Essential Building Blocks from Milk
At its core, cheese is a concentrated form of milk, with the most significant chemical players being its proteins, fats, and sugars.
Casein Protein: The Structural Foundation
Casein is the primary protein in milk, forming tiny spherical clusters called micelles. These micelles are what give milk its white, opaque appearance. In cheesemaking, enzymes like rennet or acids are used to destabilize these micelles, causing the casein to coagulate and form solid curds. This solid curd is the base of all cheese. The breakdown of casein during the aging process, known as proteolysis, is crucial for developing flavor and texture. Hard cheeses, for example, have a higher concentration of this protein relative to their moisture content.
Milk Fats: Carriers of Flavor
Milk fats, primarily triglycerides, are trapped within the casein matrix during coagulation. The enzymatic breakdown of these fats, a process called lipolysis, releases fatty acids that are vital for flavor development. This is particularly true for cheeses with piquant or sharp flavors, like aged Italian cheeses. The fat content also heavily influences the final cheese's texture and mouthfeel.
Lactose: Fuel for Fermentation
Lactose, the sugar in milk, is the primary food source for the starter cultures of bacteria. These bacteria ferment the lactose, producing lactic acid. This acidification is a fundamental chemical step that preserves the cheese and aids in curd formation. In aged cheeses, most of the lactose is converted, resulting in a product that can be low in or free of lactose.
The Role of Microbes and Enzymes
Beyond the initial milk components, microbial cultures and added enzymes are responsible for the complex chemical transformations that create the diversity of cheese.
Lactic Acid Bacteria
Starter bacteria like Lactococcus and Lactobacillus are critical for converting lactose to lactic acid, a process that lowers the pH and helps set the curd. The specific type of bacteria used greatly influences the final flavor profile. In Swiss cheeses, for example, Propionibacterium freudenreichii ferments lactic acid to produce propionic acid and carbon dioxide gas, which creates the characteristic holes or "eyes".
Rennet and Other Enzymes
Rennet, a complex of enzymes traditionally sourced from calf stomachs, is used to coagulate milk. The primary active enzyme, chymosin, specifically cleaves casein proteins, forming a strong, rubbery curd. Modern cheesemaking often uses genetically modified microorganisms to produce a more cost-effective and vegetarian-friendly rennet. Enzymes, both from rennet and microbial cultures, continue to break down proteins and fats during ripening.
Molds and Rind-Washed Bacteria
Certain cheeses rely on molds or other surface bacteria for their unique character. For instance, blue cheeses like Roquefort owe their pungent aroma and distinct veining to the mold Penicillium roqueforti, which breaks down fats into aromatic compounds like methyl ketones. Soft-ripened cheeses like Brie and Camembert use Penicillium camemberti to ripen from the outside in, while pungent, smear-ripened cheeses like Limburger rely on surface bacteria like Brevibacterium linens.
Ripening Reactions and Flavor Compounds
The ripening or aging process is where the true magic of cheese chemistry unfolds, as enzymatic and microbial activity generates a huge number of flavor-active volatile compounds. These secondary reactions lead to the specific characteristics of each cheese variety.
Proteolysis and Amino Acid Metabolism
As discussed, proteolysis is the breakdown of proteins. The resulting peptides and free amino acids serve as precursors for many flavor compounds. Different types of starter and non-starter bacteria possess varied peptidases that produce unique amino acid profiles. The subsequent metabolism of these amino acids can produce aldehydes, alcohols, esters, and sulfur compounds, each contributing to the cheese's signature taste and aroma.
Lipolysis and Fatty Acid Derivatives
Following lipolysis, the released fatty acids can be further broken down. In blue cheeses, for example, the mold Penicillium roqueforti oxidizes fatty acids into methyl ketones, which produce the characteristic blue-cheese aroma. Different enzyme systems and ripening conditions create a wide range of flavor notes, from buttery to soapy, depending on the specific fatty acids and their derivatives.
The Maillard Reaction
In some aged cheeses, a chemical reaction between amino acids and reducing sugars, known as the Maillard reaction, contributes to nutty, caramelized flavors and browning. This non-enzymatic reaction is a complex series of pathways that generate hundreds of volatile compounds, adding to the cheese's complexity.
Comparison Table: Chemical Compounds by Cheese Type
| Feature | Hard Cheeses (e.g., Parmesan) | Soft-Ripened Cheeses (e.g., Brie) | Blue Cheeses (e.g., Roquefort) |
|---|---|---|---|
| Dominant Protein | Highly concentrated casein due to low moisture. | Less concentrated casein, more hydrated, breaking down during ripening. | Casein with significant breakdown from mold enzymes. |
| Key Flavor Compounds | High levels of amino acids, peptides, and fatty acid derivatives, Maillard reaction products. | High levels of peptides, amino acids, and ammonia from mold ripening. | Distinctive methyl ketones from mold activity breaking down fat. |
| Microbial Activity | Thermophilic starter bacteria and non-starter lactic acid bacteria (NSLAB) active during prolonged aging. | Penicillium camemberti mold ripens from the surface inward, increasing pH. | Penicillium roqueforti mold spores injected to ripen internally. |
| Salt Content | Often high, contributing to flavor and low moisture levels. | Moderate; salt is added to the rind or brine. | Higher than other varieties to inhibit unwanted molds and control flavor. |
Conclusion
From the moment milk is first curdled, a fascinating cascade of chemical events begins, orchestrated by proteins, fats, bacteria, and enzymes. What starts as a simple liquid is transformed into a diverse and complex matrix through proteolysis, lipolysis, and fermentation. These chemical reactions are responsible for everything that makes cheese so varied and beloved—the texture of a hard Parmesan, the creamy mouthfeel of a Brie, and the pungent aroma of a blue cheese. Understanding the chemistry of cheese offers a deeper appreciation for this ancient and delicious food. For further information on the scientific processes, the Dairy Processing Handbook provides an extensive overview of cheesemaking procedures and chemical transformations.
