Whether you’re roasting a whole turkey or grilling a single flank steak, one of the keys to carnivorous cooking success is controlling the amount of moisture in your meat. The higher a piece of meat’s internal temperature, the tighter its muscle fibers become as proteins bind up and literally squeeze moisture out of the muscle.
Enter the brine, a salty bath in which you soak a cut of meat that uses osmotic pressure to literally push extra moisture into the muscle tissue. Brining acts as overcooking insurance, adding juiciness to meat with water and keeping it anchored there with salt. Marinades, on the other hand, are primarily about flavoring. In addition to salt and liquid, they usually feature an acidic component, which cooks often claim is there to “tenderize” the meat.
In truth, the acid in vinegar or lime juice isn’t strong enough to do any real tenderizing in the 4 to 24 hours usually reserved for marinating. But the marinade does coat the surface of the meat with salt and flavorings. Those flavors don’t wind up penetrating very far into the muscle, but even a surface treatment can add the perception of extra flavor and juiciness on your tongue. And ultimately, that’s more important than any water retention percentage.