Grades 6–8 · Everyday Life
How can liquid milk become a piece of cheese?
Milk contains water, milk sugar, fat globules and proteins. Much of its protein belongs to a family called casein. Casein proteins are grouped into tiny particles that normally remain dispersed. Cheesemaking changes how these particles interact, then separates much of the liquid from the resulting network.
In the diagramMilk proteins become a curd networkRennet-set cheese · many other cheese styles existMilkCurd and wheyCurd retains protein + fatLiquid whey drains awayProtein + fat in waterThe network reorganizes ingredients already in the milk.
In a common rennet-set cheese, enzymes in rennet cut part of a protein that helps stabilize the casein particles. The changed particles can aggregate into a network. This turns the milk into a gel containing water and trapped fat. Starter bacteria often also produce lactic acid, changing acidity and helping determine the later texture and flavor.
The network becomes curd. As curd releases liquid whey, and as whey is drained away, the remaining curd contains a higher proportion of protein and fat. Whey still contains water, milk sugar and other dissolved or dispersed components. No new milk protein has appeared: existing ingredients have been reorganized and separated.
Different cheeses follow different routes. Some use acid rather than rennet to form their curd; some use heat and acid together. Moisture, microorganisms and later ripening help explain why cheese can range from soft and fresh to firm and strongly flavored.
About this explanation
Starting knowledge: Recognize proteins and fat as milk components and understand that an enzyme changes a specific reaction.
Sources
- University of Guelph — Cultured Dairy Products and Cheese