What if milk never turned into cheese?

Milk is runny. Cheese is solid. But cheese is made from milk!

How does milk change? And what if it never could?

A glass of milk is runny and white. A slice of cheese is solid, and it might be yellow, crumbly, stretchy or full of holes. Yet every cheese started as milk. Something has to change the milk's proteins and sugar to make it.

So what if milk could never turn into cheese? There would be no cheese on pizza, in sandwiches or on crackers. And as we'll see, yogurt would be in trouble too. In this lab we follow three clues: bacteria and enzymes, curds and whey, and yogurt.

The University of Guelph's Dairy Science and Technology e-book defines cheese as the fresh or ripened product obtained after coagulation and whey separation of milk, cream or partly skimmed milk. In essence, cheese is a selective concentration of milk, and thousands of varieties have evolved in different regions of the world.

This thought experiment imagines that milk could never be coagulated into cheese. To reason about it, we look at the two drivers of coagulation, bacterial acid and the enzyme rennet, at what happens to milk's components when curds and whey separate, and at yogurt, which depends on related bacteria.

Cheese makers add tiny helpers called bacteria to milk. They turn milk sugar into acid.

Then they add rennet. The milk turns thick, like jelly.

Cow's milk is about 88 percent water. The rest is fat, protein, sugar, minerals and vitamins. To make most cheeses, cheese makers first add lactic acid bacteria. These helpful microbes eat the milk's sugar, called lactose, and turn it into lactic acid.

Next they add rennet, which contains an enzyme called chymosin. Together, the acid and the rennet make the milk's main proteins, the caseins, clump together into a soft gel, trapping the fat inside. The gel is then cut, and it starts to squeeze out liquid.

According to the Guelph e-book, cheesemaking relies on lactic acid bacteria fermenting lactose to lactic acid. The falling pH assists coagulation, promotes syneresis (expulsion of whey), inhibits spoilage and pathogenic bacteria, and contributes to texture, flavor and keeping quality.

Coagulation is the formation of a gel as casein micelles are destabilized and aggregate into a network that immobilizes water and traps fat globules. It can be achieved by enzymes, most often chymosin (rennet); by acid; or by heat and acid together. Caseins make up about 80 percent of milk protein, and kappa-casein normally stabilizes the micelles. Rennet cleaves kappa-casein at a specific bond, removing that stabilizing ability.

Imagine milk that never changes into cheese. No matter what you add, it stays runny.

No cheese on pizza! No cheese sandwiches!

Now imagine milk that could never turn into cheese. Maybe the caseins could never clump, whatever you added. The milk would stay runny forever.

There would be no cheddar, no mozzarella and no cottage cheese. But there's more to it. The Guelph food scientists explain that cheese was first made as a way to keep the goodness of milk. Cheese gathers most of milk's protein and fat into a solid, and some cheeses are ripened for months or even years. Without cheese, people long ago might have had a harder time saving milk's nutrients. That is an idea, reasoned from what cheese is for.

If casein could never coagulate, by acid, enzyme or heat-acid, every cheese and every acid-set dairy gel would be impossible. The Guelph e-book notes that cheese was traditionally made to preserve the nutrients of milk, concentrating most of its protein and fat into a solid that, depending on variety, ripens over weeks to years.

A world without coagulation might therefore have relied more on other ways of preserving milk. Since yogurt also forms a gel as lactic acid lowers the pH, it might be lost too. These consequences are reasoned from dairy chemistry, not observed.

When milk turns to cheese, it splits into two parts.

The lumpy part is curds. The watery part is whey. Cheese is made from the curds.

Once the milk has turned into a soft gel, cheese makers cut it into small pieces. The pieces, called curds, start to shrink and squeeze out a greenish liquid called whey. Warming them helps squeeze out even more.

Most of the milk's protein and fat stays in the curds, which become cheese. The whey carries away most of the sugar and water, plus some protein, minerals and vitamins. Whey is used in foods and animal feeds. Some cheeses are eaten fresh; others are ripened for weeks or even years.

After the gel reaches the right firmness it is cut, which increases the area for whey release; the curd pieces immediately shrink and expel the greenish whey (syneresis), helped by a cooking step. The curds are then separated, and handling, salting and pressing depend on the variety.

The Guelph e-book describes the balance: most of the protein, fat and some minerals and vitamins are concentrated in the curd, while whey contains most of the sugar and water and some protein, minerals and vitamins. Except for fresh cheese, the curd is then ripened, from weeks to years, as rennet, bacteria and enzymes break down lactose, proteins and fat.

Yogurt is made from milk too. Two kinds of helpful bacteria make it thick and tangy.

If milk could never change, we might not have yogurt either!

Yogurt is another food made by bacteria. In North America, most yogurt is made with a team of two bacteria: Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Together they make acid much faster than either one alone, and they give yogurt its flavor and thick texture.

In the United States, the rules for yogurt actually name these two bacteria. Without milk that could change, these tangy, thick foods might not exist.

The Guelph e-book describes the usual North American yogurt culture as a symbiotic blend of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus: S. thermophilus grows faster and produces acid and carbon dioxide that stimulate L. bulgaricus, whose protein breakdown supplies peptides and amino acids for S. thermophilus. Together they produce yogurt's characteristic flavor compounds, such as lactic acid and acetaldehyde.

The U.S. standard of identity (21 CFR 131.200) defines yogurt as dairy ingredients cultured with these two lactic acid-producing bacteria, with a finished pH of 4.6 or lower within 24 hours of filling.