Cheese making is applied chemistry that turns liquid milk into a solid food with a long shelf life and complex flavor. Milk contains water, fats, sugars, minerals, and casein proteins that can be reorganized by acid, enzymes, salt, and time. Understanding the chemistry helps explain why mozzarella stretches, cheddar becomes firm, and blue cheese develops sharp aromas.
It also shows how traditional foods depend on precise control of molecules and microbes.
Key Facts
- Rennet contains chymosin, an enzyme that cuts kappa-casein and allows casein micelles to coagulate.
- Milk sugar lactose is fermented by bacteria: lactose -> lactic acid + energy.
- Lower pH reduces casein stability and helps curds form; many cheeses reach about pH 4.6 to 5.4 during processing.
- Whey is the watery liquid removed from curds, and it contains water, lactose, soluble proteins, and minerals.
- Salt lowers water activity and slows unwanted microbial growth while also improving flavor and texture.
- Aging develops flavor through proteolysis, lipolysis, and fermentation, which produce amino acids, fatty acids, esters, ketones, and sulfur compounds.
Vocabulary
- Casein
- Casein is the main milk protein that forms micelles and becomes the structural network of most cheeses.
- Rennet
- Rennet is an enzyme mixture, often containing chymosin, that causes milk proteins to coagulate into curds.
- Curd
- Curd is the solid protein and fat network that forms when milk coagulates during cheese making.
- Whey
- Whey is the liquid portion of milk that drains away after curds form.
- Proteolysis
- Proteolysis is the breakdown of proteins into smaller peptides and amino acids during cheese aging.
Common Mistakes to Avoid
- Thinking rennet simply makes milk sour is wrong because rennet mainly acts by cutting kappa-casein, while souring is caused by acid production from bacteria.
- Ignoring pH is wrong because casein structure, microbial growth, flavor, and final texture all depend strongly on acidity.
- Adding salt only for taste is wrong because salt also controls water activity, enzyme action, and the growth rate of bacteria and molds.
- Assuming all aged cheeses age the same way is wrong because hard, soft, and blue cheeses use different moisture levels, microbes, oxygen exposure, and enzyme activity.
Practice Questions
- 1 A batch starts with 20.0 L of milk and produces 2.4 kg of cheese. What is the cheese yield in kg per liter of milk?
- 2 A cheese maker adds 45 g of salt to 1.50 kg of curds. What is the salt percentage by mass?
- 3 Explain why a blue cheese needs both specific mold cultures and oxygen exposure, while a hard cheese such as Parmigiano-Reggiano is aged under conditions that limit mold growth inside the wheel.