The classic Italian dish cacio e pepe is just grated cheese on hot pasta served with pepper. The cheese is usually Pecorino Romano (sometimes, misnamed “Romano”) and the pasta can be tonnarelli or spaghetti. Tonnarelli differs only in that it has a square cross section rather than round. And while Pecorino is the preferred cheese, some chefs add up to 30% Parmesan to the Pecorino.

The story behind this seemingly simple dish is that shepherds could carry the dry pecorino (aged sheep’s milk cheese), pasta and pepper and make themselves a hearty meal. Maybe. But despite the fact that this dish has only four ingredients (including water), it is hard to get right in the home kitchen.

In restaurant kitchens, they keep a pot of pasta water boiling, and cook all their pasta in it. Over time, this becomes quite a starchy solution that chefs often use to enhance the mouth feel of sauces. In this dish, that starch content is critical to its success.

When you stir together the pasta, cheese and pasta water, you want the result to be pasta with a creamy sauce. But this depends on there being enough starch in the mixture. If there is too little starch, the cheese is likely to form lumps rather than melting into the sauce.

And, if the temperature is too high, the cheese can go into a gooey “mozzarella phase” instead of dissolving into the sauce.

We read through six or seven different recipes that weren’t a lot different and made it seem easier than it is. Interestingly enough, Lidia Bastianich suggested crushing whole peppercorns under a heavy flat pan rather then grinding the peppercorns, to give big pieces of pepper in the sauce. This is a matter of taste and how much pepper you really want. Babish solved the sauce problem by pouring the hot pasta water over the cheese in a blender. He also adds a bit of butter to the pan where you put the drained spaghetti and after tossing the spaghetti in the butter, he just pours the blended sauce over that pasta. This isn’t quite the same as melting the cheese on the pasta, but it is foolproof.

Research on the phases of cheese

However, the ultimate research on cacio e pepe (Phase behavior of cacio e pepe) was posted on ArXiv by eight physicists who study phases of such liquids. G. Bartolucci, D. M. Busiello, M. Ciarchi, A. Corticelli, I. Di Terlizzi, F. Olmeda, D. Revignas, and V. M. Schimmenti, working at the University of Barcellona, Max Planck Institute in Dresden, University of Padova and the Institute of Science and Technology, Klosterneuberg, Austria participated in studying the phases of the cheese and starch solution. Despite their labs being spread all through Europe, they emphasized that they were all originally Italian, and thus were not messing with someone else’s national dish.

In a nutshell, they determined that if there is not enough starch in the pasta water, the cheese is likely to clump rather than dissolving. They also noted that you should let the pasta cool for about a minute before mixing with the cheese to avoid the dreaded “mozzarella phase.”
So, rather than just using starchy pasta water, they propose making up a warm starch solution to mix with the cheese. We tried this and it works exactly as they described. They recommend potato or cornstarch.

• 240 g pasta (about half a pound, 8.4 oz)
• Water to cover the pasta in a wide, shallow pan.
• 160 g pecorino cheese (5.6 oz)
• 4 g cornstarch in 40 g water
• Freshly ground pepper

1. Mix the cornstarch and water and heat it gently until the mixture becomes nearly clear. The mixture will become quite thick and gelatin-like. (This is known as “starch gelation.”)
2. Grate the pecorino until you have a bit more than 160 g. Save the excess for sprinkling on top. You can use a cheese grater or a food processor.

1. Mix the starch gel with the grated cheese in a bowl, adding room temperature water as needed to make it moist enough to make a smooth mixture. Add ground pepper to the mixture.
2. Bring the water for cooking the pasta to boil in an open, flat pan wider than the length of the pasta, and cook until al dente. Be sure to test the pasta’s doneness, as it will take longer than the suggested 10 minutes, since the small amount of water will reduce the heat of the boiling water.
3. Scoop out around half a cup of pasta water about a minute before the pasta is done.
4. Lift the pasta out of the cooking water into a serving bowl.

1. Let the pasta cool about one minute and then begin to mix in the cheese mixture, adding the slightly cooled pasta water as needed. You want to add enough water so that the cheese mixture mostly dissolves in the water.
2. Add more ground pepper.
3. Divide the pasta into two serving bowls and top with ground cheese (here you could add Parmesan if you wanted) and more pepper. Serve immediately.
   

If you have some left over, you can refrigerate it and reheat it in a microwave without the sauce coming apart, as the starch stabilizes it.

Nathan Myrhvold, the author of Modernist Cuisine, has suggested that you might also be able to prevent the cheese from clumping by adding some sodium citrate, suggesting that this anticoagulant might be more more effective than the starch which could blunt the flavor of the the cheese.