When Cook’s Illustrated test cook Annie Petito set out to rehab pasta salad's reputation as dull and flavorless, the first thing she zeroed in on was the pasta. After choosing corkscrew-shaped fusilli for its ample surface area to capture dressing, she turned to the question of texture: Why do so many pasta salads feature overly firm noodles? She found the answer when she considered the temperature at which the dish is served.
When cooking pasta for a dish that will be served hot, we recommend testing it early and often. Even a couple extra minutes of boiling could push it past the desired point of al dente. But since pasta salad is served cold, the noodles will go through a process called retrogradation as they cool, which makes al dente pasta become overly firm.
Here’s the science behind retrogradation: As pasta cooks, its starch granules absorb water and swell. The chain-like starch molecules that formerly stuck together separate, allowing water to seep in among them. Then, as the pasta cools, the starch chains creep back together, forming tight microscopic crystals. The water that was keeping the molecules separate becomes bound up inside the crystals, and the pasta becomes overly firm because the starch is more rigidly compacted and the water is trapped.
So to account for the extra firmness as the pasta retrogrades, Annie specifies cooking it about three minutes past al dente and then running it under cold water to cool it down. Once it’s cool, the pasta will be perfect.