Cheese, enjoyed globally, owes its diverse forms and flavors to microscopic organisms, primarily bacteria. These microbes are not contaminants but purposeful additions, chosen for their ability to transform milk. They are living ingredients that initiate and guide the complex biochemical changes required to convert liquid milk into the solid, flavorful product known as cheese. This reliance on microbial action underscores their significance in cheesemaking.
The Fundamental Role of Bacteria in Cheese
Bacteria begin the cheesemaking process by converting lactose, the natural sugar in milk, into lactic acid. This conversion causes the milk’s pH to drop, making it more acidic. The increased acidity causes milk proteins to destabilize and coagulate, forming a gel-like substance known as curd.
The formation of this curd is the initial separation of milk solids from the watery component called whey. As lactic acid production continues and the pH lowers, it aids in the expulsion of whey from the curd, concentrating the milk solids and setting the stage for the cheese’s eventual texture. This early bacterial activity creates the basic structure and acidic environment necessary for subsequent development and ripening of the cheese.
Primary Bacterial Groups in Cheesemaking
Cheesemaking involves several distinct groups of microbes, each contributing uniquely to the final product. Starter cultures, often composed of lactic acid bacteria (LAB) like Lactococcus and Lactobacillus species, are introduced first to initiate rapid acidification. Lactococcus lactis subspecies lactis and cremoris are commonly used as mesophilic starter cultures, thriving in moderate temperatures. Streptococcus thermophilus and Lactobacillus helveticus are examples of thermophilic starters, preferring higher temperatures.
Beyond initial acidification, non-starter lactic acid bacteria (NSLAB) naturally develop during cheese ripening and significantly influence flavor and texture development. These adventitious bacteria, primarily mesophilic lactobacilli such as Lactobacillus paracasei and Lactobacillus plantarum, grow slowly but contribute to the cheese’s unique characteristics over time. Adjunct cultures, like Propionibacterium freudenreichii used in Swiss cheese, are specifically added for distinct flavor and eye formation, working alongside starter cultures.
Surface-ripening cultures, including bacteria and molds, are applied to the exterior of certain cheeses to develop their characteristic rinds and flavors. For instance, Brevibacterium linens contributes to the reddish-orange rind and pungent aroma of washed-rind cheeses like Limburger and Muenster. Molds such as Penicillium candidum create the soft, white bloomy rind on cheeses like Brie and Camembert, while Penicillium roqueforti is responsible for the blue-green veins and piquant flavor in blue cheeses.
Bacterial Impact on Cheese Characteristics
The metabolic activities of cheese bacteria influence the characteristics of the final product, shaping its flavor, aroma, and texture. One process is proteolysis, where bacterial enzymes break down milk proteins into smaller peptides and free amino acids. This breakdown softens the cheese’s texture over time and contributes to its flavor profile, with some peptides leading to bitter notes while others serve as precursors for aromatic compounds.
Lipolysis is another process where bacterial lipases hydrolyze milk fats into free fatty acids. These fatty acids directly contribute to the cheese’s flavor and aroma, with short- and medium-chain fatty acids being impactful. For example, the lipolytic activity of Penicillium roqueforti is responsible for the piquant flavor of blue cheeses.
Bacteria also produce various volatile compounds that define specific cheese aromas. Diacetyl, a product of citrate metabolism by bacteria like Lactococcus lactis subspecies diacetylactis and Leuconostoc citrovorum, imparts buttery notes. In Swiss cheese, Propionibacterium freudenreichii ferments lactate into propionic acid and acetic acid, contributing to its nutty, sweet flavor, and also produces carbon dioxide gas, which forms the distinctive “eyes” or holes. Surface-ripening bacteria like Brevibacterium linens generate sulfur-containing compounds, which give washed-rind cheeses like Limburger and Muenster their pungent aromas.
Beneficial Microbes and Cheese Safety
The bacteria intentionally introduced into cheesemaking are beneficial and play a direct role in ensuring the safety of the final product. These microbes are considered safe for consumption, with many varieties containing live, active cultures that may offer probiotic benefits for gut health.
The growth of these desired bacteria creates an environment that actively inhibits the proliferation of spoilage organisms and harmful pathogens through competitive exclusion. Lactic acid bacteria, for instance, rapidly acidify the milk, lowering the pH to a level that is inhospitable for most undesirable microorganisms. This acidic environment acts as a natural preservative, safeguarding the cheese from spoilage. Furthermore, starter cultures can produce antimicrobial compounds, which directly suppress the growth of pathogens. The stringent hygiene standards in cheesemaking facilities, combined with practices like milk pasteurization, further control microbial populations to ensure product safety.