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Mozzarella Magic: How My Father And I Learned To Separate Curds And Whey

Hanna Walker, cheese and herdsman intern, prepares fresh milk to be filtered.
Bruce Beans for NPR
Hanna Walker, cheese and herdsman intern, prepares fresh milk to be filtered.

My father and I have a cheese habit. To feed this passion, and maybe save us some cash at the cheese counter, we decided it was time we learned to make the stuff ourselves. Our first goal: mozzarella.

On a below-freezing January morning we arrived at in eastern Pennsylvania, ready for a crash course in cheese. The instructor and farm owner, Kathy Fields, met us in the dairy shop. She took on this 26-acre farm in 1997 and a few years later began turning it into an educational center.

Walking with the confidence of a woman who knows her way around a horse drawn plow, she led us to the cheese kitchen, a small shed-like room built into one side of a barn, and presented us with a pot of glistening, yellow-hued milk. Fields's Jersey cows, Apple and Honey, each produce four to five gallons per day. The milk is rich — about one third cream. And Fields and her customers like it that way. "I couldn't drink low fat milk if I tried," she said. "I'd gag on it."

At Fields' direction, I swept my hair under a knit cap and we got to work on mozzarella. My father poured some citric acid into a double boiler and I mixed in one gallon of milk. We needed to increase the acidity for the mozzarella to stretch well, Fields explained.

Fields turned on the double boiler. Once the mixture reached about 88ºF, I added a mix of enzymes, known as rennet. Rennet causes much of the protein in milk to join together and form curds while the liquid is squeezed out as whey. It's found naturally in the stomach lining of young ruminants, like calves or lambs, and is also produced by some microbes and even plants like the cardoon thistle.

I excitedly mixed in the rennet as if I were whisking custard for a quiche, but Fields stopped me short at four whirls around the pot. "If you whisk too much," she warned, "you'll get something that looks like ricotta."

Fields told us to watch for the "curds to sink and shrink." Within four minutes, I understood what she meant. There, at the bottom of the pot, was a solid white mass –curds! And above it, a watery liquid — whey! I cut the curds into two-inch squares and we raised the temperature further. We watched in amazement as the squares shrank more.

Kathy Fields carries milk to her Farm Store and Dairy.
/ Carolyn Beans for NPR
Carolyn Beans for NPR
Kathy Fields carries milk to her Farm Store and Dairy.

My father and I took turns microwaving the curd squares in batches small enough to fit into our hands. After 90 seconds, we took out the warm gooey mass and squeezed and stretched it to drain more whey. Then we microwaved it again, and repeated until we had a warm soft ball of cheese that was stretchy, but not rubbery.

The entire process took under thirty minutes.

I know there's nothing magical about mozzarella. When acidity and the proper enzymes meet milk at the right temperature, curds are bound to form. And yet, actually watching cheese take shape was astounding.

I later called Paul Kindstedt, a food chemist at the University of Vermont and author of " Cheese and Culture: A History of Cheese and its Place in Western Civilization," to find out exactly why the curds formed so quickly after I added rennet.

Kindstedt explained that a molecule called casein makes up about 80 percent of the protein in milk. Casein proteins lump together, about 10,000 at a time, into balls called micelles. The rennet "attacks" these little protein balls, forcing them to link together into an increasingly dense "fishnet matrix."

Kathy Fields, owner of Flint Hill Farm, demonstrates how to form a mozzarella ball.
/ Bruce Beans for NPR
Bruce Beans for NPR
Kathy Fields, owner of Flint Hill Farm, demonstrates how to form a mozzarella ball.

"The rest of cheese making from then on is to create opportunity for that fishnet to squeeze out more of the water trapped in the net," he explained.

I told Kindstedt about my overzealous whisking and he agreed with Fields. The mixture couldn't actually become ricotta, but it would certainly look like it.

"If you keep stirring, any kind of movement of that milk while these net-like structures are forming will cause them to break apart and you'll end up with flecks, particles, and not the curd," he explained. "You end up with a mess, in other words."

I also learned from Kindstedt that by making mozzarella, my father and I were practicing a culinary art that may well reach back to antiquity. Mozzarella is one form of pasta filata, a group of stretchy cheeses that likely originated in the Mediterranean. The process of making pasta filata resembles the "hand cheese" described in written records from the first century A.D.

Fields also taught us to make cheddar, which required a bit more patience. Thanks to a live bacterial culture and a lower cooking temperature, it takes over two whole hours for the curd to form and shrink. Then, there's another two days in a press and a week or more to form a rind.

We left the farm that day with a pint full of mozzarella, some generous wedges of cheddar prepared earlier, and the confidence to make both cheeses ourselves.

Fields warned that cheese making doesn't always go so smoothly. "Cheese is this living creature," she said. "Some days it works. Other days you can do the same thing and the cheese says, 'Not today.'"

My father is going to take his chances. He's planning a dinner party featuring his homemade mozzarella. He'll wait until his guests arrive and then make cheese right in front of their eyes — a good party trick that is likely a couple of millennia old.

Carolyn Beans is a freelance science journalist living in Washington, D.C. She specializes in ecology, evolution, and health.

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Carolyn Beans
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