An emulsion is a scientific concept that means at least two liquids that normally won’t mix are forced to come together.
There are three crucial steps to emulsifying any kind of pasta sauce. They include:
Reserving some pasta water, Introducing fat slowly, Providing some form of agitation — which in this context means lots of stirring/mixing/flipping.
How to Emulsify Pasta Sauce
Here is what the process looks like:
Step 1: Ensure you have some butter in the fridge.
Step 2: Bring heavily salted water to a boil in a two-quart pot.
Step 3: Add 3 ounces of your spaghetti of choice, then cook until you’d consider the pasta a bit underdone. The pasta will keep cooking in the sauce later, so if you pull it out of the water later, by the time you’re done mixing everything together, it will have become overcooked.
Step 4: Before draining the pasta, reserve at least half a cup of the water it cooked in. This water, and the starch left behind from the boiling pasta, can be handy as glue for finishing sauces.
Step 5: In the now empty pot that you cooked the pasta in, pour in about half a cup of your preferred brand of tomato sauce. This should be just enough to cover the pasta without drowning it in sauce. Afterward, bring the sauce to a gentle simmer. The bubbles that form will aid the agitating we require.
Step 6: Get the butter out of the fridge, and add half a tablespoon of butter to the sauce, stirring constantly. The very cold butter will combine with the simmering sauce as it slowly melts. Keep adding half tablespoons of butter until the sauce is thick enough. Depending on how watery or thick your jarred sauce is, the amount of butter you’ll need to add may differ, but ensure you don't exceed 2 tablespoons.
Step 7: When the sauce is ready, add your drained pasta directly to the pan and mix vigorously. The mixing motion will further emulsify and thicken the sauce by pulling in bits of starch from the pasta.
If you find that your sauce is too thick, you can add some of the reserved pasta water, little drops at a time.