As we explained in our previous article, cacio e pepe is a beloved Roman dish made of pasta, pecorino romano cheese, some pasta water and pepper. We explained the research of Giacomo Bertolucci et. el., who determined that most recipes don’t have enough starch in their pasta water to prevent the cheese from clumping. Instead, they proposed added some cornstarch or potato starch solution to the cheese before adding it to the hot pasta.
This approach worked very well. However, food scientist Nathan Myrhvold, the author of the food scientists’ bible, Modernist Cuisine proposed another solution. He suggested adding some sodium citrate to the cheese mixture, which serves to emulsify and stabilize the cheese and also prevents the dreaded clumping.
Well, sodium citrate is easily available online and in cooking stores, and is well known for it uses in making smooth cheese sauces. Just to reassure you, citric acid is the sour taste in citrus fruits such as lemons, limes and all the related fruits, and you can get sodium citrate by mixing lemon juice and baking soda. But since you can get a pound of it (probably a lifetime supply) for around $10 and have it delivered the next day, why bother?
Our recipe
• 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)
• 2 tsp sodium citrate (11 g).
• Freshly ground pepper
1. Grate a little more than the 160g of pecorino cheese in a food processor. Save the excess for sprinkling on top. You can use a cheese grater if you don’t have a food processor.
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. Add the sodium citrate to this water and stir to dissolve.
4. Mix the citrate pasta water with the grated cheese.
5. Lift the pasta out of the cooking water into a serving bowl.
6. Stir in the cheese mixture a little at a time. Add more pasta water if it is too thick. We ended up adding almost another half cup. The cheese shouldn’t clump at all.
7. Grate pepper into the cheese and pasta and stir it in.
8. Serve right away. This makes enough for two hungry people.
9. Serve with a side salad with homemade blue cheese dressing.
How to make sodium citrate at home
We know that lemon juice contains 1.44g of citric acid per ounce.
1. Weigh out 7.6 grams of lemon juice and 14.3 grams of baking soda (sodium bicarbonate). This will give you about 11 g of sodium citrate (2 tsp).
2. Slowly pour the lemon juice into the baking soda and stir. The mixture will foam up as it expels carbon dioxide.
3. When the foaming subsides, your paste containing your sodium citrate is read to use. Stir it into the pasta water and add to the cheese just as above.
The chemistry behind these measurements
Citric acid has the structure
You can also write it as
HOOC-CH2-C(OH)(COOH)-CH2-COOH
Or compactly but confusingly as C6H8O7.
Each of those COOH groups represents a carboxylic acid. Note that there are three of them and we have to neutralize all three.
The molecular weight of citric acid is 192.12 g/mole. Since our 2 tsp of citric acid weighs 11g, we need 11/192 or 0.057 moles of citric acid. Sodium bicarbonate, NaHCO3 has a molecular weight of 84 g/mole, and .057 x 84 is 4.8 grams to make .057 moles. However, since there are three COOH groups, we need three times that much, or 14.4 g of baking soda.
When you mix the lemon juice with the baking soda it foams as carbon dioxide gas is released:
C6H8O7 (aq)+ 3 NaHCO3(s) = 3 CO2(g) + Na3C6H5O7(aq)
The resulting mixture still contains some lemon oil that you may taste slightly in the resulting pasta dish.