The transformation of simple milk into the complex, diverse food known as cheese is an ancient process rooted in biochemistry. This conversion relies primarily on the metabolic activity of microorganisms. These tiny helpers, mainly bacteria and fungi, develop the characteristic flavors, textures, and aromas of every cheese variety. Understanding the microbial involvement illuminates the science behind a craft often viewed as pure culinary art.
The Essential Role of Starter Cultures
The cheesemaking process begins with the deliberate introduction of starter cultures, which are specialized groups of Lactic Acid Bacteria (LAB). These bacteria, including genera like Lactococcus, Streptococcus, and Lactobacillus, initiate the first and most fundamental biochemical change: acidification. They consume lactose, the natural sugar present in milk, and convert it into lactic acid in a process called fermentation. This rapid production of acid lowers the milk’s pH, which is the necessary trigger for casein proteins to coagulate and form a solid curd.
Mesophilic cultures thrive best in moderate temperatures (between 20°C and 39°C) and are used for cheeses like Cheddar and Gouda. Thermophilic cultures function optimally at higher temperatures (from 40°C to 55°C) and are used for cheeses like Mozzarella and Parmesan. The rate and extent of acid production directly influence the curd’s final moisture content and texture. A high acid environment causes the curd matrix to demineralize, resulting in a more fragile, higher-moisture, and crumbly texture.
Beyond initial acidification, LAB drive long-term changes in flavor during aging. Their enzymes are responsible for proteolysis (the breakdown of milk proteins into smaller peptides and amino acids) and lipolysis (the degradation of milk fats). These smaller compounds are flavor precursors, and their further metabolism creates aromatic molecules that define the final taste profile of the ripened cheese.
Beyond Bacteria: The Fungi of Cheesemaking
Once the bacterial cultures have established the curd structure, fungi, primarily molds and yeasts, contribute to distinctive cheese characteristics. These organisms are introduced either directly into the curds or onto the cheese surface. They flourish in the post-acidification environment and are responsible for softening the texture and developing pungent aromas.
The characteristic blue veins in cheeses like Roquefort and Stilton are the result of the mold Penicillium roqueforti, which is often added to the milk early in the process. This mold is a powerful agent of lipolysis, breaking down fats into compounds like methyl ketones, which impart the sharp, peppery flavor and aroma associated with blue cheeses. For soft-ripened cheeses like Brie and Camembert, the surface is inoculated with Penicillium camemberti (also known as P. candidum), sometimes in conjunction with the yeast Geotrichum candidum.
Penicillium camemberti works from the outside in, producing enzymes that degrade the casein protein matrix beneath the rind. This action softens the cheese body to a buttery, yielding texture. The surface molds and yeasts also neutralize the acidity at the rind, raising the pH and creating a unique microenvironment that encourages the growth of other bacteria and results in the distinct, earthy flavor of bloomy-rind cheeses.
Controlling the Ecosystem for Flavor and Texture
Cheesemakers manipulate the environment to select for and control the activity of the desired microorganisms. The conditions maintained during production and aging are tuned to encourage the metabolism of specific starter cultures and secondary flora, ultimately determining the final cheese style. Temperature is a powerful control; most cheeses are aged at 10–15°C. Warmer conditions accelerate microbial growth leading to off-flavors, while colder temperatures slow the beneficial ripening process.
Salt content is another control mechanism that influences the microbial ecosystem. A higher salt concentration inhibits the growth of many undesirable bacteria and helps to regulate the metabolic activity of molds like Penicillium roqueforti. Humidity is tightly managed in aging rooms, ranging from 75% to 95% relative humidity. This is necessary to prevent excessive moisture loss and to encourage the growth of surface molds and yeasts like those found on Brie and Camembert. This control over temperature, salt, and moisture allows cheesemakers to guide the biochemical reactions, ensuring the appropriate microbial species flourish to create the vast spectrum of cheese flavors and textures.