Cheese making is a controlled chemical and biological transformation of milk into a solid, flavorful food. Milk contains water, fat droplets, proteins, lactose, minerals, and vitamins, but cheese making concentrates many of these components into curds. The process matters because small changes in acidity, temperature, enzymes, and microbes create very different textures and flavors.
A soft mozzarella, a crumbly cheddar, and a hole-filled Swiss cheese all begin with similar milk chemistry.
Key Facts
- Milk is an emulsion of fat droplets and a suspension of casein protein micelles in water.
- Lactic acid bacteria ferment lactose into lactic acid: C12H22O11 + H2O -> 4 C3H6O3.
- As pH falls, casein micelles lose stability and can clump into curds.
- Rennet contains chymosin, an enzyme that cuts kappa-casein and helps milk coagulate.
- Curds contain most of the casein and fat, while whey contains much of the water, lactose, and soluble proteins.
- Aging changes cheese through proteolysis and lipolysis, which break proteins and fats into smaller flavor molecules.
Vocabulary
- Casein
- Casein is the main milk protein that forms micelles and becomes the structural network of most cheeses.
- Curd
- Curd is the solid or gel-like mass formed when milk proteins coagulate and trap fat and water.
- Whey
- Whey is the liquid left after curds form, containing water, lactose, minerals, and some proteins.
- Rennet
- Rennet is a mixture of enzymes, especially chymosin, used to coagulate milk during cheese making.
- Fermentation
- Fermentation is the process in which microbes convert sugars such as lactose into acids, gases, or other molecules.
Common Mistakes to Avoid
- Confusing curds with whey is wrong because curds are the protein-rich solid phase, while whey is the watery liquid phase.
- Assuming cheese forms only by drying milk is wrong because coagulation, acid production, enzyme action, and microbial activity are central chemical steps.
- Thinking all bacteria in cheese are harmful is wrong because selected lactic acid bacteria safely ferment lactose and help create flavor, acidity, and texture.
- Ignoring pH control is wrong because acidity affects protein charge, curd firmness, microbial growth, and the final taste of the cheese.
Practice Questions
- 1 A cheese maker starts with 20.0 L of milk and obtains 2.4 kg of cheese. What is the cheese yield in kg per liter of milk?
- 2 During fermentation, the pH of milk drops from 6.6 to 5.2. By what factor does the hydrogen ion concentration increase? Use pH = -log[H+].
- 3 A cheese is aged for months and develops stronger flavor and a firmer texture. Explain how protein breakdown, fat breakdown, moisture loss, and microbial activity can cause these changes.