Kid with a plate of cheese. (Istockphoto)

The other day, I grabbed a carton of milk from the refrigerator for my bowl of Rice Krispies. When I poured the milk, I did not hear the “snap, crackle, pop” I’d been expecting. Instead, I heard a plopping sound as clumps of white, smelly goo covered the cereal. When I looked at the carton more closely, I saw that the milk was two weeks past its expiration date. So instead of milk, I had poured a crude form of cheese on my cereal.

But that mishap didn’t go to waste. It gave me the idea to write about cheese.

Most people know that cheese is made from milk. In the United States, milk usually comes from cows. However, in some parts of the world, people get dairy products from goats, buffalo, sheep, camels, reindeer and even yaks.

So how does a bucket of milk become cheese?

Milk contains two types of proteins: casein and whey. The reason expired milk becomes “cheesy” is that bacteria in the milk grow rapidly when it gets old. The bacteria digest the milk sugar (lactose), producing lactic acid as a result. Lactic acid causes the casein to curdle, or separate into lumps, and gives the milk a sour smell.

Cheese is made the same way — by curdling milk — except the milk is curdled on purpose.

Most cheese is made in factories. After milk is poured into big vats, a “starter culture” of bacteria is added to convert the lactose into lactic acid. Then an enzyme called rennet is added to curdle the milk. In the past, rennet was obtained from the stomachs of young cows. Nowadays, cheesemakers get it from bacteria and yeast that have been genetically “taught” to make the enzyme.

Once the casein has curdled, whey protein is left behind as a thin, watery liquid. (Perhaps that reminds you of a certain nursery rhyme: “Little Miss Muffet sat on a tuffet, eating her curds and whey. . . .”)

The whey is removed, salt is added, and the curds are cut into smaller pieces and heated to release more whey. The additional whey is drained off, which leaves clumps of casein. Those clumps are pressed into molds and left to age (dry) for various periods of time.

The way a cheese looks, smells and tastes depends on many factors, including the animal that produced the milk, what the animal was fed, which bacteria were used in the starter culture and how the cheese was processed.

Here are three additional fun facts about cheese:

• Mozzarella cheese has a stringy quality because it’s kneaded like dough before being formed into its final shape.

• Swiss cheese is made with bacteria that produce carbon dioxide (CO2) when they digest lactose. The bubbles of CO2 are what create the holes in the cheese.

• It’s not a coincidence that some cheese smells funky. Limburger cheese, one of the stinkiest varieties of all time, gets its smell from the same bacteria that cause stinky feet — Brevibacterium linens. Some people don’t mind the smell. But if Mom or Dad brings home a block of Limburger, be prepared to hold your nose.

Howard J. Bennett

Bennett is a Washington pediatrician. His Web site, www.howardjbennett.com, includes past KidsPost articles and other cool stuff.