Fermenting cheese is a complex process that turns liquid milk into a solid food with unique characteristics. The transformation of milk into cheese is a marriage of biology and chemistry, where microorganisms drive a series of reactions that drastically alter the original ingredients. Understanding the nature of this change requires analyzing the molecular shifts that occur during the cheesemaking process.
Understanding Physical and Chemical Changes
A physical change involves an alteration in a substance’s form, state, or appearance without changing its underlying molecular composition. For instance, melting an ice cube into liquid water is a physical change because the water molecules remain the same, just in a different state. These changes are often easily reversible.
In contrast, a chemical change results in the formation of one or more new substances with a completely different chemical makeup than the starting materials. This transformation involves the breaking of old chemical bonds and the formation of new ones. Burning a piece of wood, for example, is a chemical change because the wood is converted into ash, smoke, and heat, and it cannot be turned back into wood. Chemical changes are typically irreversible.
The Process of Lactic Acid Fermentation in Cheese
The initial step in cheesemaking involves introducing specific starter cultures, which are beneficial bacteria, into the milk. These bacteria consume lactose, the primary carbohydrate in milk, and metabolize it through a process called lactic acid fermentation.
This metabolic process converts the lactose into lactic acid, a completely new compound. The production of lactic acid lowers the pH of the milk, making it more acidic. This drop in pH triggers a significant change in the milk’s primary protein, casein, which exists in tiny suspended structures called micelles.
As the acidity increases, the stable structure of the casein micelles destabilizes, causing the proteins to aggregate and precipitate out of the liquid suspension. This results in the formation of a soft, gel-like substance known as curd. For most cheeses, this initial acidification is followed by the addition of rennet, an enzyme that further aids in the coagulation and firming of the curd structure. The complex web of changes that transform the milk’s molecular structure establishes the initial chemical characteristics of the cheese.
Classifying the Change: Why Fermentation is Chemical
Applying the principles of change to the cheesemaking process reveals that fermentation is definitively a chemical change. The formation of lactic acid from lactose represents the creation of a new chemical substance. This conversion involves a complex biochemical reaction driven by the bacterial enzymes, fundamentally altering the sugar’s molecular structure.
Furthermore, the acidification causes an irreversible alteration to the casein protein structure. The casein proteins, once aggregated into a curd, cannot be simply returned to their original, dispersed state in fresh milk. The milk’s composition has been permanently changed; the liquid milk has been transformed into a solid curd and a liquid whey byproduct.
Observable Outcomes of the Chemical Transformation
The chemical transformation initiated by fermentation yields tangible results. The most immediate physical outcome is the change in texture, as the liquid milk is converted into a solid, elastic curd. This solidification is a direct consequence of the casein proteins rearranging under the new, acidic conditions.
The production of lactic acid imparts a tangy or sour flavor to the cheese, which is absent in fresh milk. During the subsequent aging process, the bacteria and enzymes continue their work, breaking down the proteins (proteolysis) and fats (lipolysis) into smaller flavor molecules. These secondary chemical reactions produce the complex, nuanced flavors and aromas that define different cheese varieties. The low pH environment created by the lactic acid also acts as a natural preservative, inhibiting the growth of many spoilage and harmful microorganisms.