.

6. What kinds of things can go wrong in cheese-making?

A great deal of both science and craft goes into making cheese. Anything that disturbs this process can lead to defects in quality, some with food-safety implications. It is therefore important that cheese manufacturers are adequately trained and aware of the importance of strictly following time honored processes associated with cheese manufacture. There are training courses for cheese-makers to increase their skills and knowledge base to create consistently high-quality, safe cheese products. The key to quality cheese manufacture is maintaining control over the process regardless of the scale of operation. Parameters such as initial milk composition, temperature, moisture level, pH, the size of curd particles at cutting, stirring rates, and levels of salt addition all influence the cheese-making process and the quality of the resulting cheese. The activities of microbes during every step of the process are of paramount importance to the production of quality cheese, and controlling these microbial activities is in many ways the most difficult challenge faced by the cheese-maker. The activity of the starter culture is particularly important as the rate and level of acid production contributes to a very large extent in defining the final composition of the cheese and the subsequent communities that help to create it. Inadequate process control encompassing all details of the manufacturing process is the primary problem that can lower the quality of cheese. Therefore, large-scale cheese-makers must employ adequate testing, and small-scale cheese-makers must pay close attention to consistency with their particular cheese-making procedures. Additionally, bacteriophages, viruses that infect only bacteria but not humans, can also contaminate the microorganisms involved in cheese-making and inhibit the initial fermentation step (1). These viruses are hardy, and may be found naturally on our skin and within our intestines as well as in the environment, water supply, and in many foods (See p 28 for more information).

A great deal of both science and craft goes into making cheese

Figure

A great deal of both science and craft goes into making cheese.

6a. Can pathogens grow in cheese and can they make me sick?

The occurrence of pathogens in cheese that cause disease in humans is very rare, especially if the cheese is properly made. However, some pathogens can survive and grow in cheese when particular production standards are not met. How they contaminate the cheese varies widely but sources of those pathogens include the milk, the processing environment, and people working in the cheese plant. Consumers also need to be considered when weighing these risks because each consumer is different and carries a different degree of vulnerability when encountering pathogens in cheese. For example, because fresh, non-ripened cheeses such as Queso Fresco and wash rind cheeses have higher moisture contents and a higher pH than do other cheeses, these products are more prone to be contaminated with Listeria and to support its growth (See more on Listeria on p 27). Contaminated soft cheeses may therefore pose a risk to people that are particularly vulnerable to Listeria infections, including pregnant women, individuals who are immunocompromised, and the elderly. Listeria contamination can be a consequence of improperly pasteurized milk, improper cooling of the milk, or post-pasteurization contamination in the cheese plant or in the homes of consumers. Cheese-makers make every attempt to manage these and other safety risks associated with cheese production. Interestingly, there are some cheeses such as Parmigiano-Reggiano, English Cheddar, and Gruyère, that do not support the growth of pathogens if correctly manufactured. These cheeses are dry, acidic (low pH), and have a high salt content, all traits that inhibit pathogen growth.

The occurrence of pathogens in cheese that cause disease in humans is very rare…

6b. How can contaminating microbes affect the cheese outcome?

Both desirable and undesirable microbes are introduced to cheeses during production. The desirable microbes include LAB in the starter culture and the secondary microbiota that dominate during cheese ripening and aging. Depending on the characteristics of the individual contaminants, they can accelerate typical flavor development, have no direct impact on flavor development, or can lead to development of off-flavors. Undesirable contaminating microbes are any microbes that are not intentionally introduced during the cheese-making process that lead to development of off-flavors such as bitterness and rancidity, poor texture, unappetizing appearance, or pose health risks from consuming the cheese produced.

6c. How are contaminating microbes controlled during cheese-making to preserve both the flavor and safety of cheese?

Listeria monocytogenes is one of the main undesirable microbial contaminants which the cheese industry must address. It can cause a very serious illness known as listeriosis that has a mortality rate of 20-30% (23). L. monocytogenes is widespread and can survive for extended periods in the environment among soils, vegetation, and water, and can also be found on the surfaces of equipment, walls, and floors in farms and cheese-production facilities, sometimes hiding in the nooks and crannies of milking and cheese-making devices. Because Listeria is associated with vegetables, it can also hide in home refrigerators. Many measures have been implemented by cheese-making regulators to prevent contamination. As a general rule of thumb to prevent cheese-making contamination, the “3Ks” must be followed: Keep the pathogens (undesirable microbes) out (sanitation), Kill them (with heat or antimicrobial treatments), and Keep them from growing (temperature, salt content). Extensive post-processing monitoring of cheeses along with compositional testing analysis also helps to control contamination. There are also several new technologies that help with contamination control (See Text box 4).

Box Icon

Text box 4.

Technologies used for contamination control in cheese-making.

6d. How is cheese-making regulated?

In the US, 15 federal agencies share responsibilities for regulating food safety (24). Among them, the Food and Drug Administration (FDA) has regulatory jurisdiction over cheese and stipulates that cheese be made in one of two ways (1):

  1. Use milk that is pasteurized to FDA standards.
  2. If made from raw (unpasteurized) milk, cheese must be aged for a minimum of 60 days at a temperature no less than 35°F.

Cheeses produced in the US must meet strict sanitation and quality standards that also are specified by the US FDA. From the initial step of milking animals to the shipment of cheeses to consumers, the product is subjected to rigorous monitoring, quality assurance standards, and specific tests. It is estimated that approximately 30% of the world’s cheese production comes from more than 450 cheese-makers in the US, making the US the largest cheese-producer in the world (1). In addition to FDA oversight, state agencies, which may collaborate with federal agencies, also play an active role in ensuring that cheese safety is enforced in their respective states. Many professional societies worldwide support cheese safety and sustainability by offering training courses and annual certification exams for cheese-makers, helping them to become more competent in their trade.

The US cheese industry is also expanding its exports, reflecting in part the high quality and safety of the cheeses produced in the US. According to the United States Dairy Export Council (USDEC), cheese exports dramatically increased from 12,000 metric tons in 1991 to more than 123,000 metric tons in 2008 (25). In 2011, the FDA implemented the Food Safety Modernization Act (FSMA) which aims to prevent food contamination and enforce stricter risk-based food safety standards. Additionally, the FSMA states that all cheese imported to the US must conform to the FDA’s standards (1).

Quick Facts about Cheese
  • Cheese-making is an art that has been in practice for more than 9000 years. The cheeses that we enjoy today are a testament to cheese-makers throughout history for their persistence, originality, and ability to modify their skills based on their culture, climate, geography, and available technology and equipment.
  • Cheese is a living food and microbes are essential for its production; hence, “Microbes Make the Cheese.”
  • Cheese, which is produced by concentrating and changing the proteins (caseins) and fats in milk, depends on the actions of microbes to drive this transformation.
  • The manufacturing and ripening of cheeses results from microbial successions that can include bacteria, yeasts, and filamentous fungi (molds).
  • The key steps in the manufacturing of cheese include milking, fermentation, coagulation, curd and whey separation, salting, and ripening.
  • Microbes contribute to the final flavor, smell, texture, and color of cheese. Specific microbes impart the characteristics of particular cheeses (i.e., holes in Swiss cheese).
  • Extensive contamination control and quality assurance tests are maintained throughout every step of the cheese-making process for all US cheese-makers. Newer technologies are available to help in minimizing and eliminating contamination.
  • In the US, the FDA, state, and local regulators oversee the safety of cheese.
Box Icon

Text box 5.

Future research needs and questions in cheese microbiology.

Image makethecheese.June.2014.fig25

Image retrieved from: p 30) Henry Le Monnier (1893 - 1978)

Key terms used throughout text

Acidification

: Acidification is the production within or addition of an acid to the milk fermentation mixture, lowering its pH. Most commonly, cheese acidification is due to production of lactic acid from lactose via fermentation that depends on a starter culture containing lactic acid bacteria (LAB). Alternatively, the pH is lowered by adding food items such as citric acid, lemon juice, or vinegar, which are known as acidulants.

Casein

: A milk protein that gels or solidifies during coagulation.

Coagulation

: An enzymatic process that modifies the casein proteins, causing them to clump together and precipitate out of solution, creating a solid or gel-like structure.

Draining

: A process that removes and separates liquid whey from curds.

Fermentation

: A microbial process that depends on metabolic enzymes to break down complex substances into simpler ones that may be safer to consume and, typically, can be stored for longer than could the original material. This process also may lead to acidification of a substance as occurs when lactic acid bacteria (LAB) convert lactose sugar to lactic acid. Fermentations can also produce carbon dioxide (CO2) and flavor compounds in cheeses or other materials.

Microbial succession

: Changes that occur within and among populations of microorganisms, including bacteria, yeasts, and molds, during the ripening of cheeses.

Organoleptic properties

: The sensory properties of a food or chemical, including taste, color, appearance, odor, and feel.

Pasteurization

: Mild heating process, invented by Louis Pasteur, that destroys pathogens and spoilage microorganisms in foods such as milk (1).

Rennet

: Rennet is an enzyme preparation from animals, plants, or microbes that breaks down casein proteins in milk, leading them to coagulate. Traditionally, rennet was extracted by bathing the 4th stomach of calves less than 12 months old (or other mammals such as camels) in brine to remove this enzyme. Currently, calf rennet is obtained by processing frozen stomachs in a production setting. Chymosin, also referred to as rennin, is the specific enzyme in rennet that is primarily responsible for degrading caseins and causing them to coagulate. Although plant and microbial rennets are used in making some traditional cheeses, calf-based rennets are widely thought to yield the highest quality cheeses (1).

Rind

: The outer layer of a cheese that can form during the ripening process.

Ripening

: A maturation or aging step after initial cheese-making that imparts distinguishing flavors, aromas, and textures to particular cheeses.

Syneresis

: The process that allows curds to contract and expel whey, removing water and other dissolved ingredients from the gel to achieve the desired texture and moisture content of the cheese being made.

Further Reading and Information

1.
ASM Microbiology of Cheese Broadcast June 11, 2014. https://www​.youtube.com​/watch?v=nmXKhVsIItU
2.
Caldwell G. 2012. Mastering Artisan Cheesemaking: The Ultimate Guide for Home-Scale and Market Producers. Chelsea Green Publishing.
3.
Cheese and Microbes (Microbe World Video 28) with Dr. Catherine Donnelly http://www​.microbeworld​.org/podcasts/microbeworld-video​/archives​/429-cheese-and-microbesmwv28.
4.
Donnelly CW. 2014. Cheese and Microbes. ASM Press.
5.
Fletcher J. 2007. Cheese & Wine: A Guide to Selecting, Pairing, and Enjoying. Chronicle Books.
6.
Harbutt J. 2009. The World Cheese Book. DK Publishing.
7.
Herbst ST. 2007. The Cheese Lover’s Companion: The Ultimate A-to-Z Cheese Guide with More Than 1,000 Listings for Cheeses and Cheese-Related Terms. William Morrow Cookbooks.
8.
Kindstedt P. 2012. Cheese and Culture: A History of Cheese and its Place in Western Civilization. Chelsea Green Publishing.
9.
McCalman M. 2009. Mastering Cheese: Lessons for Connoisseurship from a Maître Fromager. Clarkson Potter.
10.
McCalman M. 2005. Cheese: A Connoisseur’s Guide to the World’s Best. Clarkson Potter.
11.
Monnet JC, Berodier F, Badot PM. 2000. Characterization and Localization of a Cheese Georegion Using Edaphic Criteria (Jura Mountains, France). Journal of Dairy Science 83:1692-1704. [PubMed: 10984144]
12.
Roberts J. 2007. The Atlas of American Artisan Cheese. Chelsea Green Publishing.
13.
Teubner C. 2009. The Cheese Bible. Chartwell Books, Inc.

References

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Donnelly CW. 2014. Cheese and Microbes. ASM Press, Washington, D.C.
2.
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3.
United States Dairy Export Council. http://www​.usdec.org/home​.cfm?navItemNumber=82205. Accessed 17 June 2014.
4.
The Cheese Nun: Sister Noella’s Voyage of Discovery. 2006. PBS Home Video. Alexandria, VA.
5.
Clark S, Drake M, Costello M, Bodyfelt F. 2009. The Sensory Evaluation of Dairy Products. Springer Science New York, NY.
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Paxson H. 2013. The Life of Cheese: Crafting Food and Value in America. University of California Press, Berkeley, CA.
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Broadbent JR, Steele JL. 2005. Cheese flavor and the genomics of lactic acid bacteria. ASM News 71:121-8.
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Button JE, Dutton RJ. 2012. aCheese microbes. Curr Biol 22:R587-9. [PubMed: 22877773]
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Marie-Hélène L, Viel C, Boyle B, St-Gelais D, Labrie S. 2014. Metatranscriptome analysis of fungal strains Penicillium camemberti and Geotrichum candidum reveal cheese matrix breakdown and potential development of sensory properties of ripened Camembert-type cheese. BMC Genomics 15:235. [PMC free article: PMC3986886] [PubMed: 24670012]
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Johnson ME, Steele JL. 2007. Fermented Dairy Products, p 767-781. In Doyle MP (ed), Beuchat LR (ed), Food Microbiology: Fundamentals and Frontiers (3rd Ed). ASM Press, Washington, D.C.
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Front cover bacteria image courtesy of Dr. Jeff R. Broadbent, Utah State University

Copyright 2015 American Academy of Microbiology.

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Bookshelf ID: NBK562892PMID: 33054144DOI: 10.1128/AAMCol.June.2014

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