For the best pasta sauce, think of it like salad dressing.
If you’re a regular TASTE reader, your pasta preparation has probably come a long way since the days of dumping jarred Newman’s Own tomato sauce over a bowl of noodles you rinsed in a colander after cooking. You probably dutifully follow a recipe that calls for draining cooked pasta, but saving a half cup or so of the cooking liquid, simmering the pasta in your sauce, then adding that liquid to your pan.
So why do Italians like adding cloudy and salty water to their flavorful ragouts? You can find the answer in salad dressing, specifically vinaigrettes. In a classic vinaigrette recipe, a dollop of mustard binds fatty oil and watery vinegar into a creamy emulsified liquid. Pasta water does the same thing: It binds starchy pasta to watery sauce.
As pasta finishes cooking in the sauce, starches on the surface of the pasta and the cooking water expand and burst, thickening and binding up the surrounding liquid. The result is a thicker, richer, more cohesive sauce that clings to your pasta just like vinaigrette to salad greens. Give it a try with your next spaghetti al pomodoro and taste that luxurious pasta dressing—err, sauce—for yourself.