We made three batches of our standard brine and soaked pork loin roasts for 2, 4, and 24 hours. Then we cut each roast into 1-inch-thick slices and tested the slices for salt penetration by applying first potassium chromate and then silver nitrate, which turned orange at first and then clear again where the salt penetrated; we then measured the width of the rings of unstained meat.

The rate at which salt moved into the meat slowed significantly after 2 hours. After 2 hours, salt had penetrated ⅓ inch; after 4 hours, it had penetrated ½ inch; and after 24 hours, ¾ inch.

Salt naturally moves from areas of high concentration to those of low concentration. And the larger the difference in concentration, the more quickly the sodium ions travel into the meat. At the beginning of the brining time, there’s a huge difference in concentration: The interior of the meat has none of the salt, and the surface of the meat has a lot. As the salt travels through the meat, the difference becomes smaller: The concentration on the outside is lowered, and almost all areas inside the meat have at least some sodium ions, so the rate of diffusion drops. Furthermore, while sodium ions in the brine continue to penetrate the exterior of the pork, that reaction slows as well because the outer layer of pork already contains some sodium.

(The upshot: In the first 4 hours, salt traveled farther than it did in the next 20 hours.)

Because the returns for brining diminish over time, the first few hours of brining a large roast (or the first 30 minutes or so for smaller cuts) are critical, but soaking the meat until the end of a recommended time range—or even a few hours longer—won’t produce an appreciable difference in salt penetration.