When you think about it, consuming the milk of other animals is a freakin’ weird thing to do. Curdling, flavoring, and aging it in order to make cheese? That’s even weirder. But cheese is delicious, so whether it’s weird or not I have no intention of stopping. How exactly does milk magically morph from liquid to solid?
The origin of cheese, as the legend goes, can be traced to one (un)lucky Middle Eastern shepherd, maybe as far back as 8000 BCE. Journeying across the arid plains and lacking a container to carry his milk in, this shepherd fashioned a canteen out of the stomach of one of his sheep. Later, when he went to take a sip of milk, all he found was curds… the chunky precursor to cheese.
To this day, the cheesemaking process begins in pretty much the same way as it did in 8000 BCE, only instead of relying on offal accidents, we employ some nifty biochemistry.
To begin its leap toward immortality, milk first has to make the leap out of a cow, sheep, goat, or other grazing animal. Compared to human milk, the milk of these domesticated ruminants is extremely high in protein. For reasons that will become clear shortly, the low protein content of human breast milk is why you can’t make it into cheese, should you be so inclined (although I sincerely hope you are not so inclined).
The reason that milk curdles in ruminant stomachs is because of baby ruminants. Behold the four-chambered ruminant stomach:
When a cow drinks water, or when grazing on hard-to-digest grasses, they engage all four stomachs, but the microbes that live in the top three chambers would create a dangerously unbelchable amount of gas if they were allowed to drink milk. When suckling, calves instead engage a valve that sends the milk directly to the last of their four stomachs, the abomasum.
It is there that the sugar-, fat-, and protein-laden milk curdles, which our friend the shepherd found out the hard way when he used the abomasum for a canteen. Curdling is good for the calf, because as any parent of a newborn will tell you, milk has a tendency to go right through a baby’s digestive system, if you catch my dirty-diaper-drift. Solid milk curds take longer to pass through the digestive system, so more nutrients can be extracted from the milk.
Milk’s main protein, making up more than 80% of the total, is called casein. One particular form of this protein, kappa-casein, is basically the reason that cheese exists at all. Thank you, K-casein, we owe you big-time.
K-casein isn’t very happy floating around in the aqueous environment of milk, though. Like a shark-attack survivor, it’s a bit hydrophobic. In order to hide from H2O, casein molecules huddle together in globs called micelles, binding up fat and calcium along with it.
You’ll notice that casein is more than just the globby bits, though. Its tail (a “casein point”?), coated with sugars and hydrophilic amino acids, juts out from those micelles, caging the protein in a water-loving coat and keeping your milk from becoming a curdled mess… that is, until rennet comes along.
Rennet, the mixture of enzymes added to cheese cultures to start the curdling process, was originally extracted from the stomach linings of young calves, although today it’s manufactured by genetically engineered microbes. One of those enzymes, chymosin, is what does the curdling in both calf stomachs and cheese houses.
Chymosin acts like a molecular pair of scissors, snipping off the water-loving tail of K-casein at a very specific spot (between amino acid 105, a phenylalanine, and 106, a methionine, if you’re a sucker for detail). Without that cage to keep the micelles dissolved in milk’s watery environment, the micelles clump together in massive knots called curds.
What happens to those curds next is an adventure all its own, and every type of cheese has its own well-aged story.
Whether or not the legend of the shepherd is true or just a cheesy myth, one thing is for certain: When it comes to cheese, the stomach isn’t just where cheese ends its journey, it’s also where it begins.
This post accompanies this week’s episode of It’s Okay To Be Smart on YouTube: The Science of Cheese! Watch it here to learn more about cheese-ology: